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OCR for page 11
Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
2
Nutrient Requirements of the Laboratory Rat
The laboratory rat (Rattus norvegicus) has long been favored as an experimental model for nutritional research because of its moderate size, profligate reproduction, adaptability to diverse diets, and tractable nature. It is now the species of choice for many experimental objectives because of the large body of available data and the development of strains with specific characteristics that facilitate the study of disease and other processes.
ORIGIN OF THE LABORATORY RAT
The laboratory rat is a domesticated Norway rat, which in nature is one of the most widespread and abundant of the more than 70 species of the genus Rattus (family Muridae). The albinos of the Norway rat were first domesticated in Europe in the early 19th century and came into use as experimental animals shortly thereafter (Lindsey, 1979). The Norway rat is not indigenous to Europe; however, it is believed to have originated in Asia and to have taken advantage of human movement in expanding its range worldwide (Nowak, 1991). The origin and historical development of the major strains of laboratory rats have been reviewed by Lindsey (1979).
In the wild, the Norway rat exhibits both territorial and colonial behavior and typically occupies underground burrows (Calhoun, 1963). Females produce 1 to 12 litters per year, and those in a colony nurture their young collectively. The Norway rat is omnivorous, eating a wide variety of seeds, grains, and other plant matter as well as invertebrates and small vertebrates (Nowak, 1991). Other than the fact that it lacks a gallbladder, the rat's digestive tract resembles that of other omnivorous rodents in that the stomach contains both nonglandular and glandular regions, the small intestine is of moderate length, and the cecum is relatively well developed (Bivin et al., 1979; Vorontsov, 1979).
GROWTH AND REPRODUCTIVE PERFORMANCE
Growth and reproductive performance are two key indicators of dietary adequacy. It is important that investigators monitor performance of experimental animals in relation to expected patterns of weight gain and reproduction. Given the large number of strains and different genotypes, it is not possible to describe a single growth pattern or reproductive performance applicable to all laboratory rats. Poiley (1972) summarized body weight gains from birth to about 24 weeks of age for 18 inbred strains and 3 outbred strains of rats. Examples of his mean values for 5 major inbred strains (Brown Norway, Fischer 344, Long-Evans, Osborne-Mendel, and Sprague-Dawley) are illustrated in Figure 2-1. Males gain weight more quickly and become larger than females of the same strain, but there are considerable differences in growth rates and adult body mass among the strains. In view of ongoing genetic selection of the strains of rats, and improvements in diets, the mean growth rates shown in Figure 2-1 may not represent desired performance at present. However, Reeves et al. (1993a) found similar growth rates in Sprague-Dawley rats fed a commercial rat diet for 16 weeks. Investigators should obtain expected or desired growth curves for their experimental rats from the supplier or breeding colony from which animals originate.
Reproductive characteristics such as age at the time reproduction begins, fertility, litter size, growth rates of suckling young, and preweaning mortality also vary among strains. C. T. Hansen (Veterinary Resources Program, National Center for Research Resources, National Institutes of Health, personal communication, 1993) recently summarized selected reproductive parameters of rat strains maintained in breeding colonies at the National Institutes of Health, including 37 monogamously mated inbred strains, 42 harem-mated congenic strains, 5 harem-mated mutant
OCR for page 12
Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
FIGURE 2-1 Mean body weight of male and female rats of five inbred strains: □, Brown Norway; ◇, Fischer 344; ▹, Long-Evans; ■, Osborne-Mendel; and ◆, Sprague-Dawley. SOURCE: Data adapted from Poiley (1972).
stocks, and 3 monogamously mated outbred stocks. Differences in the percentage of sterile matings, litter size, and preweaning mortality were evident among major inbred strains such as Brown Norway, Fischer 344, Osborne-Mendel, Sprague-Dawley, and Wistar (Table 2-1). Outbred strains tended to have higher reproductive performance (Table 2-1). Thus the expected reproductive performance of rats in experimental studies may vary according to strain and system of breeding.
ESTIMATION OF NUTRIENT REQUIREMENTS
Although the nutrient requirements of the laboratory rat are better known than those of other laboratory animals, there can be considerable disparity in estimated requirements as a consequence of the criteria used (Baker, 1986). For example, the amounts of nutrients required to sustain maximum growth of young rats may be different from the
TABLE 2-1 Some Reproductive Characteristics of Representative Strains of Inbred and Outbred Rat Colonies Maintained at the National Institutes of Health
Strain
Sterile
Matings
(%)
Mean
Litter
Size
Preweaning
Mortality
(%)
Inbred strain
Brown Norway
30
5.9
1
Fischer 344
14
7.6
10
Osborne-Mendel
38
7.6
9
Sprague-Dawley
16
4.4
6
Wistar
31
7.9
8
Outbred strain
Osborne-Mendel
6
7.4
6
Sprague-Dawley
2
7.1
2
NOTE: Both strains are from monogamously mated stocks.
SOURCE: Data summarized and provided by C. T. Hansen (Veterinary Resources Program, National Center for Research Resources, National Institutes of Health, personal communication, 1993).
amounts needed to maintain tissue concentrations or to maximize functional measures such as enzyme activities. Moreover, nutrient requirements are not static; they change according to developmental state, reproductive activity, and age. There is also evidence of differences in requirements between males and females as well as among various inbred and outbred strains. The nutrient requirements listed in this chapter represent average values, but they may not suffice in all circumstances. There is a need for further research that will identify the sources of variation in nutrient requirements.
Recommended nutrient concentrations in this report have not been increased to allow a margin of safety for variation in dietary ingredients or for differences among rats. The data on which requirements are based were reported from many different laboratories that used various colony management practices. One may assume that the recommendations are adequate for rats in most laboratory conditions, but particular experimental protocols such as maintenance of germ-free colonies or testing of experimental drugs (see Chapter 1) may alter the requirements for one or more nutrients. In some cases sufficient data were available to differentiate the nutrient requirements for adult maintenance from those for growing, pregnant, or lactating rats; hence, estimates of requirements are provided for maintenance, growth, and reproduction (Table 2-2). If data available were insufficient to determine requirements, adequate concentrations are reported on the basis of long-term feeding. If cited papers provided nutrient intakes per day but did not specify dietary concentrations, the values have been converted to dietary content by assuming a dietary intake of 15 g/rat/day for growing rats or adult rats at maintenance, 15 to 20 g/rat/day during pregnancy, and 30 to 40 g/rat/day during lactation.
