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12 Physical Activity SUMMARY Physical activity promotes health and vigor. Cross-sectional data from a doubly labeled water database were used to define a rec- ommended level of physical activity, based on the physical activity level (PAL) associated with a normal body mass index (BMI) range of 18.5 to 25 kg/m2. In addition to the activities identified with a sedentary lifestyle, an average of 60 minutes of daily moderate intensity physical activity (e.g., walking/jogging at 3 to 4 miles/hour) or shorter periods of more vigorous exertion (e.g., jogging for 30 minutes at 5.5 miles/hour) was associated with a normal BMI and therefore is recommended for normal-weight individuals. This amount of physical activity leads to an “active” lifestyle, corre- sponding to a PAL greater than 1.6 (see Chapter 5). Because the Dietary Reference Intakes are provided for the general healthy population, recommended levels of physical activity for weight loss of obese individuals are not provided. For children, the physical activity recommendation is also an aver- age of 60 minutes of moderate intensity daily activity. Increasing the energy expenditure of physical activity (EEPA) needs to be considered in determining the energy intake to achieve energy balance in weight stable adults, and adequate growth and develop- ment in children (Chapter 5). Body weight serves as the ultimate indicator of adequate energy intake. Increasing EEPA, or main- taining an active lifestyle provides an important means for individuals to balance food energy intake with total energy expenditure. 880

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881 P HYSICAL ACTIVITY BACKGROUND INFORMATION A distinction is made between physical activity1 and exercise;2 the latter is considered more vigorous and leads to improvements in physical fitness.3 In qualitative terms, exercise can be defined as activity sufficiently vigorous to raise breathing to a level where conversation is labored and sweating is noticeable on temperate days. As indicated in Table 5-10, cross-sectional data indicated that the average physical activity level (PAL) among adults participating in the doubly labeled water (DLW) studies included in the DLW Database (Appendix I) was about 1.7, reflecting physical activity habits equivalent to walking 5 to 7 miles/day at 3 to 4 mph, in addition to the activities required by a sedentary lifestyle. Also regular physical activity may improve mood by reducing depression and anxiety, thereby enhanc- ing the quality of life. The beneficial outcomes of regular physical activity and exercise appear to pertain to persons of all ages, and both women and men of diverse ethnic groups. Throughout history, balancing dietary energy intake and total energy expenditure (TEE) has been accomplished unconsciously by most individuals because of the large component of occupation-related energy expendi- ture. Today, despite common knowledge that regular physical activity is healthful, more than 60 percent of Americans are not regularly physically active, and 25 percent are not active at all (HHS, 1996). It seems reason- able to anticipate continuation of the current trend for reductions in occupational physical activity and other energy expending activities of daily life. If this is to be offset by deliberately increasing voluntary physical activity, it needs to be kept in mind that in previously sedentary individuals adding periods of mild to moderate intensity exercise can unconsciously be compensated for by reducing other activities during the remainder of the day, so that TEE may be less affected than expected (Epstein and Wing, 1980; van Dale et al., 1989). Hence, to increase physical activity and to thereby facilitate weight control, recreational activities and physical training programs need to add, and not substitute for, other physical activ- ities of daily life. The trend for decreased activity by adults is similar to trends seen in children who are less active in and out of school (HHS, 1996). As both lack of physical activity and obesity are now recognized as risk factors for 1Physical activity—Bodily movement that is produced by the contraction of muscle and that substantially increases energy expenditure (HHS, 1996). 2Exercise (exercise training)—Planned structured and repetitive bodily move- ment done to promote or maintain one or more components of physical fitness. 3Physical fitness—A set of attributes that people have that relates to the ability to perform physical activity.

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882 DIETARY REFERENCE INTAKES several chronic diseases, logic requires that activity recommendations accompany dietary recommendations. History of Physical Activity Recommendations United States In 1953, Kraus and Hirschland (1953) alerted health and fitness pro- fessionals, the general public, and President Dwight D. Eisenhower to the relatively poor physical condition of American youth. Their paper and other events led to the formation of the President’s Council on Youth Fitness (HHS, 1996). Under President John F. Kennedy, the council was renamed the President’s Council on Physical Fitness, and in 1965 it estab- lished five levels of physical fitness for adult men and women. Subsequently, the word “sports” was added to the title of the organization, making it the President’s Council on Physical Fitness and Sports (HHS, 1996). Recognizing relationships among blood lipids, diet, and physical activity, the American Heart Association (AHA) issued in 1972 the first of its hand- books and statements on the use of endurance exercise training and exercise testing for the diagnosis and prevention of heart disease (AHA, 1972). In 1978, the American College of Sports Medicine (ACSM) issued its position statement on cardio-respiratory fitness and body composition titled “The Recommended Quantity and Quality of Exercise for Developing and Main- taining Fitness in Healthy Adults” (ACSM, 1978). Subsequently, ACSM issued a series of guidelines for exercise testing and prescription (ACSM, 1980). In 1979, agencies of the federal government became involved when the United States Department of Heath, Education, and Welfare (DHEW) issued Healthy People: The Surgeon General’s Report on Health Promotion and Disease Prevention, which recommended endurance exercise training (DHEW, 1979). In 1988, the U.S. Department of Heath and Human Services (HHS) issued The Surgeon General’s Report on Nutrition and Health, which promoted endurance exercise as a means of weight control (HHS, 1988). Activities such as walking, jogging, and bicycling three times a week for 20 minutes were recommended. That report was followed in 1990 by the U.S. Department of Agriculture (USDA)/Department of Health and Human Services Dietary Guidelines for Americans, which evaluated the role of activity in energy balance but did not offer specific exercise recommendations (USDA/HHS, 1990). In 1995, HHS issued the report Healthy People 2000, which listed health objectives for the nation, including an objective for physical activity and fitness (HHS, 1995). That same year, USDA and HHS updated Dietary Guidelines for Amer- icans and recommended 30 minutes or more of moderate-intensity

