INTRODUCTION
Increased exercise is recommended during weight reduction by virtually every public health organization; however, the role of exercise is generally considered secondary to energy restriction for the treatment of obesity. The classic explanation for the secondary role of exercise is that exercise alone cannot generate enough energy expenditure to create a negative energy balance to the extent possible with energy restriction and thus the role of exercise is diminished for weight loss.
This review discusses the interactive components of energy balance, the magnitude of exercise that is necessary for weight loss, and the difference between genders for weight loss in response to exercise, including potential compensation in the components of energy balance. Finally, we propose a model based on previous research to test for gender differences for weight loss in response to exercise.
WHAT IS ENERGY BALANCE?
Energy balance refers to an equal amount of energy intake and energy expenditure. Obesity is the result of excess energy intake compared to energy expenditure. To lose weight, a negative energy balance must be evoked, and to maintain lost weight, energy balance must be maintained. To illustrate, Figure 1 shows energy intake, energy expenditure, and the standard components of energy balance.
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Figure 1.:
Static model of energy balance. (A) Weight gain; (B) weight loss; (C) weight maintenance. PRO, protein; CHO, carbohydrate; SPA, spontaneous physical activity; TEF, thermic effect of food; RMR, resting metabolic rate.
Although the concept of energy balance appears simple, it is deceptively so. Energy intake consists of all ingested foods and beverages with an energy value. Control of energy intake is subject to a host of environmental, behavioral, biological, and genetic influences. Measurement of energy intake is notoriously difficult and prone to error. Men and women routinely underestimate energy intake by 30 to 50%. Energy expenditure consists of resting metabolic rate (RMR), the thermic effect of foods (TEF), spontaneous physical activity (SPA), and exercise. Energy expenditure can be measured with reasonable precision in the laboratory, but the cost is appreciable, it has a high burden on the individual, and it is not suitable for population studies.
It may appear that calculating energy intake and energy expenditure is a relatively simple matter of addition and subtraction, and this is frequently the method used to calculate a negative energy balance in the attempt to lose weight. Unfortunately, it is not so simple because the components of energy balance are interactive, as shown in Figure 2. As energy expenditure increases, a second component or multiple components of energy balance may change or “compensate” and, in turn, weight loss may not occur. For example, an individual may attempt to lose weight with a 500-kcal negative energy balance using a reduced energy diet. However, diets have been shown to diminish RMR and SPA; therefore, the intended energy deficit may be less than planned or absent altogether.
Figure 2.:
Dynamic model of energy expenditure. Illustrates the possible changes in the components of energy balance that may occur with an increase in exercise. SPA, spontaneous physical activity, TEF, thermic effect of food. RMR, resting metabolic rate.
WHY USE EXERCISE TO ALTER ENERGY BALANCE FOR WEIGHT LOSS?
There are limited choices an individual can make in the attempt to evoke a negative energy balance for weight loss. The most common method for weight loss is dietary restriction; however, diet does not provide a long-term solution because more than 50% of individuals who lose weight through diet eventually regain the weight they lost. Among the reasons, “exercise should be considered for weight reduction” is the simplistic argument that it can be. That is, exercise and SPA are the only components of energy expenditure that are under voluntary control. If RMR or TEF were under voluntary control, an individual could simply overeat and increase their RMR or TEF and remain weight-stable.
Exercise can account for almost no energy expenditure or a great deal of energy expenditure across a 24-h period. Exercise is generally thought of as planned behavior that is different from SPA. Although SPA requires energy expenditure, it is apparent that many individuals do not achieve a great deal of energy expenditure with SPA on a routine basis as evidenced by the dramatic increase in overweight and obesity. Thus, during the attempt at weight loss, individuals are generally counseled to maintain SPA and increase exercise in the attempt to create a gap between energy intake and expenditure.