OCR for page 13
Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
TABLE 2-2 Estimated Nutrient Requirements for Maintenance, Growth, and Reproduction of Rats
Amount, per kg diet
Nutrient
Unit
Maintenance
Growth
Reproduction (Female)
Fat
g
50.0
50.0
50.0
Linoleic acid (n-6)
g
a
6.0a
3.0a
Linolenic acid (n-3)
g
R
R
R
Protein
g
50.0b
150.0b
150.0
Amino Acidsc
Arginine
g
ND
4.3
4.3
Aromatic AAsd
g
1.9
10.2
10.2
Histidine
g
0.8
2.8
2.8
Isoleucine
g
3.1
6.2
6.2
Leucine
g
1.8
10.7
10.7
Lysine
g
1.1
9.2
9.2
Methionine + cystinee
g
2.3
9.8
9.8
Threonine
g
1.8
6.2
6.2
Tryptophan
g
0.5
2.0
2.0
Valine
g
2.3
7.4
7.4
Other (including nonessentials)
g
f
66.0
66.0
Minerals
Calcium
g
g
5.0
6.3
Chlorideh
g
g
0.5
0.5
Magnesium
g
g
0.5
0.6
Phosphorus
g
g
3.0
3.7
Potassiumh
g
g
3.6
3.6
Sodium
g
g
0.5
0.5
Copper
mg
g
5.0
8.0
Iron
mg
g
35.0
75.0
Manganese
mg
g
10.0
10.0
Zinci
mg
g
12.0
25.0
Iodine
µg
g
150.0
150.0
Molybdenum
µg
g
150.0
150.0
Selenium
µg
g
150.0
400.00
Vitamins
A (retinol)j
mg
g
0.7
0.7
D (cholecalciferol)k
mg
g
0.025
0.025
E (RRR-α-tocopherol)l
mg
g
18.0
18.0
K (phylloquinone)
mg
g
1.0
1.0
Biotin (d-biotin)
mg
g
0.2
0.2
Choline (free base)
mg
g
750.0
750.0
Folic acid
mg
g
1.0
1.0
Niacin (nicotinic acid)
mg
g
15.0
15.0
Pantothenate (Ca-d-pantothenate)
mg
g
10.0
10.0
Riboflavin
mg
g
3.0
4.0
Thiamin (thiamin-HCl)m
mg
g
4.0
4.0
B6 (pyridoxine)n
mg
g
6.0
6.0
B12
µg
g
50.0
50.0
NOTE: Nutrient requirements are expressed on an as-fed basis for diets containing 10% moisture and 3.8–4.1 kcal ME/g (16–17 kJ ME/g) and should be adjusted for diets of differing moisture and energy concentrations. Unless otherwise specified, the listed nutrient concentrations represent minimal requirements and do not include a margin of safety. Higher concentrations for many nutrients may be warranted in natural-ingredient diets. R, required but no concentration determined; other long-chain n-3 polyunsaturated fatty acids may substitute for linolenic acid. ND, not determined.
a Females require only 2 g/kg linoleate for growth. Separate requirements for maintenance have not been determined for linoleate. Requirements presented for growth will meet maintenance requirements.
b Estimates based on highly digestible protein of balanced amino acid composition (e.g., lactalbumin).
c Asparagine, glutamic acid, and proline may be required for very rapid growth (see text).
d Phenylalanine plus tyrosine. Tyrosine may supply up to 50 percent of aromatic acid requirement.
e Cystine may supply up to 50% of the methionine plus cystine requirement on a weight basis.
f 41.3 g/kg diet as a mixture of glycine, L-alanine, and L-serine.
g Separate requirements for maintenance have not been determined for minerals and vitamins. Requirements presented for growth will meet maintenance requirements.
h Estimate represents adequate amount, rather than true requirement.
i Higher concentration is required when ingredients that contain phytate (such as soybean meal) are included in the diet.
j Equivalent to 2,300 IU/g. Requirement may also be met by 1.3 mg β -carotene/kg diet. Higher vitamin A concentration is needed under conditions of stress (e.g., surgical recovery).
k Equivalent to 1,000 IU/kg.
l Equivalent to 27 IU/kg. Higher concentration may be required if high-fat diets are fed.
m Higher concentration may be required with low-protein, high-carbohydrate diets.
n Estimate represents adequate amount, rather than true requirements.
EXAMPLES OF DIETS FOR RATS
The type of diet and its nutrient composition will vary according to experimental objectives (see Chapter 1). Most practical diets include nutrient concentrations that exceed requirements as a margin of safety. Examples of natural-ingredient diets based on detailed formula specifications and used successfully to maintain rat colonies at the National Institutes of Health and at other facilities are provided in Table 2-3. The ingredient specifications, however, have not been updated for some years and are not entirely in agreement with recommendations in this report.
As indicated in Chapter 1, natural-ingredient diets do not offer the same control over nutrient concentrations or potential contaminants as do purified diets. An example of a purified diet that has often been used in rat studies is given in Table 2-4. However, as there has been concern about the standardization and concentrations of some constituents, as well as the clinical observation of nephrocalcinosis in females when this diet is used (Reeves, 1989; Reeves et al., 1993a), a change in formulation is warranted. A committee of the American Institute of Nutrition (AIN) has reformulated and tested new purified diets for the
OCR for page 14
Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
TABLE 2-3 Examples of Natural-Ingredient Diets Used for Rat and Mouse Breeding Colonies at the National Institutes of Health
Ingredient
Conventional
(NIH-07)
Autoclavable
(NIH-31)
Basic Diet, g/kg diet
Dried skim milk
50.0
Fish meal (60% protein)
100.0
90.0
Soybean meal (48% protein)
120.0
50.0
Alfalfa meal, dehydrated (17% protein)
40.0
20.0
Corn gluten meal (60% protein)
30.0
20.0
Ground #2 yellow shelled corn
245.0
210.0
Ground hard winter wheat
230.0
355.0
Ground whole oats
100.0
Wheat middlings
100.0
100.0
Brewer's dried yeast
20.0
10.0
Dry molasses
15.0
Soybean oil
25.0
15.0
Salt
5.0
5.0
Dicalcium phosphate
12.5
15.0
Ground limestone
5.0
5.0
Mineral premix
1.2
2.5
Vitamin premix
1.3
2.5
Mineral Premix, mg/kg diet
Cobalt (as cobalt carbonate)
0.44
0.44
Copper (as copper sulfate)
4.40
4.40
Iron (as iron sulfate)
132.30
66.20
Manganese (as manganous oxide)
66.20
110.00
Zinc (as zinc oxide)
17.60
11.00
Iodine (as calcium iodate)
1.54
1.65
Vitamin Premix, per kg diet
Stabilized vitamin A palmitate or stearate
6,060.00 IU
24,300.00 IU
Vitamin D3 (D-activated animal sterol)
5,070.00 IU
4,190.00 IU
Vitamin K (menadione activity)
3.09 mg
22.10 mg
All-rac-α-tocopheryl acetate
22.10 mg
16.50 mg
Choline chloride
617.00 mg
772.00 mg
Folic acid
2.43 mg
1.10 mg
Niacin
33.10 mg
22.10 mg
Ca-d-pantothenate
19.80 mg
27.60 mg
Pyridoxine-HCl
1.87 mg
2.21 mg
Riboflavin supplement
3.75 mg
5.51 mg
Thiamin mononitrate
11.0 mg
71.7 mg
d-Biotin
0.15 mg
0.13 mg
Vitamin B12 supplement
0.004 mg
0.015 mg
NOTE: Amounts listed for mineral and vitamin premixes represent the mass or IU of the specific mineral element or vitamin rather than the added compounds.