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883 P HYSICAL ACTIVITY physical activity preferably on all days of the week (USDA/HHS, 1995). In 1996 the HHS report Physical Activity and Health: A Report of the Surgeon General was published and offered specific recommendations for physical activity: a minimum of 30 minutes of moderate intensity on most, if not all, days of the week. The 2000 Dietary Guidelines for Americans recommends that adults accu- mulate at least 30 minutes and children 60 minutes of moderate physical activity most days of the week, preferably daily (USDA/HHS, 2000). In addition, that report recommended combining sensible eating with regular physical activity and acknowledged that physical activity and nutrition work together for better health. Physical activity and fitness objectives of Healthy People 2010 seek to increase the proportion of Americans that engage in daily physical activity to improve health, fitness, and quality of life (HHS, 2000). Canada In Canada, similar recommendations have been proposed. An early initiative was the Toronto International Conference on Physical Activity and Cardiovascular Health in 1966. Toronto was also the site of the 1988 International Consensus Conference on Exercise, Fitness and Health. In 1992, coinciding with Canada’s 125th birthday, the Second International Conference on Physical Activity, Fitness, and Health was held. That meet- ing resulted in publication of the report, Physical Activity, Fitness, and Health (Bouchard et al., 1994). Most recently, in cooperation with Health Canada and the Canadian Society of Exercise Physiology, Canada’s Physical Activity Guide to Healthy Active Living has been published (Health Canada, 1998). This guide describes the benefits of regular physical activity and makes specific recommenda- tions to improve fitness and achieve particular health-related outcomes such as decreasing the risk of premature death from chronic diseases (heart disease, obesity, high blood pressure, type II diabetes, osteoporosis, stroke, colon cancer, and depression). The recommendations include 60 minutes of “light effort” exercises (e.g., light walking, easy gardening), 30 to 60 minutes of “moderate effort” exercises (e.g., brisk walking, biking, swimming, water aerobics, leaf raking), or 20 to 30 minutes of “vigorous effort” exercises (e.g., aerobics, jogging, hockey, fast swimming, fast dancing, basketball). For moderate and vigorous activities, the Canadian recom- mendations are for 4 or more days per week and also include participation in flexibility activities (4–7 days per week) and strength activities (4–7 days per week).

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884 DIETARY REFERENCE INTAKES PHYSICAL ACTIVITY LEVEL AND ENERGY BALANCE Aside from dietary energy intake, energy expenditure of physical activity (EEPA) is the variable that a person can control, in contrast to age, height, and gender (Chapter 5). Energy expenditure can rise many times over resting rates during exercise, and the effects of an exercise bout on energy expenditure persist for hours, if not a day or longer (Benedict and Cathcart, 1913; Van Zant, 1992). Thus, changing activity level can have major impacts on total energy expenditure (TEE) and on energy balance. Further, exer- cise does not automatically increase appetite and energy intake in direct proportion to activity-related changes in energy expenditure (Blundell and King, 1998; Hubert et al., 1998; King et al., 1997). In humans and other mammals, energy intake is closely related to physical activity level when body mass is in the ideal range, but too little or too much exercise may disrupt hypothalamic and other mechanisms that regulate body mass (Mayer et al., 1954, 1956). Impact of Physical Activity on Energy Expenditure and on PAL Metabolic Equivalents (METs) The impact of various physical activities is often described and com- pared in terms of METs (i.e., multiples of an individual’s resting oxygen uptake), and one MET is defined as a rate of oxygen (O2) consumption of 3.5 ml/kg/min in adults. Taking the oxygen energy equivalent of 5 kcal/L consumed, this corresponds to 0.0175 kcal/minute/kg (3.5 mL/min/kg × 0.005 kcal/mL). A rate of energy expenditure of 1.0 MET thus corresponds to 1.2 kcal/min in a man weighing 70 kg (0.0175 kcal/kg/minute × 70 kg) and to 1.0 kcal/minute in a woman weighing 57 kg (0.0175 kcal/kg/min × 57 kg) based on the reference body weights for adults in Table 1-1. Knowing the intensity of a type of physical activity in terms of METs (see Table 12-1 for the METs for various activities) allows a simple assess- ment of its impact on the energy expended while the activity is performed (number of METs × minutes × 0.0175 kcal/kg/minute). However, as men- tioned in Chapter 5, the increase in daily energy expenditure is somewhat greater because exercise induces an additional small increase in expendi- ture for some time after the exertion itself has been completed. This “excess post-exercise oxygen consumption” (EPOC) (Gaesser and Brooks, 1984) depends on exercise intensity and duration as well as other factors, such as the types and durations of activities in normal living; EPOC has been estimated at about 15 percent of the increment in expenditure that occurs during the exertion itself (Bahr et al., 1987). The thermic effect of food (TEF), which needs to be consumed to cover the expenditure associated

