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OCR for page 121
6
Body Composition Changes
During Pregnancy
In the development of standards for optimum weight gain during
pregnancy, or in the use of weight gain to identify suboptimal pregnancies,
the variability in the components of weight gain must be recognized. These
include the products of conception (fetus, placenta, and amniotic fluid),
uterine and breast tissue, extracellular fluid, and maternal fat. These
components change over the course of pregnancy and to different extents
in different individuals, markedly affecting the interpretation of weight gain.
Although measurement of weight gain can be a clinically useful screen-
ing method for identifying some pregnancies that are progressing abnor-
mally, it provides very limited information regarding changes in body com-
position of an individual pregnant woman, even when weight gain is close
to the average for normal pregnancies. Information on body composition
would add substantially to understanding of the meaning of a given weight
gain. Fetal growth may be influenced more by specific maternal tissue
changes, for example, by accretion of lean tissue, fat, or body water, than
by total gestational weight gain. Body composition studies in appropri-
ate animals could provide valuable information in this regard. However,
even if changes in lean tissue should prove to be more important for fe-
tal outcomes, methods would still be needed to determine accurately the
net amount of \fat stored during normal pregnancy for estimating energy
requirements, since fat is the most calorie-dense substance deposited.
121
OCR for page 122
122
NUTRITIONAL STAN-US AND WEIGHT GAIN
STANDARI) METHODS
In the most widely used model for examining body composition, the
body is regarded as being composed of only two compartments fat and
lean. In this usage, lean body mass represents a mixture of all the nonfat
tissues of the body. Most techniques currently used to estimate body
composition are based on measuring the qualities of the lean body tissues.
Of the commonly used methods, only density measurements are dependent
on both fat and lean tissue, but the fat estimate is still highly influenced by
the variability of the lean tissue density. In the two compartment model,
the weight of fat is the difference between two large masses" body weight
and lean body mass. Therefore, a small relative error in the lean body mass
estimation will produce a much larger relative error in calculated body fat.
There are three standard methods for estimating lean body mass:
measurement of total body water, determination of total body potassium
content, and underwater weighing, which permits estimation of total body
density, thereby allowing simultaneous estimation of both fat and lean
tissue. Inherent in each of these methods are assumptions relating the
actual measurements to specific body compartments. The assumptions are
discussed here to assist in later interpretation of the data.
Total Body Water
1b calculate lean tissue from total body water, the water content of
the lean tissue must be known. Although the average percentage of water
in lean tissue is known with fair accuracy in adult women, the nonfat
tissues added during pregnancy (edema fluid, fetus, amniotic fluid, plasma)
contain a high percentage of water. Thus, pregnancy may increase the
water content of lean tissue from approximately 72.5% at 10 weeks of
gestation to about 75.0 at 40 weeks in women with generalized edema
(van Raaij et al., 1988~. A difference of this magnitude can cause fat to be
underestimated by 50% or more in women gaining 3 to 4 kg of fat.
Since gestational changes in lean tissue hydration in individual women
have not been measured in body water studies, only approximate correc-
tions are possible. Theoretical corrections for dilution of the lean tissues
during pregnancy may improve estimates of body composition changes for
a population; lean tissue estimates for an individual (which are impor-
tant for relating body composition to pregnancy outcome) may still be
inexact, although they are useful for identifying markedly aberrant cases.
Interpretation of body water changes might be improved with a measure
of extracellular water. Variation in extracellular water can be substantial.
Hytten (1980) estimated that pregnant women with generalized edema have
more than 3 kg (6.6 lb) of additional extracellular fluid compared with that
OCR for page 123
BODY COMPOSITION CHANGES DURING PREGNANCY
123
in women with no edema or leg edema only. Extracellular water can be
determined either with the use of an extracellular tracer such as bromide
or by estimation of intracellular water from measurement of total body
potassium and determination of extracellular water by difference from total
body water. There have been few studies in which extracellular water has
been measured with tracers appropriate for use in pregnant women. Three
small studies (Emerson et al., 1975; Forsum et al., 1988; Pipe et al., 1979)
combined total body water and total body potassium measurements. These
are discussed below.