SOURCES: Knapka (1983), Knapka et al. (1974), and National Institutes of Health (1982).
growth (AIN-93G) and maintenance (AIN-93M) of rats and mice (Reeves et al., 1993a,b; Reeves et al., 1994). These diets are included in Table 2-5.
ENERGY
Purified diets containing 5 to 10 percent fat have gross
TABLE 2-4 Example of a Commonly Used Purified Diet (AIN-76A) for Rats
Ingredient
Amount, g/kg diet
Basic Diet
Sucrose
500.0
Casein (≥85% protein)
200.0
Cronstarch
150.0
Corn oil (including 0.01-0.02% antioxidanta)
50.0
Fiber source (cellulose-type)
50.0
Mineral mix (listed below)
35.0
Vitamin mix (listed below)
10.0
DL-Methionine
3.0
Choline bitartrate
2.0
Mineral Premix
Calcium phosphate, dibasic (CaHPO4)
500.00
Potassium citrate, monohydrate (K3C6H5O7·H2O)
220.00
Sodium chloride
74.00
Potassium sulfate
52.00
Magnesium oxide
24.00
Ferric citrate (16-17% Fe)
6.00
Manganous carbonate (43–48% Mn)
3.50
Zinc carbonate (70% ZnO)
1.60
Chromium potassium sulfate [CrK(SO4)2·12H2O]
0.55
Cupric carbonate (53–55% Cu)
0.30
Potassium iodate (KIO3)
0.01
Sodium selenite (Na2SeO3·5H2O)
0.01
Sucrose, finely powdered
118.03
Vitamin premix
Nicotinic acid or nicotinamide
3.000
Calcium d-pantothenate
1.600
Pyridoxine-HCL
0.700
Thiamin-HCl
0.600
Riboflavin
0.600
Folic acid
0.200
d-Biotin
0.020
Cyanocobalamin (vitamin B12)
0.001
Retinyl palmitate or acetate (vitamin A)
+b
α-Tocopheryl acetate (vitamin E)
+c
Cholecalciferol (vitamin D3)
0.0025
Menaquinone (vitamin K)
0.005
Sucrose, finely powdered
To make <1,000 g
SOURCE: American Institute of Nutrition (1977, 1980).
a Betahydroxytoluene or Santoquin.
b As stabilized powder to provide 400,000 IU vitamin A activity (120,000 retinol equivalents).
c As stabilized powder to provide 5,000 IU vitamin E activity.
energy (GE) values of about 4.0 to 4.5 Mcal/kg (17 to 19 MJ/kg). The digestible energy (DE) of most purified diets ranges from 90 to 95 percent of GE (Hartsook et al., 1973; McCraken, 1975; Deb et al., 1976). The metabolizable energy (ME) varies from 90 to 95 percent of DE (Hartsook et al., 1973; Pullar and Webster, 1974; McCraken, 1975; Deb et al., 1976). These values may be somewhat lower when diets formulated from natural-ingredients are used
OCR for page 15
Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
TABLE 2-5 Examples of Recently Tested Purified Diets for Rapid Growth of Young Rats and Mice or for Maintenance of Adult Rats and Mice
Ingredient
Growth
(AIN-93G)
Maintenance
(AIN-93M)
Basic Diet, g/kg diet
Cornstarch
397.486
465.692
Casein (≥85%)
200.000
140.000
Dextrinized cornstarcha
132.000
155.000
Sucrose
100.000
100.000
Soybean oil (no additives)
70.000
40.000
Fiber source (cellulose)b
50.000
50.000
Mineral mix (listed below)
35.000
35.000
Vitamin mix (listed below)
10.000
10.000
L-Cystine
3.000
1.800
Choline bitartrate (41.1% choline)
2.500
2.500
Tert-butylhydroquinone (TBHQ)
0.014
0.008
Mineral Premix, g/kg mixc
Calcium carbonate, anhydrous (40.0% Ca)
357.00
357.00
Potassium phosphate, monobasic
(22.8% P, 28.7% K)
196.00
250.00
Potassium sulfate (44.9% K, 18.4% S)
46.60
46.60
Potassium citrate tri-K, monohydrate, (36.2% K)
70.78
28.00
Sodium chloride (39.3% Na, 60.7% Cl)
74.00
74.00
Magnesium oxide (60.3% Mg)
24.00
24.00
Ferric citrate (16.5% Fe)
6.06
6.06
Zinc carbonate (52.1% Zn)
1.65
1.65
Manganous carbonate (47.8% Mn)
0.63
0.63
Cupric carbonate (57.5% Cu)
0.30
0.30
Potassium iodate (59.3% I)
0.01
0.01
Sodium selenate anhydrous (41.8% Se)
0.01025
0.01025
Ammonium paramolybdate 4 hydrate (54.3% Mo)
0.00795
0.00795
Sodium meta-silicate 9 hydrate (9.88% Si)
1.4500
1.4500
Chromium potassium sulfate 12 hydrate (10.4% Cr)
0.2750
0.2750
Lithium chloride (16.4% Li)
0.0174
0.0174
Boric acid (17.5% B)
0.0815
0.0815
Sodium fluoride (45.2% F)
0.0635
0.0635
Nickel carbonate (45% Ni)
0.0318
0.0318
Ammonium vanadate (43.6% V)
0.0066
0.0066
Powdered sucrose
221.0260
209.8060
Vitamin Premix, g/kg mixd
Nicotinic acid
3.000
3.000
Calcium pantothenate
1.600
1.600
Pyridoxine-HCI
0.700
0.700
Thiamin-HCI
0.600
0.600
Riboflavin
0.600
0.600
Folic acid
0.200
0.200
d-Biotin
0.020
0.020
Vitamin B12 (cyanocobalamin 0.1% in mannitol)
2.500
2.500
All-rac-α-tocopheryl acetate (500 IU/g)e
15.000
15.000
Retinyl palmitate (500,000 IU/g)e
0.800
0.800
Vitamin D3 (cholecalciferol 400,000 IU/g) Vitamin K1 (phylloquinone)
0.075
0.075
Powdered sucrose
974.655
974.655
a Ninety percent tetrasaccharides or higher oligosaccharides such as Dyetrose (Dyets Inc., Bethlehem, PA) Lo-Dex (American Maize Co., Hammond, NJ) or equivalent.