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885 P HYSICAL ACTIVITY TABLE 12-1 Intensity and Impact of Various Activities on Physical Activity Level (PAL) in Adultsa Metabolic Equivalents ∆PAL/10 minc ∆PAL/hc (METs) b Activity Leisure Mild Billiards 2.4 0.013 0.08 Canoeing (leisurely) 2.5 0.014 0.09 Dancing (ballroom) 2.9 0.018 0.11 Golf (with cart) 2.5 0.014 0.09 Horseback riding (walking) 2.3 0.012 0.07 Playing Accordion 1.8 0.008 0.05 Cello 2.3 0.012 0.07 Flute 2.0 0.010 0.06 Piano 2.3 0.012 0.07 Violin 2.5 0.014 0.09 Volleyball (noncompetitive) 2.9 0.018 0.11 Walking (2 mph) 2.5 0.014 0.09 Moderate Calisthenics (no weight) 4.0 0.029 0.17 Cycling (leisurely) 3.5 0.024 0.14 Golf (without cart) 4.4 0.032 0.19 Swimming (slow) 4.5 0.033 0.20 Walking (3 mph) 3.3 0.022 0.13 Walking (4 mph) 4.5 0.033 0.20 Vigorous Chopping wood 4.9 0.037 0.22 Climbing hills (no load) 6.9 0.056 0.34 Climbing hills (5-kg load) 7.4 0.061 0.37 Cycling (moderately) 5.7 0.045 0.27 Dancing Aerobic or ballet 6.0 0.048 0.29 Ballroom (fast) or square 5.5 0.043 0.26 Jogging (10-min miles) 10.2 0.088 0.53 Rope skipping 12.0 0.105 0.63 Skating Ice 5.5 0.043 0.26 Roller 6.5 0.052 0.31 Skiing (water or downhill) 6.8 0.055 0.33 Squash 12.1 0.106 0.63 Surfing 6.0 0.048 0.29 Swimming 7.0 0.057 0.34 Tennis (doubles) 5.0 0.038 0.23 Walking (5 mph) 8.0 0.067 0.40 continued

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886 DIETARY REFERENCE INTAKES TABLE 12-1 Continued Metabolic Equivalents ∆PAL/10 minc ∆PAL/hc (METs) b Activity Activities of daily living Gardening (no lifting) 4.4 0.032 0.19 Household tasks, 3.5 0.024 0.14 moderate effort Lifting items continuously 4.0 0.029 0.17 Light activity while sitting 1.5 0.005 0.03 Loading/unloading car 3.0 0.019 0.11 Lying quietly 1.0 0.000 0.00 Mopping 3.5 0.024 0.14 Mowing lawn (power mower) 4.5 0.033 0.20 Raking lawn 4.0 0.029 0.17 Riding in a vehicle 1.0 0.000 0.00 Sitting 0.0 0.000 0.00 Taking out trash 3.0 0.019 0.11 Vacuuming 3.5 0.024 0.14 Walking the dog 3.0 0.019 0.11 Walking from house to 2.5 0.014 0.09 car or bus Watering plants 2.5 0.014 0.09 a PAL is the physical activity level that is the ratio of the total energy expenditure to the basal energy expenditure. b METs are multiples of an individual’s resting oxygen uptakes, defined as the rate of oxygen (O2) consumption of 3.5 mL of O2/min/kg body weight in adults. c In the PAL shown here, an allowance has been made to include the delayed effect of physical activity in causing excess postexercise O2 consumption and the dissipation of some of the food energy consumed through the thermic effect of food. SOURCE: Adapted from Fletcher et al. (2001). with a given activity, must also be taken into account. The TEF dissipates about 10 percent of the food energy consumed. The impact of a given activity on daily energy expenditure under conditions of energy balance thus includes the intensity of the physical activity in terms of METS, the EPOC, and the TEF and expressed as: # of METs × min × 0.022 kcal/kg/min × kg body weight, where 0.022 kcal/kg/min = 0.0175 kcal/kg/min × 1.15 percent (EPOC) ÷ 0.9 percent (TEF). Bijnen and coworkers (1998) found that activities with METs greater than 4 are more effective than less intensive activities in reducing cardio-