Underwater Weighing
Underwater weighing is based on the assumption that the weight of
fat and lean tissue can be estimated from total body density by using
standard values for the average densities of fat and lean tissues. Because
of the increased hydration of lean tissue during pregnancy, and especially
because added tissue includes little bone, which is dense, the density of
the lean body mass is likely to decline during pregnancy. Using theoretical
estimates of body composition during pregnancy, Fidanza (1987) estimated
that the density of the fat-free body declines from 1.100 kg/m3 at 10
weeks of gestation to 1.087 at 40 weeks of gestation. If nonpregnancy
lean tissue density values are used for pregnant subjects, lean body mass
will be underestimated and fat will be overestimated, perhaps by as much
as 2.5 kg at term (see van Raaij et al., 1988~. In addition, the true mean
density of the lean body tissue differs among individuals because of differing
proportions of the organs, muscle, and bone comprising the lean body and
may also change to varying degrees over the course of pregnancy because
of the differential growth of various tissues, especially those with a high
water content and no bone.
Total Body Potassium
Measurement of total body potassium can be used to estimate lean
body mass if a standard value for the concentration of potassium in the
lean tissues is assumed. The vast majority of the body's potassium (approx-
imately 98%) is intracellular; therefore, total body potassium is actually a
reflection primarily of the intracellular compartment. Substantial changes
in the extracellular compartment can go undetected. For this method to
give a good estimate of the weight of total lean tissue, the ratio of intra-
cellular to extracellular tissue must be either close to the norm or assessed
independently. This ratio of intra- to extracellular water decreases during
pregnancy, resulting in overestimation of fat if not corrected. Independent
OCR for page 124
124
NUTRITIONAL STATUS AND WEIGHT GAIN
measures of hydration (total body water, extracellular water) allow correc-
tion for individual variation, as has been done in some studies (Forsum et
al., 1988; Pipe et al., 1979~.
Despite their limitations, total body water, underwater weighing, and
total body potassium are the three best methods for studying body compo-
sition in pregnant women. However, measurement of total body potassium
and underwater weighing require special, large equipment and considerable
patient cooperation, and estimation of total body water requires special iso-
topes that are expensive to use and measure. These considerations have
encouraged the use of simpler methods, such as measurement of skinfold
thicknesses with calipers.
SKINFOLD THICKNESS MEASUREMENT
Changes in skinfold thickness have been widely used to estimate
changes in the fat content of pregnant women. Skinfold thickness mea-
surements suggest that more maternal fat is accumulated centrally than
peripherally (Taggart et al., 1967~. Skinfold thickness can be measured
quickly with relatively inexpensive equipment. As one early researcher
cautioned, however, ". . . skinfold measurements are relatively inaccurate
and . . . a high degree of standardization is required to obtain reliable
comparisons, even with one observer" Braggart et al., 1967, p. 441~. Proper
use requires extensive training and monitoring to consistently achieve re-
producible measurements.
1b convert skinfold thickness measurements to estimates of body fat,
standard regression equations are used. Generally, these are based on
studies correlating skinfold thickness to body fat measured by total body
water, body density, or total body potassium. Of special importance is the
fact that the most widely used regression equations for interpreting skinfold
thicknesses in pregnant women (Durnin and Womersley, 1974) have been
developed in studies of nonpregnant subjects. Longitudinal studies of
skinfold thickness in pregnant women (Taggart et al., 1967) suggest that
skinfold thickness in late pregnancy may be increased by water retention.
Therefore, an observed increase in skinfold thickness may not indicate an
increase in body fat. The magnitude of this hydration effect may also vary
from one measurement site to another, as indicated by a decrease in some
skinfold thicknesses between the final weeks of pregnancy and the first
month post partum. This has been studied by Adair et al. (1984) in Taiwan,
by Taggart et al. (1967) in Scotland, and by Forsum et al. (1989) in Sweden.
Thus, especially during late pregnancy, skinfold thickness measurements
may be less indicative of body fat content. Because skinfold measurements
are used (despite their limitations) in many clinical settings, it would be of
OCR for page 125
BODY COMPOSITION C~4NGES DURING PREGNANCY
125
great value to develop calibration equations derived from pregnant women
whose body fat was estimated with the best methods and models available.