b Solka-Floc 200 FCC (FS&D Corp., St. Louis, MO) or equivalent.
c Mineral mix for growth referred to as AIN-93G-MX and for maintenance as AIN-93M-MX.
d Vitamin mixes referred to as AIN-93-VX
e Dry gelatin-matrix form.
SOURCE: Adapted from Reeves et al. (1993b).
(Yang et al., 1969; Peterson and Baumgardt, 1971a). The addition of cellulose to natural-ingredient diets decreases energy digestibility (Yang et al., 1969; Peterson and Baumgardt, 1971a) even though 15 to 60 percent of the cellulose is digested (Conrad et al., 1958; Yang et al., 1969; Peterson and Baumgardt, 1971b). Some of the decrease in energy digestibility is caused by the low digestibility of cellulose and some is caused by increased fecal nitrogen losses (Meyer, 1956).
In general the rat will consume food to meet its energy requirement (Brody, 1945; Mayer et al., 1954; Sibbald et al., 1956, 1957; Yoshida et al., 1958; Peterson and Baumgardt, 1971b; Kleiber, 1975). Yoshida et al. (1958) reported that the daily caloric intake remained constant when the diet contained from 0 to 30 percent fat. A proportionate increase in consumption of diet occurs when the diet is diluted with inert materials. A maximum concentration of 40 percent dilution of the diet could be made for weanling female rats before caloric intake was reduced, whereas 50 percent dilution could be made for mature females (Peterson and Baumgardt, 1971b). The energy requirement of the lactating female is high, and dilution of the diet with only 10 percent of inert material results in a significant decrease in DE intake (Peterson and Baumgardt, 1971b). Inadequate dietary protein may decrease energy intake (Menaker and Navia, 1973).
Temperature, age, and activity influence the energy requirement of the rat. The lower critical temperature of the fasting rat is 30°C (Swift and Forbes, 1939; Brody, 1945; Kleiber, 1975). The lower critical temperature is that environmental temperature below which heat production must be increased to maintain body temperature. The basal metabolic rate of the rat can be estimated from the general formula
where Hkcal is the heat production in kcal per day, BW is the body weight in kilograms, and 72 is the average heat production (kcal) per kg0.75 of 26 groups of rats studied (Kleiber, 1975). This formula is valid only for estimating the basal heat production of mature animals. If kilojoules (kJ) are used as the unit of energy, the formula is Hj = 301 BWkg0.75.
OCR for page 16
Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
MAINTENANCE
The maintenance energy requirement can be generally defined as that portion of the total energy requirement that is separate from the needs for growth, pregnancy, and lactation. Animals fed at maintenance are in energy equilibrium. Advantages and disadvantages of the methods by which the maintenance energy requirement may be estimated have been reviewed by van Es (1972). The maintenance energy requirement is usually expressed as energy required per unit of body weight in kilograms to the 0.75 power (BWkg0.75), and is based on the work of Brody (1945) and Kleiber (1975).
An estimate of the maintenance requirement for the adult rat (300 g) can be made by increasing the value for basal heat production (300 BWkg0.75; Kleiber et al., 1956) by approximately 20 percent (Morrison, 1968) to cover the expected requirement for activity in a laboratory setting. This value, which is in net energy units, can be converted to ME units by dividing by 0.75, based on an efficiency of 75 percent in conversion of ME to net energy (McCraken, 1975). The daily maintenance energy requirement in ME units for adult rats, based on these assumptions, is 114 kcal ME/BWkg0.75 (477 kJ ME/BWkg 0.75). This value is remarkably similar to direct estimates of the maintenance energy requirement in ME units at 100 kcal (418 kJ; Pullar and Webster, 1974), 106 kcal (444 kJ; McCraken, 1975), 130 kcal (544 kJ; Deb et al., 1976), and 91 kcal (381 kJ; Ahrens, 1967). However, the requirement for fat rats (e.g., obese Zucker) is approximately 15 percent lower than the requirement for normal rats (Pullar and Webster, 1974; Deb et al., 1976). As this difference in maintenance energy requirement between the adult normal and adult obese Zucker is not determined by body weight, the practice of estimating maintenance energy requirement solely from body weight must be used with caution (Pullar and Webster, 1974; Webster et al., 1980).
It is well established that resting heat production per BWkg0.75 is greater in working and producing animals than in nonworking animals (Brody, 1945). A portion of this difference is attributable to differences in amount of intake. Walker and Garrett (1971) demonstrated that a decrease in food intake of rats results in a decrease in energy expenditure and in the maintenance energy requirement.
Previous plane of nutrition also influences maintenance energy requirement. Rats fed at a high plane of nutrition (39.6 g/BWkg0.75/day) for a 3-week period had a 38 percent greater heat production when compared to rats on a low plane (28.8 g/BWkg0.75/day) of nutrition (Koong et al., 1985). Experiments with sheep indicate that changes in organ mass associated with amount of intake and sequence of feeding may be largely responsible for altered heat production (Koong et al., 1985). Thus an accurate prediction of the maintenance energy requirement of the rat requires consideration of the amount of intake, previous nutritional history, physiological state, and other factors including the composition of the body (see Baldwin and Bywater, 1984, for a review). Nonetheless, data suggest that maintenance energy requirements of the rat will be met in most cases by consumption of 112 kcal/BW kg0.75/day (470 kJ/BWkg0.75/day).