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887 P HYSICAL ACTIVITY vascular mortality. A rate of energy expenditure of 4.5 METs corresponds to the upper boundary for moderate activities (Table 12-1) and elicits an exertion that falls into the upper range of the percent of Vo2max consid- ered to reflect light physical activity intensity for 20- to 39-year-old adults, but falls into the lower range of moderate intensities in 40- to 64-year-old adults (Fletcher et al., 2001). A rate of exertion of 4.5 METs is reached, for example, by walking at a speed of 4 mph (Table 12-1). Physical Activity Level (PAL) While METs describe activity intensities relative to a resting metabolic rate (RMR), the physical activity level (PAL) is defined as the ratio of total energy expenditure (TEE) to basal energy expenditure (BEE). Thus, the actual impact on PAL depends to some extent on body size and age, as these are determinants of the BEE (Figure 12-1). The impact of these factors can be judged by examining the ratio of MET (extrapolated to 24 hours) to BEE. It is noteworthy that the errors that this introduces in the calculation of PAL values, at least over the normal range of body weights, is of minor importance in comparison to the very large uncertain- ties generally inherent in the assessment of the duration and intensity of physical activities in individuals and populations. For a typical 30-year-old reference man and woman 1.77 m and 1.63 m in height and weighing 70 kg and 57 kg (Chapter 1, Table 1-1), BEEs are 1,684 and 1,312 kcal/day, respectively (calculated from the predictive BEE equations in Chapter 5. These correspond to 0.95 and 0.91 times the 1,764 and 1,436 kcal/day obtained by extrapolating a rate of 1.0 MET4 to 24 hours for reference men and women (1,764 kcal/day = 1 MET × 1,440 min × 0.0175 kcal/kg/min × 70 kg and 1,436 kcal/day = 1 MET × 1,440 min × 0.0175 kcal/kg/min × 57 kg). The following equations, derived for refer- ence body weights of 70 kg for men and 57 kg for women, were utilized to determine the change in PAL for each of the activities in Table 12-1. ∆PAL = (# of METs – 1) × 1.34 × (min/1,440 min), Men: where 1.34 = 1.15 percent (EPOC) ÷ 0.9 percent (TEF) ÷ 0.95 percent.5 Women: ∆PAL = (# of METs – 1) × 1.42 × (min/1,440 min), where 1.42 = 1.15 percent (EPOC) ÷ 0.9 percent (TEF) ÷ 0.91.5 4Defined as 0.0175 kcal/kg/min. 5Correction to cover EPOC and TEF.

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888 DIETARY REFERENCE INTAKES 3000 2500 2000 kcal/day 1500 1000 500 0 40 50 60 70 80 90 100 110 120 130 Body Weight (kg) 1.20 1.00 .80 Rate of 1.0 BEE/MET .60 .40 .20 0.00 40 50 60 70 80 90 100 110 120 130 Body Weight (kg) FIGURE 12-1 Relationship of basal energy expenditure (BEE), metabolic equiva- lents rate and body weight in 30-year-old adults. The upper panel shows the impact of body weight on BEE in men ( ) and women (▫) and on a MET-rate of 1.0 (×) extrapolated to 24 h. Points with body mass indexes (BMIs) from 18.5 up to 25 kg/m2 are filled in. The lower panel shows the ratio of BEE divided by an MET rate of 1.0 for a given body weight for men ( ) with reference heights of 1.75 m or reference height ± 1 standard deviation (i.e., 1.64 or 1.86 m), and for women (▫) with reference heights of 1.62 m or reference height ± 1 standard deviation (i.e., 1.55 or 1.70 m), and BMI of 18.5, 22.5 (men) or 21.5 (women), 25, 30, and 35 kg/m2.

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889 P HYSICAL ACTIVITY The coefficients given in Table 12-1 can then be used to arrive at an estimate of an individual’s PAL by cumulating the effects of the various activities performed on the basis of their duration and intensities (see below, “Physical Activity for Adults”). Because it is the most significant physical activity in the life of most individuals, walking/jogging is taken as the reference activity, and the impact of other activities can be considered in terms of exertions equiva- lent to walking/jogging, to the extent that these activities are weight bear- ing and hence involve costs proportional to body weight. The effect of walking/jogging on energy expenditure at various speeds is given in Table 12-1 in terms of METs and is also shown in the upper panel of Figure 12-2. The middle panel describes the energy expended in kcal/hour for walking or jogging at various speeds by individuals weighing 70 or 57 kg (the reference body weights for men and women, respectively from Table 1-1. The figure’s lower panel describes the total cost of walking or jogging one mile at various speeds, including the increments in energy expenditure above the resting rate during and after walking or jogging plus a commensurate increase in TEF. The energy expended per mile walked or jogged is essentially constant at speeds ranging from 2 to 4 miles/hour (1 kcal/mile/kg for a man [70 kcal/mile/70 kg] to 1.1 kcal/mile/kg for a woman [65 kcal/mile/57 kg], or approximately 1.1 kcal/mile/kg body weight; lower panel, Figure 12-2), but increases progressively at higher speeds. According to the formulas shown above, walking at a speed of 4 mph (4.5 METs, upper panel, Figure 12-2) for 60 minutes causes an increase in the daily ∆PAL of 0.195 ([4.5 METs – 1] × 1.34 × 60 min/1,440 min) in men and 0.204 ([4.5 METs – 1] × 1.42 × 60 min/1,440 min) in women, or a ∆PAL of approximately 0.20 as given in Table 12-1. Walking or jogging at speeds of 4.5 mph raises the metabolic rate to 6 METS (upper panel, Figure 12-2), increasing the impact on changing the daily PAL by half to 0.30 for sixty minutes (∆PAL in men = [6 METs – 1] × 1.34 × 60 min/ 1,440 min = 0.279, ∆PAL in women = [6 METs – 1] × 1.42 × 60 min/ 1,440 min = 0.296). Indeed, walking or jogging to cover 4.5 miles in 60 minutes, at a cost of 107 kcal/mile (lower panel, Figure 12-2) or 1.53 kcal/mile/kg (107 kcal/mile ÷ 70 kg) in men, or performing some equally demanding activity for 60 minutes, will cause an increase in PAL of approximately 0.30. Impact of Body Weight on Energy Expenditure The impact of body weight on energy expenditure while walking at various speeds is illustrated in Figure 12-3, while Figure 12-4 describes how body weight affects the total increase in energy expenditure caused by