The applicability of the equations may also be affected by differences
in age, ethnic background, and exercise patterns of the reference and study
populations. The usefulness of a regression equation depends in part on
the comparability of the measurement techniques used in the population
under study and in the reference population from which the equation was
derived. Different or less experienced workers in the same research group
may obtain different values or measurement variabilities (braggart et al.,
1967~. Therefore, even very exact regression equations obtained by one
group of investigators may give less accurate estimates of body fat when
applied in a new study.
DIRECT COMPARISONS OF SKINFOLD THICKNESS
MEASUREMENT S
In several studies, skinfold thickness values themselves have been used
without calculating body fat content. In this approach, investigators used
either individual skinfold thicknesses (Frisancho et al., 1977; Maso et al.,
1988; Viegas et al., 1987) or a sum of several different skinfold thicknesses
(Arroyo et al., 1978; Lawrence et al., 1984; Prentice et al., 1981; Taggart
et al., 1967), but they did not assume that all the measured change reflects
changes in body fat. This approach may be conceptually more justifiable
than relating skinfold thicknesses to body fat. Furthermore, by combining
skinfold thickness measurements with arm circumference measurements, it
is possible to estimate arm muscle area, which reflects the amount of lean
tissue. This could be of value, since it is not known whether maternal fat
or lean tissue increments are more important for fetal growth. Frisancho et
al. (1977) observed that maternal arm fat in Peruvian women was related
to infant fatness but not birth weight, whereas arm muscle area was related
to infant length. In contrast, Maso et al. (1988) observed that arm fat area
and arm circumference changes between weeks 22 and 32 of gestation were
correlated with birth weight in a U.S. black population, but arm muscle
area was not.
Changes in total body water and plasma volume also reflect com-
ponents of the lean tissue. Both these measurements have been related
to birth weight (Duffus et al., 1971~. Thus, although fat changes may
contribute most to gestational calorie needs, lean tissue may influence
important aspects of fetal growth. Clearly, better understanding of the
importance of lean tissue changes during pregnancy is needed. Develop-
ment of this understanding will require further study of total body water,
total body potassium, extracellular water, and plasma volume and their
relationship to pregnancy outcome.
OCR for page 126
126
NUTRITIONAL STATUS AND WEIGHT GAIN
TABLE 6-1 Changes in Total Body Water from Studies Covering
Different Periods of Gestation
Change in
Country and Number of Total Body Period of Comments on Study
Reference Subjects Water, liters Gestation, wk Subjects
Scotland 82-91 7.74 + 2.63 (SD)a 10 to 38 Mixed parities
Hytten et al., 84 1.88 10 to 20
1966 82 2.80 20 to 30
91 3.03 30 to 38
Scotland 48 7.3 10 to 38 Estimated from their
Taggart et reported dry
al., 1967b weight
Scotland 35 3.2 3~32 to 3~39 Primiparous, under
Duffus et al., age 30
1971
United States 5 4.4 20 to 40 Four subjects
(Boston) restricted their
Emerson food intake; one
et al., 1975 was obese
England 27 7.2 1(}14 to 3~38 Normal
Pipe et al., 3.3 1~14 to 2~28 prepregnancy
1979 3.9 2~28 to 3~38 weight for height
Scotland 81 3.8 30 to 38 Obese women
Campbell,
1983
Sweden 22 5.7C Prepregnancy Twenty-one
Forsum to 36 weeks multiparous
et al., 1988 4.2 1~18 to 30
2.0 30to36
a SD = Standard deviation.
b It is not clear whether the patients in the study by Taggart et al. (1967) are a subset of
those in the study by Hytten et al. (1966~. Hytten's group had 39 primiparas and 54 multiparas
and Taggart's group had 23 and 25, respectively, drawn from the same research site, at about
the same time, with the same coauthors.
c All other studies used deuterium oxide dilution to determine total body water. This study
used water labeled with Oxygen.
