Although most weight loss studies have used aerobic training, a number of studies have used resistance training and have shown impressive decreases in body fat and increases in fat-free mass. The evidence shows that exercise is an important part of any weight loss program. But to maximize losses in body weight and body fat, it is necessary to combine exercise with decreased caloric intake.
Since the 1990s, abdominal visceral fat (figure 22.8 on p. 556) has been identified as a major independent risk factor for cardiovascular diseases and obesity. There is now substantial evidence that physical activity reduces the rate of accumulation of visceral fat and that exercise training actually reduces visceral fat stores.38 This could be one of the most important health benefits of an active lifestyle!
Mechanisms for Change in Body Weight and Composition
When attempting to explain how exercise causes such changes in body weight and composition, it is necessary to consider both sides of the energy-balance equation. Evaluating energy expenditure requires that we consider each of the three components of energy expenditure: RMR, TEM, and TEA. Evaluating energy intake requires that we also consider the energy that is lost in the feces (energy excreted), which is generally less than 5% of the total caloric intake. Keeping this balance in mind, in the next section we examine some of the possible mechanisms through which exercise might affect body weight and body composition.
Exercise and Appetite
Some believe that exercise stimulates the appetite to such an extent that food intake is unconsciously increased to at least equal that expended during exercise. In 1954, Jean Mayer, a world-famous nutritionist, reported that animals exercising for periods of from 20 min to 1 h per day had a lower food intake than nonexercising control animals.29 He concluded from this and other studies that when activity is less than a certain minimal level, food intake does not decrease correspondingly and the animal (or human) begins to accumulate body fat. This led to the theory that a certain minimal level of physical activity is necessary for the body to precisely regulate food intake to balance energy expenditure. A sedentary lifestyle may reduce this regulatory ability, resulting in a positive energy balance and weight gain.
Exercise does, in fact, appear to be a mild appetite suppressant, at least for the first few hours following intense exercise training. Furthermore, studies have shown that the total number of calories consumed per day does not change when a person begins a training program. Although some people interpret this as evidence that exercise does not affect appetite, a more accurate conclusion might be that appetite was affected, in fact suppressed, because caloric intake did not increase in proportion to the additional caloric expenditure from the exercise program. In studies conducted on rats, male rats appear to reduce food intake with exercise training, whereas female rats tend to eat the same or even more than nonexercising control rats.33 There is no obvious explanation for this sex difference. Also, it is unclear whether this sex difference is present in humans.
The decrease in appetite might occur only with intense levels of exercise, in which the resulting increased catecholamine (epinephrine and norepinephrine) levels might suppress the appetite. The increased body temperature that accompanies either high-intensity activity or almost any activity performed under hot and humid conditions also might suppress appetite. We all know from experience that we desire less food when the weather is hot or when our body temperatures are elevated because of illness. This also might explain why a hard running workout in the heat results in little or no desire to eat, yet a hard swimming workout in a cool swimming pool elicits a relatively strong craving for food. In the pool, provided that the water temperature is well below body core temperature, the heat generated by exercise is lost very effectively, so core temperature typically is not elevated to the same extent.
Exercise and Resting Metabolic Rate
The effects of exercise on the components of energy expenditure became a major topic of interest among researchers in the late 1980s and early 1990s. Of obvious interest is how exercise training might affect the RMR, since RMR represents 60% to 75% of the total calories expended each day. For example, if a 25-year-old man’s total daily caloric intake was 2,700 kcal and his RMR accounted for just 60% of that total (0.60 2,700 = 1,620 kcal RMR), a mere 1% increase in his RMR would require an extra 16 kcal expenditure each day, or 5,840 kcal per year. This small increase in RMR alone would account for the equivalent of a 0.8 kg (1.7 lb) fat loss per year!
The role of physical training in increasing RMR has not been totally resolved. Several cross-sectional studies have shown that highly trained runners have higher RMRs than untrained people of similar age and size. But other studies have not been able to confirm this.34 Few longitudinal studies have been conducted to determine the change in RMR in untrained people who undergo training for a period of time. Some of these suggest that RMR might increase following training.8 However, in a study of 40 men and women 17 to 62 years of age (HERITAGE Family Study), a 20-week aerobic training program (three times per week, 35-55 min per day, at 55-75% of V.O2max) failed to increase RMR even though V.O2max increased by nearly 18%.47 Because RMR is closely related to the fat-free mass of the body (fat-free tissue is more metabolically active), interest has increased in the use of resistance training to increase fat-free mass in an attempt to increase RMR.8
Exercise and the Thermic Effect of a Meal
Several studies have examined the role of individual bouts of exercise and exercise training in increasing the TEM. A single bout of exercise, either before or after a meal, increases the thermic effect of that meal. Less clear is the role of exercise training on the TEM. Some studies have shown increases; others have shown decreases; and yet others have shown no effect at all. As with measuring changes in RMR accompanying exercise training, measurement of the TEM must be timed carefully with the last exercise bout. When measurements are made within 24 h of the last bout, the TEM is typically lower than it is three days afterward.39
Exercise and Mobilization of Body Fat
During exercise, fatty acids are freed from their storage sites to be used for energy. Several studies suggest that human growth hormone may be responsible for this increased fatty acid mobilization. Growth hormone levels increase sharply with exercise and remain elevated for up to several hours in the recovery period. Other research has suggested that, with exercise, the adipose tissue is more sensitive to either the sympathetic nervous system or the increasing levels of circulating catecholamines. Either situation would increase lipid mobilization. More recent research suggests that this mobilization occurs in response to a specific fat-mobilizing substance that is highly responsive to elevated levels of activity. Thus, we cannot state with certainty which factors are of greatest importance in mediating this response.
Read more about Physiology of Sport and Exercise, Fifth Edition by W. Larry Kenney, Jack Wilmore, and David Costill.