Weight gain resulting from excess storage of fat is ultimately explained by an intake of calories that exceeds those expended. However, the physiological mechanisms that govern the metabolic balance of food consumption and energy extraction with the energy costs of basal metabolism, digestion, and physical exertion are complex and incompletely understood. The factors that influence the behaviors of eating and physical activity, which determine energy balance and gains or loss of body fat, are even less well understood. Two common theories about the etiology of overweight and obesity, set point and settling point, address how physical activity can play a role in the treatment or prevention of overweight and obesity. The concept of reasonable weight, discussed later in this chapter, provides an alternative view of weight goals for obese people.
Set Point Theory
Set point theory hypothesizes that the body has an internal control mechanism, that is, a set point, located in the lateral hypothalamus of the brain, that regulates metabolism to maintain a certain level of body fat. Though evidence in rats has supported the theory, there is no scientific consensus that such a metabolic set point exists in humans for fat maintenance. Though weight losses after the use of stimulant drugs, nicotine, and exercise seem consistent with the concept of an altered set point, these effects can also be explained by the alteration of basal metabolism in ways that do not require a change in the set point. Studies using mainly dietary restriction have shown that weight loss is accompanied by a decrease in fat-free body mass and basal energy expenditure (Sum et al. 1994). Severe caloric restriction has been shown to depress resting metabolism by as much as 45% (McArdle, Katch, and Katch 1991).
Settling Point Theory
Interventions designed to alter diet and reduce weight have used principles of behavior therapy with modest success, particularly among the obese (Foreyt and Goodrich 1993). The most successful weight loss programs incorporate physical activity (Pavlou, Krey, and Steffee 1989; Perri et al. 1986). The settling point theory was proposed by obesity researcher James Hill of the University of Colorado to help explain why overweight and obesity are more than problems of metabolism (Hill, Pagliassotti, and Peters 1994). His idea is that weight loss and gain in most humans are more related to the patterns of diet and physical activity that people “settle” into as habits based on the interaction of their genetic dispositions, learning, and environmental cues to behavior. Evidence suggests that obese people are more sensitive to food-related stimuli in the social and physical environment, which influence their energy intake, than to the stimuli for energy expenditure.
The Role of Physical Activity
Physical activity has an important role in the prevention and treatment of overweight and obesity, even if that role is not yet completely understood.
The association between moderate-to-vigorous physical activity and the prevalence of overweight and obesity was estimated from self-reports in a cross-sectional survey of 137,593 youth (10-16 years of age) from 34 countries (including a number in Europe, as well as Israel, Canada, and the United States) that participated in the 2001-2002 Health Behaviour in School-Aged Children Study (Janssen et al. 2005). The international child BMI standards were used to project youth as overweight (BMI ≥25) or obese (BMI ≥30) at age 18. Overweight and obesity prevalence was particularly high in countries located in North America, Great Britain, and southwestern Europe (Greece, Italy, Malta, Portugal, and Spain). The two countries with the highest prevalence of obese and overweight youth were Malta (7.9% and 25.4%) and the United States (6.8% and 25.1%). Physical activity levels were lower in overweight youth in 30 of 34 countries. After adjustments for age, gender, TV watching, computer use, current attempts to lose weight, and dietary factors (intake of fruit, vegetables, sweets, and soft drinks), each day of weekly physical activity reduced the odds of being overweight or obese rather than normal weight. Odds ranged from 0.80 (95% CI: 0.76-0.85) in Austria to 0.95 (95% CI: 0.90-1.00) in Greece and were 0.90 (95% CI: 0.87-0.94) in the United States.
The relation of physical activity with body fatness in adolescents is complex, though. A study of over 600 black and white U.S. teens 14 to 18 years of age found that regardless of age or race, boys and girls with the lowest percent body fat spent the most time in vigorous physical activities but also had the highest daily intake of calories (Stallmann-Jorgensen et al. 2007). Also, the vigorously active consumed about 440 kcal a day more than those who said they got no vigorous activity, but their moderate physical activity and TV watching were the same. So, the researchers speculated that the lower percent body fat in those who were vigorously active resulted from elevated metabolic rate and fat oxidation, similar to the concepts of energy flux and subsistence efficiency introduced at the beginning of this chapter. If this is true, the study suggests that prevention of obesity in children should focus on increasing vigorous physical activity rather than restricting energy intake (Gutin 2008).