Several newer methods (e.g., total body electrical conductivity, bioim-
pedance analysis, and computerized axial tomography) for measuring fat or
lean tissue may produce accurate results quickly and relatively easily. Some
have gained acceptance by being validated against more familiar methods.
However, there are no formal reports of their application to pregnant
women for consideration by this subcommittee.
OCR for page 127
BODY COMPOSITION CHANGES DURING PREGNANCY
TABLE 6-2 Estimated Total Maternal Weight Gain, Corrected to 40
Weeks of Gestation,a from Six Studies
127
Total Weight
Reported Mean Gain, kg
Weight Gain, Period of (corrected to 40
Reference kg + SD Gestation, wk weeks of gestation)
Hytten et al., 1966 11.15 + 3.34 1~38 13.0
Taggart et al., 1967b 11.0C 1~38 12.8
Emerson et al., 1975 9.2c NRd 9.2e
Pipe et al., 1979 10.4C 1~14 to 3~38 12.0
Campbell, 1983 9.2c 2~38 13.8
Forsum et al., 1988 13.6 + 3.0 Prepregnancy 13.6
to delivery
a See text for method used to correct to 40 weeks of gestation.
b See footnote b in Table 6-1.
c Standard deviations (SD) were not reported.
d NR = Not reported.
e Not adjusted. Mean of "maximum gain" as reported by the authors, based on reported
prepregnancy weights. Some of the five subjects were limiting their food intake.
RESULTS OF STUDIES
Total Body Water
Table ~1 presents data from studies of total body water during preg-
nancy. The tabulated results illustrate the interpretive problems: wide
variations in weight gains among studies (see Table 6-2), different periods
used to compute the gestational increment in body water, and small sample
sizes.
A better comparison of the tabulated studies, which cover different
gestational periods, can be made by normalizing all the weight gain figures
to 40 weeks of gestation (Table 6-2~. A graph of maternal weight gain
(Hytten and Leitch, 1971) was used to estimate the weight that would have
been gained before and after the measurement periods listed in Able 6-2.
On the basis of this standard, expected additional weight gains are 0.5 kg
(1.1 lb) before 5 weeks, 1 kg (2.2 lb) before 10 weeks, 4 kg (8.8 lb) before
20 weeks, 1.3 kg (2.9 lb) after 37 weeks, and 0.8 kg (1.8 lb) after 38 weeks of
gestation. Able 6-2 shows the estimated total weight gains for six studies.
These numbers suggest a consistency in average weight gain among the
studies, whereas the values given for the various periods actually measured
did not.
The data provided by Pipe et al. (1979) and Hytten et al. (1966), which
include measurements at three and four gestational periods, respectively,
OCR for page 128
128
NUTRITIONAL STATUS AND WEIGHT GAIN
TABLE 6-3 Estimations of Fat Gain from Body Water Studies
Change in Body Equivalent Change in
Number of Water, in Lean Body Fat Gain
Subjects kg + SDa b Mass,C kg Weight, kg Estimate, kg
Hytten et al., 75 7.74 + 2.63 8.5 ll.~:Sa 2.65
1966
Taggart et al., 48 7.3d 8.0 ll.Oa 3.0
1967
Emerson et 5 6.3d (2.0 kg 6.9 8.2e 1.3
al., 1975 added for 1~20
wk)
Pipe et al., 27 7.2d (1(}12 to 3~ 7.9 10.4 2.5
1979 38 wk)
Campbell, 81 8 5d (4 7 kg 9.3 12.8e 3.5
1983 added for 1~30
wk)
Forsum et al., 22 5.7d,/ 6.3 11.7
1988
NOTE: All data are normalized to a period ranging from 10 to 38 weeks or more of
gestation and are corrected for the extra hydration of pregnancy.
a From 10 to 38 weeks of gestation.
b SD = Standard deviation.