EXERCISE AND MAGNITUDE OF WEIGHT LOSS
The existent literature appears to provide little evidence that exercise alone is a potent strategy for weight loss. Epstein and Wing (5) completed the first review of weight loss studies using meta-analytic techniques in 1980 and found that exercise alone showed a 1.15-kg weight loss in a typical 12-wk program. More recently, The American College of Sports Medicine Position Stand, “Appropriate intervention strategies for weight loss and prevention of weight regain for adults,” acknowledges that there is little evidence to suggest exercise alone will provide the amount of weight loss similar to that generally achieved by diet restriction (7). The lack of efficacy for exercise to promote weight loss may in part be caused by the relatively low levels that have been used in exercise studies. Frequently, levels are derived from exercise standards that are intended to promote cardiovascular fitness, not to promote weight loss. For example, a recent 12-month randomized controlled trial (RCT) used an energy equivalent of approximately 1200 kcal·wk−1 (low) or approximately 2000 kcal·wk−1 (high) with overweight men and women (13). The group with the low energy equivalent lost only approximately 1% of baseline weight and the group with high energy equivalent lost approximately 3.5% of baseline weight, both below the NHLBI recommendations.
Current evidence indicates that higher levels of exercise than previously recommended may be necessary to promote weight loss. The Institute of Medicine has issued guidelines suggesting that 1 h of moderately vigorous exercise may be necessary for weight management. This is supported by two studies from Ross et al., who determined that energy expenditure of exercise of 500–700 kcal per day for 12 wk provided weight loss of approximately 6 kg for women (11) and approximately 8 kg for men (10). In both of these studies, energy intake was closely controlled to baseline levels and dietary counseling was provided throughout.
Similar findings have been shown in men, but not women, in a long-term RCT that provided a minimum of 400 kcal·d−1 of energy expenditure of exercise with ad libitum diet (3). Using identical exercise prescription criteria (intensity, frequency, duration), men achieved approximately 3000 kcal energy expenditure of exercise per week, and this was effective to provide a weight loss of approximately 6% for men (Fig. 3). Women undergoing an identical exercise program achieved a verified energy equivalent of approximately 2000 kcal·wk−1 and were weight-stable, but did not lose weight (Fig. 3). Thus, with increased levels of energy expenditure of exercise, weight loss has been shown in men, but to a lesser extent in women. This suggests a gender difference or differences in compensation in response to exercise and this possibility is further discussed.
Figure 3.:
Body weight across 16 months between exercisers and controls by gender. (A) Men. (B) Women. EX, exercise; CON, control. Means with different letters are different within the exercise group, P ≥ 0.05. *Significant differences between exercise and control groups.
DO MEN AND WOMEN RESPOND DIFFERENTLY TO EXERCISE FOR WEIGHT LOSS?
Although men generally show greater weight loss than women in response to exercise (1), men and women are frequently reported together when expressing the results from exercise trials, and this makes it difficult to discern gender effects. When data for men and women are combined, the only comparison that can be made is a between-group comparison. If gender data are combined and there are significant differences between the exercise and control groups, it is not possible to determine whether one of the genders is primarily responsible for between groups differences or if both genders contributed equally. The ability to determine gender effects is further compromised because approximately 70 published studies of exercise for weight loss since 1960 do not report final sample size by gender (6).
Although there are difficulties interpreting the literature for differences for genders in response to exercise for weight loss, there is reason to believe the difference in the energy expenditure of exercise for men and women plays an important role. However, investigation of this premise is compromised because the energy expenditure of exercise is rarely measured and reported in investigations of exercise for weight loss. In addition, it is not adequate to use an energy expenditure of exercise that has been estimated from a test of maximal oxygen consumption completed at baseline and that is not updated across the duration of the study. The energy expenditure of exercise will decrease as the individual becomes more efficient, as fitness improves, and as weight is lost. In turn, this may diminish the magnitude of weight loss.
In most investigations, exercise is prescribed and reported as intensity, frequency, and duration. Frequency and duration are easily quantified; however, intensity is generally reported either as a target heart rate or at a subjective level such as “brisk walking.” With this prescription, heavier individuals will usually have greater energy expenditures compared to lighter individuals, as will men compared to women, even when all individuals are performing the same relative levels of exercise. For example, in a 16-month RCT of exercise alone for weight loss, men and women received identical exercise prescriptions based on intensity, duration, and frequency, and the energy expenditure of exercise was measured at 4-month intervals using indirect calorimetry (3). Verified exercise heart rates for men were 154 ± 11 bpm and 156 ± 9 bpm for women. Using 24-h energy expenditure from doubly labeled water (DLW) and RMR from indirect calorimetry, the Physical Activity Index (PAI) was calculated as 24-h energy expenditure divided by RMR. Men and women showed increases from baseline to 16 months of PAI 1.51 ± 0.34 to PAI 1.83 ± 0.55 and from PAI 1.60 ± 0.23 to PAI 1.74 ± 0.15 at 16 months, respectively. Men achieved an average of 667 ± 116 kcal per exercise session compared with women, who achieved an average of 438 ± 88 kcal per exercise session, for a mean difference of 229 kcal per exercise session. When kilocalories were expressed per unit of fat-free mass, men had an energy expenditure of exercise of 6.7 ± 0.8 kcal·kg−1 fat-free mass and women had 5.4 kcal·kg−1 fat-free mass (P < 0.05). As a result of this exercise program prescribed at relative intensity, men lost 5.2 ± 4.7 kg, and women gained 0.6 ± 3.8 kg. Thus, either a gender difference exists and women compensate for exercise to a greater extent than men (i.e., SPA, energy intake, etc.) or the difference was caused by the different levels of energy expenditure of exercise found when prescribing exercise by relative amount rather than targeting the desired energy expenditure of exercise. The discrepancy between the energy expenditure of exercise for men and women may contribute to the observation that women generally lose less weight than men in response to exercise.