GROWTH
It is difficult to estimate the energy requirement for growth because of variation in the composition of weight gain (Meyer, 1958; Schemmel et al., 1972; Hartsook et al., 1973; McCraken, 1975; Deb et al., 1976) and in the energetic efficiency of net protein and fat synthesis. Kielanowski (1965) used a multiple regression model that partitioned total ME intake (MEI) into a component proportional to body weight, representing maintenance energy requirement, a component proportional to energy gain as fat, and a component proportional to energy gain as protein or lean body mass. Separation of the efficiencies of energy gain in fat and lean is problematic, however, leading to inconsistent and sometimes biologically impossible results (see Chapter 3, which deals with the mouse). Pullar and Webster (1974) attempted to circumvent some of these difficulties by using obese and lean Zucker rats, which partition energy differently between gain in fat and in lean at all stages of growth. The energetic efficiencies of fat and net protein synthesis were estimated at 65 and 43 percent, respectively. These estimates assumed that the maintenance energy requirement remained constant. In a subsequent experiment that did not require assumptions about maintenance energy requirements, Pullar and Webster (1977) determined the energetic efficiencies of fat and net protein deposition as 73.5 and 44.4 percent, respectively. Kielanowski (1976) concluded from a review of previous work that the energetic efficiency of net protein deposition in growing rats is approximately 43 percent.
The requirement of the rat for maintenance and growth can be met by diets with a wide range of energy densities. Peterson and Baumgardt (1971b) reported that weanling and mature rats consumed 225 and 150 kcal DE/BWkg0.75 (940 and 630 kJ/BWkg0.75), respectively, when the energy density of the diet varied from 2.5 to 5.0 kcal DE/g (10.5 to 20.9 kJ/g). When the energy density in the diet fell below 2.9 kcal/g (12.1 kJ/g), the weanling rat could not meet its energy requirement. The mature rat could meet its energy requirement until DE density fell to values below 2.5 kcal DE/g (10.5 kJ/g). These values are equivalent to diluting the diet with 40 and 50 percent inert material for weanling and mature rats, respectively. During the 4-week growth period after weaning at 21 days postpartum, the average daily energy requirement is at least 227 kcal/BWkg0.75) and may be greater. A diet con-
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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
taining at least 3.6 kcal ME/g (15.0 kJ ME/g) will meet the energy requirement for maintenance and growth if rats are allowed free access to food and the diet is not deficient in other nutrients.
GESTATION AND LACTATION
The energy requirement for gestation appears to be 10 to 30 percent greater than that of the mature but nonreproductive female rat (Morrison, 1956; Menaker and Navia, 1973). Food intake of rats fed diets adequate in protein increased 10 to 20 percent (Menaker and Navia, 1973) or 20 to 30 percent during the first days of gestation and up to 140 percent by days 16 to 18 of gestation (Morrison, 1956). Total heat production in pregnant rats increased approximately 10 percent above that of nonpregnant female rats (Brody et al., 1938; Kleiber and Cole, 1945; Morrison, 1956; Champigny, 1963). Approximately one-third of the 100 to 201 kcal (420 to 840 kJ) stored during gestation is deposited in fetal tissues (Morrison, 1956). Restriction of the diet during gestation decreases the size and viability of the young and may induce resorption (Perisse and Salmon-Legagneur, 1960; Berg, 1965). Protein appears to be more critical than energy for satisfactory reproduction (Hsueh et al., 1967; Menaker and Navia, 1973).
The daily ME requirement of the rat is about 143 kcal/BWkg0.75 (600 kJ/BWkg0.75) in early gestation and may increase to 265 kcal/BWkg0.75 (1,110 kJ/BWkg0.75) during the later stages of gestation. Lactating rats consume from two to four times more energy than nonlactating female rats, and the magnitude of increase depends on the number of offspring being nursed (Nelson and Evans, 1961; Peterson and Baumgardt, 1971b; Menaker and Navia, 1973; Grigor et al., 1984). Some of the increase in measured intake late in the lactation period may be caused by the consumption of diet by the litter, but this is not significant until about 15 to 17 days postpartum. In spite of the large increase in feed consumption, however, rats are generally in negative energy balance at peak lactation. Losses of both body fat and protein can occur. In general it appears that lipid is stored in the maternal body during gestation and then mobilized to support the lactation process (Sampson and Janson, 1984). Naismith et al. (1982) concluded that hormonal factors were responsible for the storage of lipid during gestation and the mobilization of body fat during lactation. They suggested that body fat supplies a major portion of the energy required to support lactation. Sainz et al. (1986) fed lactating rats diets containing 12, 24, and 36 percent protein and 4.50, 4.37, and 4.04 kcal ME/g (18.8, 18.3, and 16.9 kJ/g). Although fed ad libitum, the lactating rats lost body fat from days 7 to 14 of lactation regardless of diet. It is difficult to establish an energy ''requirement" for the lactating rat based on body energy change. At peak lactation, rats rearing 8 pups produced about 41 g milk/day, representing a milk energy output of about 239 kcal/BWkg0.75/day (1,000 kJ/BWkg0.75 /day) (Kametaka et al., 1974; Oftedal, 1984). It is likely that during peak lactation, the dam's daily ME requirement to support lactation will be at least 311 kcal/BWkg0.75 (1,300 kJ/BWkg0.75), but the ME requirement will vary with litter size.
LIPIDS
Lipid is an important component of the rat diet because it provides essential fatty acids (EFA) and a concentrated energy source, aids in the absorption of fat-soluble vitamins, and enhances diet acceptability.
ESSENTIAL FATTY ACIDS
n-6 Fatty Acids
The rat requires fatty acids from the n-6 family as a component of membranes, for optimal membrane-bound enzyme function, and to serve as a precursor for prostaglandin formation (Mead, 1984; Dupont, 1990; Clandinin et al., 1991). Linoleic acid [18:2(n-6)] cannot be synthesized endogenously but can be elongated and desaturated to form arachidonic acid [20:4(n-6)]; thus, the requirement for n-6 fatty acids can be met through dietary linoleic acid. Early estimates of the requirement for n-6 fatty acids were made by using growth and skin condition as response criteria. The requirement for growing male rats was estimated to be 50 to 100 mg/day and that of growing female rats to be 10 to 20 mg/day (Greenberg et al., 1950). That the requirement differs between sexes is in agreement with early observations that male rats are more susceptible than female rats to the development of EFA deficiency signs (Loeb and Burr, 1947). A series of later experiments indicated that other lipid components of the diet [e.g., oleic acid (Lowry and Tinsley, 1966) and cholesterol (Holman and Peiffer, 1960)] increased the need for n-6 fatty acids. Also, Holman (1960) demonstrated that more linoleic acid is needed in high-lipid diets versus low-lipid diets in order to meet the EFA requirement of the rat. Therefore it has become common practice to express the requirement as a percent of dietary ME rather than as a percent of diet weight. The Δ6 desaturase enzyme involved in the synthesis of long-chain polyunsaturated fatty acids uses the fatty acid substrates in the following order: n-3 > n-6 > n-9 (Johnston, 1985).