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890 DIETARY REFERENCE INTAKES 10 8 Equivalents Metabolic 6 4 2 0 0 1 2 3 4 5 6 mph 800 700 600 500 kcal/h 400 300 200 100 0 0 1 2 3 4 5 6 mph 160 140 120 kcal/m 100 80 60 40 20 0 0 1 2 3 4 5 6 7 mph FIGURE 12-2 Relationships of energy expenditure and walking/jogging speeds. The upper panel shows the rate of energy expenditure as a function of walking/ jogging speed. The middle panel shows the energy expended by a 70-kg man ( ) and by a 57-kg woman (▫) while walking/jogging 1 h at various speeds. The lower panel shows the increase in daily energy expenditure induced by walking/jogging 1 m at various speeds for a 70-kg man (●) and a 57-kg woman ( ).

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925 P HYSICAL ACTIVITY Resistance Exercise and General Physical Fitness Initial efforts by health professionals to reduce FM involved endurance exercise protocols mainly because of the large impact on total energy expenditure and links to coronary heart disease risk amelioration. More recent efforts using resistance exercise training, or combinations of resis- tance and endurance exercises, have been tried to maintain the interest of participants as well as to positively affect body composition through stimu- lation of anabolic stimuli (Grund et al., 2001). Practitioners of speed, power, and resistance exercises can change body composition by means of the muscle-building effects of such exertions. Moreover, exercises that strengthen muscles, bones, and joints stimulate muscle and skeletal devel- opment in children, as well as assist in balance and locomotion in the elderly, thereby minimizing the incidence of falls and associated complica- tions of trauma and bed rest (Evans, 1999). While resistance training exercises have not yet been shown to have the same effects on risks of chronic diseases, their effects on muscle strength are an indication to include them in exercise prescriptions, in addition to activities that pro- mote cardiovascular fitness and flexibility. Supplementation of Water and Nutrients As noted earlier, carbohydrate is the preferred energy source for work- ing human muscle (Figure 12-7) and is often utilized in preference to body fat stores during exercise (Bergman and Brooks, 1999). However, over the course of a day, the individual is able to appropriately adjust the relative uses of glucose and fat, so that recommendations for nutrient selection for very active people, such as athletes and manual laborers, are generally the same as those for the population at large. With regard to the impact of activity level on energy balance, modifications in the amounts, type, and frequency of food consumption may need to be considered within the context of overall health and fitness objectives. Such distinct objectives may be as varied as: adjustment in body weight to allow peak performance in various activities, replenishment of muscle and liver glycogen reserves, accretion of muscle mass in growing children and athletes in training, or loss of body fat in overweight individuals. However, dietary considerations for active persons need to be made with the goal of assuring adequate overall nutrition. Following the recently released joint position statement of the American College of Sports Medicine, American Dietetics Association, and Dietitians of Canada (ACSM et al., 2000), water and fluids containing carbohydrates and electrolytes may be consumed immediately prior to, during, and after physical activity. For instance, a collegiate swimmer arriving on an empty

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926 DIETARY REFERENCE INTAKES stomach at the training site should be provided with fluids during and immediately after training as well as food after training. Similarly, follow- ing competition or training for competition, athletes should rehydrate and consume a high carbohydrate meal (ACSM, 2000). For the healthy individual, the amount and intensity of exercise recommended is unlikely to lead to glycogen depletion, dehydration, or water intoxication. None- theless, timing of post-exercise meals to promote restoration of glycogen reserves and other anabolic processes can benefit resumption of normal daily activities. ADVERSE EFFECTS OF EXCESSIVE PHYSICAL ACTIVITY Adverse Effects Overuse Injuries Physical exercise has the potential to cause overuse injuries to muscles, bones, and joints as well as injuries caused by accidents. Additionally, pre- existing conditions can be aggravated upon initiation of a physical activity program, and chronic, repetitive activities can result in injuries. For instance, running can result in injuries to muscles and joints of the lower limbs and back, swimming can cause or irritate shoulder injuries, and cycling can cause or worsen problems to the hands, back, or buttocks. Fortunately, the recommendation in this report to accumulate a given amount of activity does not depend on any particular exercise or sports form. Hence, the activity recommendation can be implemented in spite of possible mild, localized injuries by varying the types of exercise (e.g., walk- ing instead of jogging). Recalling the dictum of “do no harm,” the physical activity recommendations in this report are intended to be healthful and invigorating. Activity-related injuries are always frustrating and often avoid- able, but they do occur and need to be resolved in the interest of long- term general health and short-term physical fitness. Dehydration and Hyperthermia Physical activity results in conversion of the potential chemical energy in carbohydrates and fats to mechanical energy, but in this process most (~ 75 percent) of the energy released appears as heat (Brooks et al., 2000). Evaporative heat loss from sweat is the main mechanism by which humans prevent hyperthermia and heat injuries during exercise. Unfortunately, the loss of body water as sweat during exercise may be greater than what can be replaced during the activity, even if people drink ad libitum or are on a planned diet. Hence, exercise may result in dehydration that increases