POTENTIAL COMPENSATION IN ENERGY BALANCE IN RESPONSE TO EXERCISE
For exercise to be effective for weight loss, the real question is, “will exercise provide a negative energy balance without substantial compensation from other components of the energy balance equation?” It is apparent that the expected weight loss when calculated from energy expenditure of exercise is usually greater than what is actually observed. That is, when using a static model of energy balance it is possible to calculate the energy deficit (in theory) created by the energy expenditure of exercise and an energy value for a pound of weight loss may be assigned, and this is generally 3500 kcal. For example, if an individual were to have a calculated energy expenditure of 7000 kcal, the expected weight loss would be 2 lb (7000 kcal/3500 kcal = 2 lb).
It is now known that energy balance cannot be accurately determined from a calculation using a static model. The components of energy balance are dynamic and likely react to changes in one component with compensation in another component. For example, it is well known that when energy intake is restricted, RMR decreases. Likewise, Liebel et al. (9) demonstrated that when overfed, men and women did not gain as much weight as calculated, and when underfed they did not lose as much weight as calculated.
Resting Metabolic Rate
Although RMR decreases during energy restriction, there is considerable evidence that RMR is preserved when weight loss is caused by exercise (15). Thus, the components of energy balance most likely to be responsible for compensation during exercise for weight loss are energy intake and SPA. It is well known that energy intake is underreported by as much as 20 to 50%, that women underreport more so than men, and that underreporting increases as overweight and obesity increases (12). For example, Donnelly et al. (3) reported a small increase in weight in response to 438 ± 88 kcal per exercise session (approximately 2000 kcal·wk−1) for 16 months in women. During this time, energy intake was monitored for six 2-wk periods in a cafeteria, and energy and macronutrient content was calculated using weigh-and-measure techniques. Additionally, 24-h energy expenditure was measured by DLW. Women had no changes for energy or macronutrient content of their diet and increased 24-h energy expenditure by 209 ± 555 kcal, yet lost no weight. Any compensatory mechanisms for increased energy expenditure of exercise by other components of energy expenditure (i.e., ↓resting metabolic rate, SPA) were not sufficient to diminish the effects of exercise as evidenced by the increase in 24-h energy expenditure. Thus, in women, compensation through increases in energy intake is suspect.
Spontaneous Physical Activity
SPA is difficult to measure with accuracy. For example, Jakicic et al. (8) have reported that 45% of overweight and obese individuals overestimate the amount of SPA they achieve when self-report was compared to data from accelerometry. Perhaps the best estimation of SPA is derived from the estimation of 24-h energy expenditure from DLW combined with measurements of RMR, an estimation of TEF (i.e., 10% of 24-h energy expenditure), and measurement of planned exercise. In this fashion, SPA is estimated as the energy left when these components are subtracted from 24-h energy expenditure derived from DLW. Washburn et al. (14) compared SPA obtained from DLW in 45 overweight sedentary men and women and found no significant differences across 16 months of exercise. When SPA from DLW was compared to SPA from physical activity questionnaires, women estimated SPA more accurately compared to men, and both men and women overestimated SPA with increased levels of overweight.