C Assuming 91.1% water in added lean tissues at term; see van Raaij et al. (1988~. This
corrects for the added hydration of pregnancy, which would otherwise result in an under
estimation of fat.
d Standard deviation not reported or not applicable.
e One kilogram was subtracted from the estimate of weight gain for the entire pregnancy
to eliminate gains before 10 weeks of gestation.
f The values for body water and pregnancy weight gain are the reported values from
prepregnancy to 36 weeks of gestation.
provide a means of evaluating the body water values obtained in the other
studies. Both data sets show a larger increment of body water later in
pregnancy than early in pregnancy. In general, other studies that provide
fewer longitudinal data are in agreement (Campbell, 1983; Duffus et al.,
1971~. The early increments obtained by Pipe et al. (1979) or Hytten et al.
(1966) can be used in combination with data from studies that only provide
measurements for the last half of gestation to estimate total increases in
body water and body fat. Table 6-3 presents the results of six studies, along
with estimations where needed.
The highest estimate for fat gain was obtained by Forsum et al. (1988~.
Their result was influenced by the value they found for body water incre-
ments, which was much lower than those reported by Hytten et al. (1966)
and Campbell (1983~. Of these three studies, only the one reported by
Forsum and colleagues involved the use of water labeled with the isotope
Oxygen, rather than deuterium, and saliva, rather than blood, to moni-
tor dilution of the isotonically labeled water. Unless a correction is made,
OCR for page 129
BODY COMPOSITION CHANGES DURING PREGNANCY
129
the use of deuterium oxide will lead to a higher estimate of body water
compared with that from the use of Oxygen water (which is considered
the more accurate method), because deuterium exchanges with nonaqueous
hydrogen to a small extent. These methodologic differences may contribute
to the lower body water values obtained by Forsum and colleagues. How-
ever, since deuterium oxide values are corrected for hydrogen exchange, it
is unlikely that this factor completely explains the reported differences.
Nevertheless, the study by Forsum and colleagues is provocative, be-
cause it is the first longitudinal body water study that includes actual
prepregnancy measurements. In addition, both the body water and total
body potassium values they obtained suggest a loss of lean tissue in early
pregnancy, as discussed below. Other considerations suggest that caution
be exercised in accepting the data of Forsum and colleagues. For example,
Clapp et al. (1988) showed an increase of both fat and lean tissue during
the first 7 weeks of gestation and from weeks 7 to 15 of gestation (based on
skinfold thicknesses). I5ggart et al. (1967), however, reported no increase
in skinfold thicknesses in a group of women followed from before concep-
tion through early pregnancy. The finding by Forsum et al. (1988) that total
body water at 6 months pOSt partum is lower than the prepregnancy value,
while body fat is 3.2 kg (7 lb) above prepregnangy levels, is surprising. It
is difficult to accept the fact that women were retaining so much fat while
they lost enough weight to place them below their prepregnancy weight.
van Raaij et al. (1988) found that a 1.7-kg (3.7 lb) increase in weight from
the prepregnancy to the postpartum period was associated with a 1.5-kg
(3.3 lb) increase in fat, as determined by densitometry. Further studies will
be needed to provide certainty about the changes that occur during early
pregnancy.
Body Density (by Underwater Weighing)
Body density has been measured in two studies of pregnant women. In
the most recent study (van Raaij et al., 1988), a new approach was used to
interpret the measurements, correcting for changing density as pregnancy
advances. This approach can also be applied to the earlier study by Seitchik
et al. (1963), who reported individual values.
When consistent methods of calculation are used, the two studies on
body density are in excellent agreement (Bible 6-4~. All the differences
in fat gain could be due to the differences in weight gains and gestation
periods studied. These two studies give corrected fat estimates that are
within the range of values obtained from total body water measurements.
Total Body Potassium
Results from studies of total body potassium in adult pregnant women
are shown in Figure 6-1. Comparison of the three data sets reveals large
differences in the absolute values obtained for total body potassium, espe
OCR for page 130
130
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OCR for page 131
BODY COMPOSITION CHANGES DURING PREGNANCY
3.4
U.
o
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Weeks After Conception
131
Emerson et al., 1973 (N = 5)
---I-- Pipe et al, 1979 (N = 27)
- -is- Forsum et al., 1988 (N = 22) /
-
~ ' 1 1
0 10 20 30 40 50
Postpartum
FIGURE 6-1 Changes in total body potassium during pregnancy based on data from three
studies.
cially in late pregnancy-differences that do not correspond to the reported
differences in body weight. For example, weight gain was lowest in the
report of Emerson et al. (1975), but total body potassium changes were the
highest.