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927 P HYSICAL ACTIVITY the stress and relative difficulty of subsequent activity. This can be aggra- vated by environmental conditions that increase fluid losses, such as heat, humidity, and lack of wind (Barr, 1999). Therefore, as already described, people should consume water before, during (if possible), and after exer- cise (ACSM et al., 2000). A weight loss of 1 to 2 percent of body weight on a day following exercise cannot be attributed to a loss of body fat, but reflects some degree of hypohydration that needs to be compensated for by the consumption of fluids (ACSM et al., 2000). Individuals who have lost more than 2 percent of body weight are to be considered physiologically impaired (Naghii, 2000) and should not exercise, but rehydrate. Hypothermia Hypothermia can result from water exposure and during winter sports. Even exposure to cool, damp environments can be dangerous to inade- quately clothed and physically exhausted individuals. Accidental immersion due to capsizing of boats, poor choice of clothing during skiing, change in weather, or physical exhaustion leading to an inability to generate ade- quate body heat to maintain core body temperature can all lead to death, even when temperatures are above freezing. Prevention of hypothermia and its treatment are beyond this report; however, hypothermia is unlikely to accrue from attempts to fulfill the physical activity recommendation. Because water and winter sports are gaining popularity and do provide means to enjoyably follow the physical activity recommendation, safe par- ticipation in such activities needs special instruction and supervision. Cardiac Events While regular physical activity promotes cardiovascular fitness and reduces risks associated with cardiovascular diseases (CVD), heavy physical exertion can trigger the development of arrhythmias or myocardial infarctions (Mittleman et al., 1993; Thompson, 1982; Willich et al., 1993) or, in some instances, can lead to sudden death (Kohl et al., 1992; Koplan, 1979; Siscovick et al., 1984; Thompson, 1982). Thus, while it is true that compared to the population at large, individuals who exercise regularly have reduced risk of CVD and sudden cardiac death, there is a transient increase in risk in this group during and immediately after vigorous exercise (Kohl et al., 1992; Siscovick et al., 1984). However, Manson and colleagues (2002) recently reported that both walking and vigorous activity were associated with marked reductions in the incidence of cardiovascular events.

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928 DIETARY REFERENCE INTAKES Female Athlete Triad Although loading the skeleton through resistance (e.g., weight train- ing, weight-bearing exercises) and impact activities (e.g., jumping) increases bone mineral density (BMD) (Fuchs et al., 2001; Welten et al., 1994), athletic women who undereat and/or overtrain can develop a condition, or cluster of conditions (disordered eating, amenorrhea, and osteoporosis) termed the “female athlete triad” (ACSM, 1997; Thrash and Anderson, 2000; West, 1998). In this triad, disordered eating and chronic energy deficits can disrupt the hypothalamic-pituitary axis, leading to loss of menses, osteopenia, and premature osteoporosis (Loucks et al., 1998), increasing the possibility of hip, spine, and forearm fractures. While dangerous in themselves, skeletal injuries can predispose victims to a cascade of events including thromboses, infections, and physical deconditioning. Prevention of Adverse Effects The possibility that exercise can result in overuse injuries, dehydration, and heart problems has been noted above. Consequently, a prudent approach to initiating physical activity or exercise by previously sedentary individuals is recommended. Men over 40 years of age and women over 50 years of age, those with pre-existing conditions, known or suspected risk factors or symptoms of cardiovascular and other chronic diseases (physical inactivity being a known risk factor) should seek medical evalua- tion as well as clinical exercise testing, clearance, and advice prior to initi- ating an exercise program (ACSM, 2000). The evaluation should include a stress electrocardiogram and blood pressure evaluation. Ideally, respiratory measurements should be performed to evaluate Vo2max. For all individuals initiating an exercise program, emphasis should be placed on the biological principle of stimulus followed by response. Hence, easy exercises must be performed regularly before more vigorous activities are conducted. Similarly, exercise participants need to rest and recover from previous activities prior to resuming or increasing training load. Also, as already noted, conditions of chronic soreness or acute pain and insomnia could be symptoms of over-training. Hence, activity progression should be discontinuous with adequate recovery periods to minimize chances of injury and permit physiological adaptations to occur. Those adaptations are elicited during exercise but occur during recovery. Thus, physical activity recommendations for healthful living, whether a minimum of 30 minutes for most days, as recommended in the Surgeon General’s report (HHS, 1996), or 60 minutes a day, should not be construed as the starting point for an adult wishing to change from a sedentary lifestyle to a more active form of living. Depending on the individual, as little as 5 to

