Role of Physical Activity in Prevention and Treatment of Excess Weight
From the preceding discussion, it is clear that a decrease in physical activity contributes to the increased prevalence of obesity worldwide. Accordingly, it is intuitive to suggest that an increase in physical activity levels would be associated with a decrease in obesity. Indeed, evidence from population-based studies with long-term follow-up confirms that age-related weight gain is attenuated in physically active adults compared with sedentary adults (Saris et al. 2003). However, while the experts agree that an increase in physical activity is associated with a lower prevalence of obesity, precisely how much physical activity is required to prevent age-related weight gain is the subject of considerable debate.
Physical activity guidelines for adults were initially formulated for the prevention of morbidity and mortality. Indeed, there is now a large body of evidence suggesting that the accumulation of 30 min or more of moderate-intensity physical activity on most days of the week provides substantial benefits across a broad range of health outcomes. Although 30 min of daily physical activity may prevent unhealthy weight gain in some individuals, it is now generally reported that this volume of physical activity may be insufficient for the prevention of the age-related weight gain in many if not most adults. Table 12.3 summarizes the reports from various expert groups that have considered how much physical activity is required to prevent weight gain. In general, the expert groups derived their recommendations from analysis of longitudinal, population-based studies that related estimates of self-reported physical activity levels over time to corresponding changes in body weight. The principal exception was the report from the Institute of Medicine, which based its recommendations on a cross-sectional analysis of data that used the doubly labeled water method to estimate total energy expenditure. Combined with the measurement of basal metabolic rate, the total energy expenditure values derived by doubly labeled water can be used to calculate an individual’s physical activity level (PAL). The PAL is also used in population-based studies as a way of standardizing the various approaches used to determine physical activity energy expenditure. In short, the higher the PAL, the higher the level of physical activity performed on a daily basis.
The PAL is defined as the ratio of total energy expenditure to 24-h basal energy expenditure. Thus, PAL depends to a certain degree on body size and age, because these variables contribute to basal energy expenditure. Individuals can be placed into one of four activity categories based on their PAL, as shown in table 12.4.
The four PAL categories in the table correspond roughly to quartiles in the population. Thus, the Sedentary category is the lowest 25% of the population, whereas the Very Active category is the highest 25%. The Sedentary category was defined according to basal energy expenditure, the thermic effect of food, and the energy expended in physical activities that are required for independent living. Incorporating about 40 min per day of walking at a speed of 4.8 to 6.4 km/h (3 to 4 mph), in addition to the activities that are part of daily living, raises the PAL to the Low Active level in the average 70 kg (154 lb) person. To reach a PAL of 1.7 to 1.8—which is currently recommended for the prevention of age-related weight gain—the average 70 kg person must incorporate about 2 h per day of walking at 3 to 4 mph, in addition to the activities that are part of daily living.
The distances and times required to move to the more active categories vary considerably by body weight. Thus, more walking is required for lean individuals and less for obese individuals. People can reduce the times substantially by walking faster or performing other, more vigorous activities. For example, if the average person walked 30 min per day at 6.4 km/h (4 mph), cycled moderately for another 25 min, and played tennis for 40 min, the PAL would increase to about 1.75 (Active).
The consensus opinion at present is that the prevention of weight gain in both developed and undeveloped countries is associated with a PAL of about 1.7 to 1.8 (table 12.3). To achieve a PAL of 1.8 would require a physical activity habit equivalent to walking 8 to 11 km (5 to 7 mi) per day at 4.8 km/h (3 mph) in addition to the habitual activity required by a sedentary lifestyle. For most inactive people, this would require adding more than 60 min of physical activity to their daily routine.
On the other hand, the guidelines for prevention of weight gain have been derived in large measure from population-based cohorts of men and women, and thus the implementation of the guidelines may vary substantially among individuals. In other words, some people will maintain body weight by accumulating only 30 min of daily physical activity, whereas others may find it necessary to accumulate 60 to 120 min or more to maintain energy balance and to prevent weight gain. The point is that the quantity of physical activity required to maintain body weight (e.g., energy balance) varies depending on the individual. Also note that the guidelines for prevention of weight gain through physical activity are derived from studies that used primarily Caucasian adults. Accordingly, the potential influence of race on these guidelines is unknown.
On average, a PAL of about 1.75, which is equivalent to about 60 to 90 min of daily leisure-time physical activity, is recommended to prevent age-related weight gain.