Comparison of the incremental gestational changes in total body potas-
sium also reveals substantial differences among the studies. The incremen-
tal value of Forsum and colleagues (1988) from early to late pregnancy
is approximately one-third that of the other two studies. The total body
potassium data provided by Forsum's group, if correct, also suggest the
loss of lean tissue during early pregnancy. In fact, the reported increment
from prepregnancy to term is so low that it is insufficient to account for the
amount of potassium others have estimated to be required for the contri-
bution of the conceptus alone. However, all three studies show postpartum
figures that are very close to their early pregnancy figures. If one ignores
the prepregnancy figures of Forsum and colleagues, the pattern of the data
is more consistent with those from the other studies.
The total body potassium value and an estimate of the conceptus
contribution to it (i.e., 169 mmol of potassium at 36 to 38 weeks of gestation;
Pipe et al., 1979) can be used to calculate the amount of potassium gained
OCR for page 132
132
NUTRITIONAL STATUS AND WEIGHT GAIN
by the mother herself. In turn, this value can be used to estimate the change
in maternal lean tissue in the three studies, assuming that the incremental
lean tissue has 92 mmol of potassium per kilogram (Pipe et al., 1979~. The
results indicate a maternal gain of 1.2 to 2.1 kg (2.6 to 4.6 lb) of lean tissue
between early and late pregnancy, excluding the conceptus. This would
include blood, mammary gland, and uterine increments. Subtracting this
estimate of the mother's lean tissue gain and an estimate of the weight
of the conceptus from the weight gained during the corresponding period
provides an estimate of the increment in maternal stores, largely fat, gained
by the mother. This method of estimation indicated a loss of 1.6 kg (3.5
lb) of fat (weeks 20 to 40 of gestation) in the study of five women who
were restricting food intake (Emerson et al., 1975~; these women may have
gained weight earlier. When data from the other two studies and total
body potassium changes are used, the estimated maternal stores are 3.6 kg
(7.9 lb) (Forsum et al., 1988) or 4.6 kg (10.1 lb) (Pipe et al., 1979~. Both
groups of workers have combined their data on total body water and their
data on total body potassium to calculate a corrected value for total body
fat. In this approach, changes in hydration are taken into consideration in
computing lean tissue from the total body potassium. Their corrected fat
estimates were 1.6 kg (3.5 lb) (Forsum et al., 1988) and 1.87 kg (4.1 lb)
(Pipe et al., 1979) from early to late pregnancy, which are quite different
from the uncorrected values calculated from the total body potassium
value only. The values for fat increments in women on unrestricted diets
estimated from the three standard methods suggest very different calorie
requirements for fat storage-i.e., nearly a 30,000-kcal difference between
the low and high values. This represents a substantial portion of the
estimated energy requirement for pregnancy and indicates the need for
a better definition of the changes that occur in calorie requirements and
energr partitioning in successful pregnancies.
Skinfold Thicknesses
Skinfold thicknesses have been used to describe normal body fat
changes throughout gestation, to determine whether skinfold thickness is
associated with fetal outcome or with supplementation in undernourished
women, to identify women with unusually small or large changes in body fat
during pregnancy, and to estimate the initial body fat content. Measured
mean triceps values range from a low of approximately 10 mm (at term
in ~iwanese women; Adair et al., 1984) to a high of 18.9 mm (at 22
weeks of gestation, in black teenagers having appropriate-for-gestational-
age newborns; Maso et al., 1988~. Mean values for the sum of triceps,
biceps, subscapular, and suprailiac skinfold thicknesses ranged from a low
of 31.3 mm (4 to 6 weeks post partum; Gambian data reported by Durnin,
OCR for page 133
BODY COMPOSITION CHANGES DURING PREGNANCY
133
TABLE 6-5 Estimations of Total Body Fat and Increase in Fat During
Gestation Based on Skinfold Thickness Measurements and the Equation
of Durnin and Womersley (1974)
Study and Country
Estimated Total Body
Fat in First Trimester,
kg + SDa
Fat Increment, kg
Pipe et al., 1979 15.4 + 2.9 2.8b
England
Dibblee and Graham, 1983 14.1 + 3.9 4.4b
England
Langhoff-Roos et al., 1987 Not available 4.0C
Sweden
Durnin, 1987
Scotland 15.1 + 4.6 2.3d
The Netherlands 17.7 + 4.9 2.0d
The Gambia 10.3 + 2.5 0.6d
Thailand 11.3 + 2.8 1.4d
Philippines 11.2 + 3.4 1.3d
a SD = Standard deviation.