Holman (1960) found that the ratio of 20:3(n-9) to 20:4(n-6) (also called the triene:tetraene ratio) was relatively constant in tissues until clinical signs of EFA deficiency began to develop. Because this biochemical measure was both objective and relatively stable (the precise ratio varied
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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
slightly among tissues), Holman (1960) initiated the use of the triene:tetraene ratio to indicate inadequate EFA status. Diets high in n-3 fatty acids also lower the triene concentration (Morhauer and Holman, 1963; Rahm and Holman, 1964) because of the competition for desaturation mentioned above. Therefore the triene:tetraene ratio is a valid indicator of EFA status only when n-6 fatty acids are the principal unsaturated dietary fatty acids. Pudelkewicz et al. (1986) used this technique (in conjunction with dermatitis and growth) to estimate the linoleic acid requirement of growing female and male rats to be 0.5 percent and 1.3 percent of dietary ME, respectively.
The requirement for pregnancy is met by diets adequate for growth, while that for lactation is somewhat higher. Deuel et al. (1954) conducted two experiments to estimate these requirements. In the first experiment, 0, 10, 40, 100, 200, 400, and 1,000 mg cottonseed oil (approximately 50 percent linoleic acid) were fed daily. The number of litters born and the average number of pups per litter increased and then plateaued at 10 and 100 mg dietary cottonseed oil (e.g., 5 and 50 mg linoleate), respectively. When pure linoleate was fed as the fat source in a second experiment in which rats were provided with 0, 2.5, 5.0, 10, 20, 40, and 80 mg linoleate daily, the maximum litters born and average number of pups per litter increased and then plateaued at 2.5 and 20 mg linoleate, respectively. This higher amount of linoleic acid intake will be achieved if the pregnant rat consumes 18 g/day and the diet contains 0.25 percent of dietary ME as linoleic acid.
The requirement for lactation also can be estimated from these experiments. Indicators of successful lactation (the maximum average weight per pup in a litter at 21 days and minimum percent mortality between 3 and 21 days) reached a plateau between 100 to 200 mg cottonseed oil (50 to 100 mg linoleate) in the first experiment. In the second experiment, these criteria for successful lactation reached a maximum at 80 mg of linoleate. Because greater amounts of linoleate were not tested, it is not possible to determine whether the optimal requirement for lactation is more than 80 mg/day. In addition, the authors did not indicate the range of experimental error; therefore, a precise requirement cannot be established. Assuming a requirement of 100 mg linoleate/day, a lactating rat consuming 35 g diet/day will consume sufficient linoleate for lactation (100 mg/day) if the diet contains 0.68 percent of dietary ME as linoleate (approximately 0.30 percent of the diet).
Bourre et al. (1990) proposed that the n-6 fatty acid requirement be established as the percent of dietary linoleate that results in a constant concentration of tissue arachidonic acid. They found that constant concentrations of arachidonic acid were achieved at 150 mg (nerve tissue), 300 mg (testicle), 800 mg (kidney), and 1,200 mg (liver, lung, heart) linoleate/100 g diet. The minimal requirement then was expressed as 1,200 mg linoleic acid/100 g diet (or approximately 2.5 percent of dietary ME) because some tissues require a higher concentration of arachidonic acid to reach a plateau. Because the authors did not relate the arachidonic acid concentration plateau to any functional phenomena, the validity of this method of estimating the n-6 fatty acid requirement remains to be established.
n-3 Fatty Acids
The essentiality of the n-3 fatty acids has been equivocal until recently. It was initially demonstrated that the n-3 fatty acid, α-linolenic acid [18:3(n-3)], could substitute, in part, for the requirement of n-6 fatty acids (Greenberg et al., 1950). Tinoco et al. (1971) failed to show any change in growth of rats raised for three generations on diets lacking in n-3 fatty acids in comparison to litter mates fed diets containing 1.25 percent linoleate and 0.25 percent linolenate. However, the sequestering of n-3 fatty acids in specific tissues (retina, cerebral cortex, testis, sperm; Tinoco, 1982) and the tenacity with which they retain these fatty acids, despite the variation in dietary concentration (Crawford et al., 1976), led many researchers to speculate that n-3 fatty acids were required for some function in the body.
Bernsohn and Spitz (1974) fed rats lipid-free diets for 4 months and measured slightly decreased amounts (38 percent of control values) of monoamine oxidase and 5'-mononucleotidase in cerebral cortex that responded to dietary α-linolenic but not to linoleic acid. Retinal function may be negatively impacted in offspring of rats fed a low linolenate oil for two to three generations (Okuyama et al., 1987). Lamptey and Walker (1976) found reduced exploratory behavior in second generation rats fed safflower oil. A large percent of safflower oil is linoleic acid, and a very small percent is linolenic acid. This was confirmed in studies by Enslen et al. (1991) who showed a reduction in exploratory behavior in 16- to 18-week-old rats from dams fed safflower oil 6 weeks prior to mating and throughout gestation and lactation. These researchers found that when rats were switched to α-linolenic acid at weaning, they did not recover exploratory behavior, further suggesting a specific requirement for n-3 fatty acids during development.
Yamamoto et al. (1987, 1988) found a reduction in brightness discrimination learning in offspring from rats fed safflower oil through two generations in comparison to rats similarly fed perulla oil, a rich source of α-linolenic acid.
Bourre et al. (1989) measured impairment of nerve terminal Na+, K+ -ATPase activity, brain 5'-nucleotidase, and 2',3'-cyclic nucleotide-3'-phosphodiesterase in offspring from rats fed 1.8 percent sunflower oil in comparison to those from dams fed 1.9 percent soybean oil through two generations. They estimated the requirement for n-3 fatty
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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
acids as the least amount of α-linolenic acid in the diet that resulted in a higher concentration of brain n-3 fatty acids. Using this methodology, they estimated a requirement of 2 g/kg food (or 0.4 percent of the total dietary ME). These experiments indicate that n-3 fatty acids are required; further study is needed to determine the amount below which functional impairment occurs.
Although a requirement has not been defined, it may be advisable to include a source of n-3 fatty acids when dietary oils such as sunflower or safflower are fed through two or more generations. Homeostatic mechanisms appear to sequester n-3 fatty acids to protect the rat from n-3 deficiency during short-term dietary deprivation.