Treatment of Obesity
The independent role of physical activity as a treatment strategy for obesity has received considerable attention. Early reviews of the literature suggested that the reduction in body weight (1-2 kg [2.2-4.4 lb]) associated with physical activity alone (e.g., no caloric restriction) was marginal, and thus physical activity in the absence of caloric restriction was not a particularly useful strategy for the treatment of obesity (NIH National Heart Lung and Blood Institute 1998). Subsequently, a careful inspection of the exercise studies revealed that, for the most part, few of the early studies prescribed an exercise program that would be expected to lead to meaningful weight loss (Ross and Janssen 2001). On the other hand, in studies in which the prescribed exercise program did result in a meaningful negative energy balance, weight loss was substantial (Ross and Janssen 2001). In other words, weight loss is positively related to the volume of physical activity performed. This point is illustrated in figure 12.4, showing a dose–response relationship between caloric expenditure and the time spent exercising with the corresponding reductions in body weight and total fat. However, daily exercise is not always associated with reductions in body weight or body fat. Some investigators report a resistance to weight loss in response to daily exercise performed for about 30 to 40 min for several months (Donnelly et al. 2003). Nevertheless, the majority of studies suggest that regular physical activity without restriction of caloric intake is associated with weight loss and a reduction in total fat in overweight men and women.
From figure 12.4, it is also clear that exercise performed for as little as 200 min per week is associated with weight loss. In fact, weight loss on the order of 0.5 kg (1 lb) per week is achieved in response to exercise performed for between 300 and 400 min per week or about 50 min per day. This observation is consistent with the position of the American College of Sports Medicine, which recommends that overweight and obese persons seeking weight loss should exercise between 200 and 300 min per week, the equivalent of about 8,374 J (2,000 kcal) per week (American College of Sports Medicine 2001).
Treatment of Abdominal Obesity
Whether an increase in physical activity is associated with a significant reduction in abdominal obesity is an important question. Minor reductions (~2 cm [~0.8 in.]) in waist circumference (a surrogate for abdominal fat) are observed in response to exercise-induced weight loss on the order of 2 to 3 kg (4.4-6.6 lb) (NIH National Heart Lung and Blood Institute 1998). In other words, similar to body weight, waist circumference is reduced a small amount in response to a small amount of physical activity. Although it is unclear whether a dose–response relationship exists between the amount of physical activity and the reduction in waist circumference, it is evident that larger reductions in waist circumference are observed in response to a significant amount of daily exercise. Indeed, exercise performed for 300 to 400 min per week, or about 60 min per day, is associated with reductions of about 0.5 cm per week. In fact, exercise performed for about 60 min per day for three to four months is associated with reductions in waist circumference that approach 5 to 6 cm (2-2.4 in.) in both men and women, as illustrated in figure 12.5.
Whether exercise-induced weight loss is associated with corresponding reductions in abdominal subcutaneous and visceral fat has also been considered (Ross and Janssen 2001). It is generally observed that exercise is associated with a substantial reduction in abdominal subcutaneous and visceral fat independent of gender and age. An example of this effect is shown in figure 12.6, where a 10% reduction in body weight is associated with a reduction in abdominal subcutaneous and visceral fat that approximates 25% and 35%, respectively. Figure 12.6 also shows that the greater the exercise level expressed in minutes per week, the greater the reduction in both abdominal subcutaneous and visceral fat.
Because visceral fat is such an important predictor of health risk, practitioners have questioned whether this fat depot is selectively reduced in response to exercise-induced weight loss. The answer depends on how the reduction is presented. That is, for a given weight loss, a greater reduction in abdominal subcutaneous fat is observed if the reduction is expressed in absolute values (e.g., cm2 at the L4-L5 image); the reason is that most adults have more abdominal subcutaneous fat than visceral fat. On the other hand, if the reduction in abdominal subcutaneous and visceral fat is expressed in relative terms (e.g., relative to the initial size of the depot), then the reduction in visceral fat is greater than the reduction in subcutaneous fat.