b Fat gain from first to last trimester.
c Fat gain from weeks 17 to 37 of gestation.
d Fat gain from week 10 of gestation to 4 to 6 weeks postpartum.
1987) to a high of 64.8 mm (at 17 weeks of gestation in Swedish women;
Langhoff-Roos et al., 1987~. Differences of this magnitude may partly
resect methodologic differences.
Table 6-5 shows the values obtained for body fat in the four studies in
which the regression equation of Durnin and Womersley (1974) was used
to compute body fat changes from skinfold thickness measurements. In
light of the fact that the equation was derived from data on nonpregnant
women, the general consistency of these findings with those from more
complex methods is reassuring. The values for women from industrialized
countries are in the same range as those found by the methods discussed
previously, but there is nearly a twofold difference between the highest and
lowest estimates of body fat content changes. The range is even wider if
the values for women from developing countries are included. The data
presented by Durnin (1987) are based on skinfold thicknesses measured
4 to 6 weeks post partum. This may partly explain why they are lower
than the values from the other studies. For example, Dibblee and Graham
(1983) estimated a 4.4 kg (9.7 lb) fat increment between the first and third
trimesters, based on skinfold thickness changes. Yet, only 1.3 kg (2.9 lb)
of that estimated fat gain was retained at 4 weeks post partum. Although
some fat may be lost post partum, it is likely that the increase in body
water contributes to the increase in skinfold thickness during pregnancy;
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134
NUTRITIONAL STATUS AND WEIGHT GAIN
the water loss post partum may contribute to the decrease in the skinfold
thickness.
A study by Clapp et al. (1988) has provided some information on
changes in skinfold thickness that occur very early in pregnancy. Six skinfold
thicknesses were measured serially in 20 women, starting before pregnancy.
The data indicate that body weight increased by 2 kg (4.4 lb) and body fat
increased by 1.54 kg (3.4 lb) between the prepregnancy measurement and
the seventh week of pregnancy. Thus, this study supports the possibility
that maternal fat may already be increased above prepregnancy levels by
the time most studies of body composition during pregnancy are begun. If
so, then when measurements begin after the first trimester, increments in
total body fat may be underestimated. Alternatively, these findings may
indicate that the relationship between skinfold thickness and total body
fat is altered very early in pregnancy. This possibility cannot be evaluated
until more studies using serial measurements of total body water, total
body density, or total body potassium are done during the periconceptional
period.
SUMMARY
Issues to consider when examining results of pregnancy body compo-
sition studies include the following:
.
Each body composition method is based on underlying assumptions,
and correction factors are needed to adjust for changes in the lean body
during pregnancy. Without these corrections, total body water tends to
underestimate total body fat and both underwater weighing and total body
potassium tend to overestimate it.
· In the future, multicompartment models of body composition need
to be used in studies of larger numbers of pregnant women. Attention must
also be given to differences in the gestational period studied, weight gain,
initial weight, maternal age, ethnic background, and parity.
.
Skinfold thickness may be useful for research purposes, but the
currently used reference equations may not permit calculation of actual
total body fat changes. Because of its potential for clinical as well as
research use, measurement of skinfold thickness needs to be standardized
against reference methods in a large number of pregnant women.
· For the development of dietary and weight gain recommendations,
more information is needed on the relationship of weight gain to body fat
gain in individual women. Studies of the effects of composition changes on
other outcomes are also needed.
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BODY COMPOSITION CHANGES DURING PREGNANCY
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Representative terms from entire chapter:
body water