The scientific advisory committee for the 2005 Dietary Guidelines for Americans concluded that moderate physical activity for an hour each day can increase energy expenditure by about 150 to 200 calories, which can be helpful in preventing unhealthy weight gain if the extra calories used aren’t offset by an equal increase in calorie intake. The committee also concluded that adults who’ve lost weight may need 60 to 90 min of daily activity to help avoid regaining weight and that children and adolescents need at least 60 min of moderate-to-vigorous physical activity most days of the week for healthy weight gain as they mature (Dietary Guidelines Advisory Committee 2005). Likewise, the scientific advisory committee for the 2008 Physical Activity Guidelines for Americans recognized that recommendations about the amount of physical activity needed for weight maintenance, weight loss, or prevention of weight regain after weight loss must allow for energy intake. The committee also noted that in most weight loss studies that included dieting, the contribution of physical activity to the overall calorie reduction and weight loss was small. Finally, the committee noted that increased use of labor-saving tools at work and home, as well as more time spent in sedentary activities (e.g., Internet use and online social networking) during discretionary leisure, has shifted downward the energy cost of activities of daily living in ways that put extra importance on planned physical activities above daily routines in order to achieve energy balance (Physical Activity Guidelines Advisory Committee 2008).
The American College of Sports Medicine (ACSM) recommends that adults participate in at least 150 min/week of moderate-intensity physical activity to protect against excessive weight gain and reduce chronic disease risk factors, especially in adults with a BMI ≥25 or a waist circumference above 88 cm (35 in.) in women and 102 cm (40 in.) in men. The ACSM recommends a weight loss of 5% to 10% of body weight for those people, with additional health benefits expected for those who sustain weight loss of more than 10%. Overweight and obese individuals will likely benefit from 250 or more minutes each week to experience greater weight reduction and prevent weight regain. The ACSM also recommends strength training to increase or maintain fat-free mass and further reduce health risks. Although resistance exercise added to aerobic exercise doesn’t typically enhance weight loss, it can increase basal metabolic rate by retarding loss of muscle (sarcopenia) during aging and dieting.
Even among people who are considered sedentary during their work or leisure, there can be meaningful differences in daily energy expenditure that results from the routine activities of daily living, maintaining an upright posture (e.g., standing rather than sitting), and spontaneously arising movements (e.g., pacing or fidgeting), which has been termed nonexercise activity thermogenesis, or NEAT (Levine, Eberhardt, and Jensen 1999). People vary widely in weight gain when they overeat. A groundbreaking study measured changes in weight gain and naturally occurring energy expenditure in young, nonobese adults who were fed 1000 kilocalories more than they needed for weight maintenance each day for eight weeks. Body fatness was measured with DXA, and total daily energy expenditure was measured using doubly labeled water. Exercise was estimated by accelerometer counts and interviews and did not change during overfeeding. On average, 432 of the extra calories were stored as fat, and 531 calories were burned by increased NEAT. Fat gain varied among people by 10-fold, ranging from a gain of about 0.40 kg to 4 kg. The amount gained was inversely related to how much people increased their NEAT, which also varied widely from 298 to 1692 calories each day. The average increase in NEAT was 336 calories a day, which accounted for two-thirds of the increase in daily energy expenditure. The authors surmised that when people overeat, some increase NEAT to preserve leanness, while others are vulnerable to weight gain because they do not naturally increase NEAT.
A related laboratory study showed that NEAT is substantially reduced by the common use of labor-saving devices (Lanningham-Foster et al. 2003). Not surprisingly, the energy cost (kcal/day) of several household chores and work-related transportation was less when they were carried out using a machine instead of by hand (washing clothes: 27 vs. 45; washing dishes: 54 vs. 80) or by foot (riding instead of walking nearly a mile to work: 25 vs. 83; taking an elevator instead of climbing stairs while at work: 3 vs. 11). The total energy cost of daily physical activities lost to the use of labor-saving devices was about 110 kcal a day!
Evidence shows that regular physical activity or physical fitness can (1) reduce health risks in people who are overweight, (2) protect against excessive weight gain, (3) help overweight and obese people lose weight, and (4) help people maintain stable weight after they lose it.