To further complicate the issue, anecdotally it has been observed that when an exercise program is initiated, SPA temporarily declines because of fatigue but returns to baseline levels at some time point and may increase as fitness increases. Because it is the objective of an exercise program to add to total 24-h energy expenditure, it would be of value to know the time course for any potential changes in SPA; however, these data are not available in the literature. Thus, although exercise may evoke compensatory changes in the components of energy expenditure that would diminish the energy gap for weight loss, there is little evidence that this is the case and a more likely suspect for compensation is energy intake, especially in women.
INDIVIDUAL VARIATION FOR MEN AND WOMEN
All individuals do not respond to exercise in an identical fashion for weight loss. Bouchard et al. (2) reported the results from a highly controlled study in which participants were sequestered in an isolated research station and energy intake and exercise were tightly controlled for a 4-month period. Energy intake was prescribed at a level to maintain baseline weight and exercise was prescribed to create a 1000-kcal daily energy deficit. Reductions in body weight ranged from 3 to 12 kg, or 5 to 12% of initial body weight.
Even more variations for weight loss may be shown between genders for the response to exercise. Our laboratory has completed two long-term exercise studies: one with women only (4) and one with men and women (3). In both studies women showed an almost equal response of either weight loss or weight gain, whereas men almost universally showed weight loss (Fig. 4). Thus, in addition to the potential for compensation caused by the interactive nature of the components of energy balance, there is wide individual variation in weight loss in response to exercise, and women appear to have less weight loss in response to exercise compared to men.
Figure 4.:
Individual 16-month weight change in exercise groups by gender. (A) Women. (B) Men.
TARGETED STRATEGIES WITH EQUAL ENERGY EXPENDITURE OF EXERCISE
In our estimation, it appears that women may show a reduced weight loss compared to men in response to exercise because it is likely women have a smaller energy expenditure of exercise and show compensation for increased energy intake in response to exercise. Men have shown greater than 5% weight loss in response to exercise; thus, it can be argued that exercise alone is an effective public health strategy for men. Behavioral strategies that are designed to increase exercise in men are warranted to increase the number of men who exercise for weight loss.
For women, it is important to determine the level of energy expenditure of exercise that will not be overcome with compensation from the other components of the energy balance equation and that will therefore provide weight loss of approximately 5% of baseline weight or more. It is a logical step to determine whether women can show weight loss in response to approximately 600 kcal energy expenditure of exercise per day, 5 d·wk−1, or approximately 3000 kcal·wk−1 as shown by men. This will help determine whether a gender difference in the response to exercise alone for weight loss is shown by women. In Figure 5, we have predicted that it is unlikely that compensation will come from decreases in either RMR or SPA as previously discussed. It is likely that some level of compensation to exercise will occur with increases in energy intake. However, if compensation in energy intake is not complete, weight loss will result. For example, if the energy expenditure of exercise is 400 kcal and compensation for energy intake is 400 kcal, then men and women will be weight-stable. If the energy expenditure of exercise is increased to 600 kcal and compensation for energy intake remains less than 600 kcal (i.e., 400 kcal), then weight loss will occur. Thus, in this model, the question to be tested is, “with ad libitum diet and at 600-kcal energy expenditure of exercise, 5 d·wk−1, will women fail to completely compensate with increased energy intake, resulting in weight loss as previously shown by men?” If there is no gender effect, men and women will remain weight-stable at 400 kcal of exercise and will lose weight in similar fashion at 600 kcal of exercise.
Figure 5.:
Theoretical model of body weight response to supervised exercise. Wt, weight; EX, exercise; RMR, resting metabolic rate; TEF, thermic effect of food; SPA, spontaneous physical activity.
SUMMARY
Exercise is recommended to alter energy balance in conjunction with weight loss programs, although it receives a secondary role to energy restriction. The diminished role for exercise may be because of the perception that exercise does not provide a great enough energy gap to generate a significant amount of weight loss. Surprisingly little information is available for the ability of exercise to reduce risk of overweight and obesity for men and women consuming ad libitum diets from randomized controlled trials in which exercise is supervised, the energy expenditure of exercise is known, and the interactive components of energy balance are measured. Compensation for the energy expenditure of exercise is likely and actual weight loss is generally diminished from the expected amount of weight loss. Differences between men and women for weight loss in response to exercise may come from increased energy intake for women and a lower energy expenditure of exercise shown in women compared to men, although a direct test of this notion is lacking and is warranted. A better understanding of the components of energy balance likely to show compensation for men and women in response to exercise will allow for targeting of weight loss intervention strategies and should result in improved weight loss outcomes.
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