Exercise-Induced Reduction in Adiposity Without a Change in Body Weight
Emerging evidence suggests that regular exercise can reduce total and abdominal obesity in the absence of any change in body weight. This is supported by at least two lines of evidence. First, for any given level of BMI between 18 and 35 kg/m2, adults who are physically active (e.g., have a higher level of cardiorespiratory fitness) have a lower waist circumference and lower levels of abdominal subcutaneous and visceral fat compared with their sedentary counterparts (lower level of cardiorespiratory fitness) (Janssen et al. 2004). Second, results from well-controlled, randomized trials reveal that obese men and women who participate in exercise programs for three to four months can experience significant reductions in both waist circumference (figure 12.5) and abdominal subcutaneous and visceral fat despite no change in BMI (Ross et al. 2000, 2004). These observations are important because they suggest that those who seek obesity reduction by increasing physical activity should be educated about the possibility that reductions in waist circumference, total fat, and abdominal fat can occur with or without a corresponding weight loss. On the other hand, it is equally important to note that the reduction in both total and abdominal fat depots is much greater in response to exercise with weight loss than to exercise without weight loss (Ross and Bradshaw 2009). These observations highlight the importance of monitoring obesity reduction using both BMI and waist circumference.
Exercise in the absence of weight loss is associated with significant reductions in total, abdominal, and visceral fat in obese men and women. These reductions are, however, smaller than those associated with exercise-induced weight loss.
The prevalence of obesity is already high and is increasing worldwide. This poses a major threat to public health; and innovative, multidisciplinary strategies are required to combat the problem. The information presented in this chapter provides strong support for the recommendation that physical activity should be an integral component in the strategies developed to both prevent and treat the obesity epidemic. Current guidelines suggest that adults should accumulate about 60 min of moderate-intensity physical activity daily to prevent unhealthy weight gain. Results from shorter obesity treatment studies in which dietary intake was carefully controlled suggest that 60 min of moderate-intensity exercise without a change in energy intake is associated with substantial reductions in total and abdominal obesity in obese men and women.
Although it is now clear that an increase in daily physical activity is required for most individuals, the challenge that remains is how to engage in and maintain a physically active lifestyle. Increasing
physical activity to the levels recommended for obesity prevention and reduction will require a multi-disciplinary approach that includes such components as educating allied health care providers about the benefits of physical activity, reestablishing daily physical education programs in our school systems, and working with urban planners to develop environments that encourage physical activity. Although the societal challenge to increase physical activity levels to an appropriate amount to combat the obesity epidemic is immense, the benefits are many, and thus the problem must be approached with vigor, step by step.
abdominal subcutaneous fat—Layer of fat that lies directly underneath the skin in the abdominal region.
adipocyte—An adipose tissue or fat cell that stores lipids.
ad libitum—At one’s pleasure; as one wishes.
body mass index (BMI)—A simple index of weight for height, calculated as weight in kilograms divided by the square of height in meters (kg/m2), that is commonly used to determine overweight and obesity status in research and clinical settings.
cytokines—For definition, see page 160.
ectopic fat—Fat that is stored outside of the adipose tissue depots.
lipolysis—Lipid breakdown reaction in adipocytes whereby the triglyceride molecule is hydrolyzed in the cell’s cytosol into its components glycerol and three fatty acid molecules.
obesity—A condition of excessive fat accumulation to the extent that health may be impaired.
physical activity level (PAL)—Total daily caloric expenditure divided by total calories from resting metabolism. This term is being increasingly used as an overall indicator of energy expenditure.
visceral fat—Internal fat in the abdominal region that surrounds the organs of the gastrointestinal tract. Visceral fat consists of omental and mesenteric adipocytes and is contained within the visceral peritoneum.
waist circumference—A measurement of abdominal circumference commonly obtained at the top of the iliac crest. Waist circumference is used to characterize levels of abdominal obesity in research and clinical settings.
1. What is BMI, how is it calculated, and what cut points are used to define overweight and obesity in adult men and women?
2. List five major chronic diseases that are associated with obesity.
3. Provide two examples of ectopic fat deposition in obesity, and explain their relationship to obesity-related disease.
4. Describe how the average dietary intake, average leisure-time physical activity levels, and average total physical activity levels have changed in the past three decades and how these changes have contributed to the obesity epidemic.
5. What is PAL, how is it calculated, and what levels are currently recommended for the prevention of unhealthy weight gain?
6. What changes occur in abdominal subcutaneous and visceral fat in obese individuals in response to daily exercise performed for about 60 min at a moderate intensity?
7. Discuss the importance of waist circumference in monitoring success in obesity reduction programs.
8. What changes occur in waist circumference in response to exercise with or without weight loss?
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