SIGNS OF EFA DEFICIENCY
A deficiency of EFA results in a plethora of gross clinical signs, anatomical changes, and physiological changes as discussed by Holman (1968, 1970). Classical overt signs include diminished growth, dermatitis, caudal necrosis, fatty liver, impaired reproduction, increased triene:tetraene ratio in the tissue and blood, and increased permeability of skin, with impaired water balance. There are many other less noticeable but equally severe changes that have been reported, including kidney lesions and a decrease in urine volume (Sinclair, 1952), lipid-containing macrophages in the lung (Bernick and Alfin-Slater, 1963), increased metabolic rate (Wesson and Burr, 1931), decreased capillary resistance (Kramar and Levine, 1953), and aberrant ventricular conduction (Caster and Ahn, 1963).
The EFA content of the rat's diet prior to the feeding of EFA-deficient diets affects reserve stores of EFA (Guggenheim and Jurgens, 1944). Weanling rats rapidly exhibit signs of EFA deficiency after consuming a lipid-free diet for 9 to 12 weeks, while mature rats may require an extensive period of starvation, then refeeding of a lipid-free diet in order to develop EFA deficiency signs (Barki et al., 1947). Dermal signs resulting from EFA deficiency were reported after feeding adult rats a lipid-free diet for 35 weeks (Aaes-Jorgensen et al., 1958). Pups from dams fed EFA-deficient diets exhibit the most severe signs of EFA deficiency and usually die within 3 days to 3 weeks after birth, depending on the duration of the EFA feeding to the dam (Guggenheim and Jurgens, 1944; Kummerow et al., 1952). Other dietary factors [e.g., cholesterol (Holman and Peiffer, 1960) and 18:2(n-6) trans,trans fatty acid (Hill et al., 1979; Kinsella et al., 1979)] accelerate the development of EFA deficiency in rats fed EFA-deficient diets. Males may develop signs of EFA deficiency more quickly than females because males have a greater EFA requirement than do females (Morhauer and Holman, 1963; Pudelkewicz et al., 1968). Prevention of coprophagy will accelerate the development of EFA deficiency in rats fed lipid-free diets (Barnes et al., 1959a).
The relative capacities of n-6 and n-3 fatty acids to alleviate some of the deficiency signs are shown in Table 2-6. The classical signs of EFA deficiency appear to be more amenable to amelioration by n-6 fatty acids. It was hypothesized that the sole function of linoleic acid was as a precursor to arachidonic acid [20:4(n-6)] (Rahm and Holman, 1964; Yamanaka et al., 1980). Linoleic acid concentration is much lower than that of arachidonic acid in membrane lipids (Sprecher, 1991). However, linoleate-rich O-acyl sphingolipids have been identified in the epidermis of pigs and humans (Gray et al., 1978). The structures of pig and human epidermal acyl ceramide and acyl glucosyl ceramide were confirmed by Wertz et al. (1986). Hansen and Hensen (1985) fed EFA-deficient rats oleic [18:1(n-9)], linoleic [18:2(n-6)], columbinic [18:3(n-6)], α-linolenic [18:3(n-3)], and arachidonic [20:4(n-6)] acid esters and measured epidermal sphingolipids and trans-epidermal water loss. Only n-6 fatty acid esters restored the water barrier; however, among n-6 fatty acids, only linoleate was esterified in substantial amounts in the sphingolipids. The authors suggested that columbinate and arachidonate result in linoleate mobilization from other tissues for incorporation into the epidermal sphingolipids. The relationship between linoleate-containing epidermal sphingolipids and trans-epidermal water loss awaits further study.
DIGESTIBILITY OF LIPIDS
Most commercial sources of dietary lipid consist exclusively of triglycerides and contain a high percentage of 18-carbon fatty acids. During digestion, lipase activity releases fatty acids from the 1 and 3 positions of the triglyceride. Free fatty acids and 2-monoacyl glycerol are absorbed.
Digestibility differs among lipid sources. Crockett and Deuel (1947) demonstrated that lipid digestibility was reduced when the melting point of the lipid was greater than 50° C. Fatty acid composition also may affect digestibility. Mattson (1959) showed that digestibility was reduced with increasing content of simple triglycerides (same fatty acid at each position) composed of 18-carbon saturated fatty acids. Saturated fatty acids and monounsaturated trans-fatty acids of 18-carbon chain length or longer are poorly
TABLE 2-6 Relative Ability of n-6 and n-3 Fatty Acids to Alleviate Several Signs of EFA Deficiency in Rats
Sign
Ability to Alleviate Sign
Diminished growth
n-6 better able than n-3
Impaired reproduction
n-6 will alleviate; n-3 ineffective
Dermatitis
n-6 will alleviate; n-3 ineffective
Increased triene:tetraene ratio in tissues and blood
n-6 equal to n-3
Capillary fragility
n-6 equal to n-3
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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
absorbed as free fatty acids but easily absorbed in the 2-monoacyl glycerol form (Linscher and Vergroeson, 1988). Increasing the number of double bonds in the fatty acid improved absorption; increasing the fatty acid chain length decreased absorption (Chen et al., 1985, 1987a). Very long-chain n-3 fatty acids were poorly hydrolyzed in in vitro experiments (Brockerhoff et al., 1966; Bottino et al., 1967) but were well absorbed in unesterified forms (Chen et al., 1985).
The rate of triglyceride digestion and subsequent absorption of fatty acids depends on the nature of the fatty acid (chain length and number and position of double bonds) and its molar frequency and position in the triglyceride (Apgar et al., 1987; Nelson and Ackman, 1988). Tables of fatty acid composition of dietary fats that include their positional specificity are available (Small, 1991).
Utilization of dietary lipid may be affected by other dietary components (Vahouny, 1982; Carey et al., 1983). Estimates of utilization by using the lipid source in the absence of other dietary ingredients may not correctly define the utilization of lipid in a complex diet. Nelson and Ackman (1988) reviewed the literature on the use of ethyl esters of lipid to study absorption and concluded that absorption and transport may not be identical to naturally occurring (triglyceride) lipid sources. The digestibility of many dietary lipids has been determined and certain of these experimental results are summarized in Table 2-7.
DIETARY LIPID CONCENTRATION
Better growth, reproduction, and lactation performance result when rats are fed diets in which lipid content is increased from 5 to 40 percent (Deuel et al., 1947). These observations and those of Forbes et al. (1946a,b) led Deuel (1950, 1955) to conclude that 30 percent lipid is the optimal dietary concentration. These response criteria alone are no longer considered sufficient to establish the optimal amount of dietary lipid.