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929 P HYSICAL ACTIVITY 10 minutes a day may represent an appropriate starting point, undertaken under professional supervision for those with cardiovascular risk or ortho- pedic problems. Attention also needs to be given to stretching and strengthening activities as part of the physical activity core to healthful living. RESEARCH RECOMMENDATIONS • More information is needed on the effect of exercise (i.e., endur- ance, resistance, other), frequency, intensity, and duration on body fatness in young and elderly adults and children. • More information is needed on the effects of exercise on substrate utilization and the roles of various energy depots (liver glycogen, muscle glycogen, adipose triacylglycerol, intramuscular triacylglycerol) in exercise and recovery in children, adults, and the elderly. • Research is needed to determine whether the timing of meals and exercise can be used to optimize changes in, or to maintain favorable Body Mass Indexes and body compositions of moderately and very active individuals. • Research is needed to determine whether there are dietary compo- sitions that optimize accretion of lean tissue in growing children and physically active adults. • More information is needed to identify the mechanisms by which acute and chronic physical activity alter substrate utilization and body composition. • Efforts need to be undertaken to develop reliable, noninvasive, and clinically appropriate measurements of body composition, cardiovascular function, and physical fitness. • Efforts should be directed at developing practical, yet reliable methods to assess habitual levels of physical activity. REFERENCES ACOG (American College of Obstetricians and Gynecologists).1994. Exercise during pregnancy and the postpartum period. Tech Bull 189. Washington DC. ACOG (American College of Obstetricians and Gynecologists). 1995. Planning for pregnancy, birth and beyond. Tech Bull. Washington, DC. ACSM (American College of Sports Medicine). 1978. The recommended quantity and quality of exercise for developing and maintaining fitness in healthy adults. Med Sci Sports 10:vii–x. ACSM. 1980. Guidelines for Graded Exercise Testing and Prescription, 2nd ed. Philadelphia: Lea and Febiger. ACSM. 1997. Position Stand: The female athlete triad. Med Sci Sports Exercise 29:I-xi. ACSM. 2000. ACSM’s Guidelines for Exercise Testing and Prescription, 6th ed. Philadelphia: Lippincott, Williams and Wilkins.

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930 DIETARY REFERENCE INTAKES ACSM, American Dietetic Association, Dietitians of Canada. 2000. Joint position statement. Nutrition and athletic performance. Med Sci Sports Exerc 32:2130–2145. AHA (American Heart Association). 1972. Exercise Testing and Training of Apparently Healthy Individuals: A Handbook for Physicians. New York: AHA. Anthony J. 1991. Psychologic aspects of exercise. Clin Sports Med 10:171–180. Bahr R, Ingnes I, Vaage O, Sejersted OM, Newsholme EA. 1987. Effect of duration of exercise on excess postexercise O2 consumption. J Appl Physiol 62:485–490. Barr SI. 1999. Effects of dehydration on exercise performance. Can J Appl Physiol 24:164–172. Benedict FG, Cathcart EP. 1913. Muscular Work: A Metabolic Study with Special Refer- ence to the Efficiency of the Human Body as a Machine. Publication No. 187. Wash- ington, DC: Carnegie Institution of Washington. Bengtsson B-Å, Brummer R-J, Bosaeus I. 1990. Growth hormone and body compo- sition. Horm Res 33:19–24. Bergman B, Brooks GA. 1999. Respiratory gas-exchange ratios during graded exer- cise in fed and fasted trained and untrained men. J Appl Physiol 86:479–487. Bijnen FCH, Caspersen CJ, Feskens EJM, Saris WHM, Mosterd WL, Kromhout D. 1998. Physical activity and 10-year mortality from cardiovascular diseases and all causes: The Zutphen Elderly Study. Arch Intern Med 158:1499–1505. Blaak EE, Westerterp KR, Bar-Or R, Wouters LJM, Saris WHM. 1992. Total energy expenditure and spontaneous activity in relation to training in obese boys. Am J Clin Nutr 55:777–782. Blair SN, Kohl HW, Barlow CE. 1993. Physical activity, physical fitness, and all-cause mortality in women: Do women need to be active? J Am Coll Nutr 12:368–371. Blair SN, Kohl HW, Barlow CE, Paffenbarger RS, Gibbons LW, Macera CA. 1995. Changes in physical fitness and all-cause mortality. A prospective study of healthy and unhealthy men. J Am Med Assoc 273:1093–1098. Blundell JE, King NA. 1998. Effects of exercise on appetite control: Loose coupling between energy expenditure and energy intake. Int J Obes Relat Metab Disord 22:S22–S29. Borer KT. 1995. The effects of exercise on growth. Sports Med 26:375–397. Bouchard C, Shephard RJ, Stephens T. 1994. Physical Activity, Fitness, and Health: International Proceedings and Consensus Statement. Champaign, IL: Human Kinetics. Brooks GA. 1987. Amino acid and protein metabolism during exercise and recovery. Med Sci Sports Exerc 19:S150–S156. Brooks GA. 1997. Importance of the ‘crossover’ concept in exercise metabolism. Clin Exp Pharmacol Physiol 24:889–895. Brooks GA, Mercier J. 1994. Balance of carbohydrate and lipid utilization during exercise: The ‘crossover’ concept. J Appl Physiol 76:2253–2261. Brooks GA, Fahey TD, White TP, Baldwin KM. 2000. Exercise Physiology: Human Bioenergetics and Its Applications, 3rd ed. Mountain View, CA: Mayfield Publishing. Brown CH, Harrower JR, Deeter MF. 1972. The effects of cross-country running on pre-adolescent girls. Med Sci Sports 4:1–5. Campbell SE, Angus DJ, Febbraio MA. 2001. Glucose kinetics and exercise perfor- mance during phases of the menstrual cycle: Effect of glucose ingestion. Am J Physiol 281:E817–E825. CDC (Centers for Disease Control and Prevention). 2000. Youth risk behavior surveillance—United States, 1999. Mor Mortal Wkly Rep CDC Surveill Summ 49(SS-5):1–96. Chad KE, Quigley BM. 1991. Exercise intensity: Effect on postexercise O2 uptake in trained and untrained women. J Appl Physiol 70:1713–1719.