Maximum growth also is associated with a decrease in longevity (French et al., 1953). Rats fed diets containing 10 or 20 percent corn oil had higher growth rates of a transplantable mammary tumor than those fed diets containing 2 or 5 percent corn oil (Kollmorgen et al., 1983). Rolls and Rowe (1982) demonstrated diminished growth and survival of pups suckled by dams fed high-lipid diets. Another study showed that rats consuming diets containing 3 or 20 percent lipid had superior reproduction (total numbers of offspring; percent of young weaned) compared to rats consuming diets containing 36 and 50 percent lipid (Richardson et al., 1964). It appears, then, that maximum growth should not be the only predictor of optimal dietary lipid content.
Data from the following experiments serve as justification for maintaining the previously recommended amount of 5 percent dietary lipid for both males and females during rapid growth and for adult females during reproduction and lactation (National Research Council, 1978). Swift and Black (1949) showed that the greatest improvement in energy retention occurred when dietary lipid content was increased from 2 to 5 percent; additional increments in energy retention were smaller when lipid content was above 5 percent. Deuel et al. (1947) reported that the greatest reduction in number of days required to reach puberty occurred when the percentage of lipid in the diet was increased from 0 to 5 percent. Relatively small changes occurred when lipid was greater than 5 percent of the diet. Burns et al. (1951) demonstrated that 5 percent lipid was satisfactory for absorption of carotene and vitamin A. Loosli et al. (1944) reported only slight improvement in weight gain of rat pups when lactating females were fed diets that contained more than 5 percent lipid. Furthermore, many lipids provide sufficient EFA when included in the diet at these concentrations. Reeves et al. (1993b: pp. 1941-1942) noted the following:
Bourre et al. (1989, 1990) used the method of dietary titration of 18:2(n-6) and 18:3(n-3) to determine linoleic and linolenic acid requirements, respectively. They used tissue saturation of 20:4(n-6) and 22:6(n-3) to make the assessments and concluded that 12 g of linoleic acid and 2 g of α-linolenic acid per kilogram of diet were the minimal requirements for the rat. This amounts to approximately 3 percent soybean oil in the diet. However, to reach the plateau for maximal concentrations of these fatty acids in many tissues of growing rats, an amount of fat equivalent to 5-6 percent soybean oil was required.
Lee et al. (1989) suggest that a n-6:n-3 ratio of five and a polyunsaturate:saturate (P:S) ratio of two are the points of greatest influence on tissue lipids and eicosanoid production. Bourre et al. (1989) suggested that the optimal n-6:n-3 ratio is between one and six. Soybean oil is a source of dietary fat that may meet these criteria. The oil contains about 14 percent saturated fatty acids, 23 percent monounsaturated fatty acids, 51 percent linoleic acid, and 7 percent linolenic acid. This gives a n-6:n-3 ratio of seven, and a P:S ratio of approximately four. The fatty acid composition of commercial lipid sources must be monitored because of the widespread practice of hydrogenation and the emergence of new cultivars with different fatty acid compositions.
CARBOHYDRATES
Although no definite carbohydrate requirement has been established, rats perform best with glucose or glucose precursors (such as other sugars, glycerol, glucogenic amino acids) in their diets. Diets containing 90 percent of dietary ME from fatty acids and 10 percent from protein were unable to support growth of young male rats. The substitution of neutral fats (soybean oil) for fatty acids or the addition of glycerol equivalent to that in the triglyceride allowed growth but not at rates equivalent to that achieved
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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995
TABLE 2-7 Digestibility of Some Selected Dietary Fats
Test Fat
Fat in Diet
Fat Digestibility, %
Test Animal and Method
Experiment 1 (Hoagland and Snider, 1943)
Butterfat
5%
87.9*
Male albino weanling rats
15%
90.2*
Mutton, tallow
5%
74.2†
15%
85.0*¹
Cocoa butter
5%
63.3§
15%
81.6‡
Soybean oil
5%
98.7¶
15%
98.3¶
Corn oil
5%
97.5‡
15%
98.3¶
Coconut oil
5%
98.9¶
15%
96.5¶
Experiment 2 (Crockett and Devel, 1947)
Margarine (mp = 34° C)
15%
97.0¶
Adult female rats; calculations similar to Experiment 1
Bland lard (mp = 43° C)
15%
94.3¶
Hydrogenated lard (mp = 55° C)
15%
63.2*
Experiment 3 (Apgar et al., 1987)
Corn oil
5%
92.9*
Male weanling Sprague-Dawley rats; calculations similar to Experiment 1
10%
96.7¶
20%
96.3¶
Cocoa butter
5%
58.8†
10%
60.3†
20%
71.7‡
Experiment 4 (Chen et al., 1987b)
Corn oil
170 mg
100.0¶
Adult male Wistar rats, 250–350 g; lipid emulsion infused via duodenal catheters; lipid collected as lymph from thoracic duct catheters; method measures absorption; corn oil set as control = 100%
Palm kernal oil
170 mg
82.3¶
Cocoa butter
130 mg
63.0*
Experiment 5 (Chen et al., 1989)
Corn oil
170 mg
100.0¶
Same as Experiment 4
Menhaden oil
170 mg
56.6*
Max EPA® (eicosapentanoic acid-containing fish oil concentrate)
170 mg
47.1*
NOTE: Different symbols following measurements in the "Fat Digestibility" column indicate that digestibility mean differs significantly (P < 0.5).
with a 78 percent starch diet (Konijn et al., 1970; Carmel et al., 1975). When carbohydrate-free diets containing 80 percent of dietary ME from fatty acids and 20 percent from protein were fed, rats were capable of weight gain, but growth increased when the diet was supplemented with glucose or neutral fats (Goldberg, 1971; Akrabawi and Salji, 1973). Rats fed low-protein (10 percent of dietary ME), carbohydrate-free diets were hypoglycemic and demonstrated abnormal glucose tolerance curves (Konijn et al., 1970; Carmel et al., 1975); rats fed higher protein (18 percent of dietary ME), carbohydrate-free diets had normal blood glucose concentrations but still demonstrated slightly abnormal glucose tolerance curves (Goldberg, 1971). When neutral fats replaced fatty acids in carbohydrate-free diets (20 percent of dietary ME from protein), growth did not improve when rats were allowed to eat ad libitum but was greater with diets containing the neutral fats when rats were meal-fed once daily (Akrabawi and Salji, 1973).
Over wide ranges of dietary fat:carbohydrate ratios (0.2 to 1.4, ME basis), the heat increment was found to be constant at 47.5 percent of ME, indicating that carbohydrate and lipid are used with equal efficiency (Hartsook et al., 1973).
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