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931 P HYSICAL ACTIVITY Chaouloff F. 1997. The serotonin hypothesis. In: Morgan WP, ed. Physical Activity and Mental Health. Washington, DC: Taylor and Francis. Pp. 179–198. Colditz GA, Coakley E. 1997. Weight, weight gain, activity, and major illnesses: The Nurses’ Health Study. Int J Sports Med 18:S162–S170. Craft LL, Landers DM. 1998. The effect of exercise on clinical depression and depression resulting from mental illness: A meta-analysis. J Sport Exerc Psychol 20:339–357. Dela F, Mikines KJ, Von Linstow M, Galbo H. 1991. Twenty-four-hour profile of plasma glucose and glucoregulatory hormones during normal living condi- tions in trained and untrained men. J Clin Endocrinol Metab 73:982–989. DHEW (U.S. Department of Health, Education, and Welfare). 1979. Healthy People: The Surgeon General’s Report on Health Promotion and Disease Prevention. DHEW (PHS) Publication No. 79-55071. Rockville, MD: Public Health Service. Dishman RK. 1997. The norephinephrine hypothesis. In: Morgan WP, ed. Physical Activity and Mental Health. Washington, DC: Taylor and Francis. Pp. 199–212. Eliakim A, Burke GS, Cooper DM. 1997. Fitness, fatness, and the effect of training assessed by magnetic resonance imaging and skinfold-thickness measurements in healthy adolescent females. Am J Clin Nutr 66:223–231. Epstein LH, Wing RR. 1980. Aerobic exercise and weight. Addict Behav 5:371–388. Evans WJ. 1999. Exercise training guidelines for the elderly. Med Sci Sports Exerc 31:12–17. Fletcher GF, Balady GJ, Amsterdam EA, Chaitman B, Eckel R, Fleg J, Froelicher VF, Leon AS, Piña IL, Rodney R, Simons-Morton DG, Williams MA, Bazzarre T. 2001. Exercise standards for testing and training. A statement for healthcare professionals from the American Heart Association. Circulation 104:1694–1740. Friedlander AL, Casazza GA, Horning MA, Buddinger TF, Brooks GA. 1998a. Effects of exercise intensity and training on lipid metabolism in young women. Am J Physiol 275:E853–E863. Friedlander AL, Casazza GA, Horning MA, Huie MJ, Piacentini MF, Trimmer JK, Brooks GA. 1998b. Training-induced alterations of carbohydrate metabolism in women: Women respond differently from men. J Appl Physiol 85:1175–1186. Friedlander AL, Casazza GA, Horning MA, Usaj A, Brooks GA. 1999. Endurance training increases fatty acid turnover, but not fat oxidation, in young men. J Appl Physiol 86:2097–2105. Fuchs RK, Bauer JJ, Snow CM. 2001. Jumping improves hip and lumbar spine bone mass in prepubescent children: A randomized controlled trial. J Bone Miner Res 16:148–156. Gaesser GA, Brooks GA. 1984. Metabolic bases of excess post-exercise oxygen con- sumption: A review. Med Sci Sports Exerc 16:29–43. Gallo L, Maciel BC, Marin-Neto JA, Martins LEB. 1989. Sympathetic and para- sympathetic changes in heart rate control during dynamic exercise induced by endurance training in man. Braz J Med Biol Res 22:631–643. Graham TE, Adamo KB. 1999. Dietary carbohydrate and its effects on metabolism and substrate stores in sedentary and active individuals. Can J Appl Physiol 24:393–415. Greenleaf JE, Kozlowski S. 1982. Physiological consequences of reduced physical activity during bed rest. Exerc Sport Sci Rev 10:84–119. Grund A, Krause H, Kraus M, Siewers M, Rieckert H, Müller MJ. 2001. Association between different attributes of physical activity and fat mass in untrained, endurance- and resistance-trained men. Eur J Appl Physiol 84:310–320.

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