Learning Objectives

The reader will be able to:

• Describe the three types of physical activity.
  
• Explain why calorie restriction must also be included in a plan for weight loss.
  
• Describe how weight loss maintainers in the National Weight Control Registry are able to maintain their lower weight for long periods.
  
• Name the five stages of change proposed in the transtheoretical model of behavior change.
  
• Propose strategies to enhance the self-efficacy and confidence needed to increase physical activity.
  
• List several factors that are associated with, and may explain, differences in obesity in different racial and ethnic groups.

Introduction

Both healthcare providers and the public often believe that exercise alone results in significant weight loss. The reality is that few can maintain the level of activity necessary to produce a significant amount of weight loss without also restricting calorie intake. Physical activity is important because it helps prevent loss of lean muscle mass and helps maintain energy balance during weight loss maintenance (1). Indeed, physical activity is perhaps the best predictor of sustained healthy weight loss (2–4). As a whole, the scientific literature suggests that physical activity is a necessary but not sufficient component to healthy weight management efforts among individuals who are overweight or obese.

In this chapter, we focus on the energy expenditure side of the energy balance equation and review the effect of physical activity on preventing weight gain, initial weight loss, and weight loss maintenance. Our discussion will include mechanisms of energy expenditure associated with energy storage (e.g., exercise, lifestyle physical activity, and non-exercise activity thermogenesis (NEAT)) and outline theories and methods designed to measure and encourage physical activity. The last section will highlight differences among diverse groups in the United States.

Relationship between Physical Activity and Weight Change

Concept of Energy Balance

The storage or loss of energy in the form of body fat is determined by energy balance. That is, energy storage and the resulting accumulation of body fat occur only when energy intake (calories from food) exceeds energy expenditure (calories burned during body function). Similarly, when energy expenditure exceeds energy intake, body fat is lost. Imagine a simple scale when thinking about energy balance. Body fat is lost when the scale tips into negative energy balance (energy expenditure is greater than energy intake). Body fat is accumulated when the body is in positive energy balance (energy expenditure is less than energy intake). Although this sounds like a very simple concept, the behavioral and physical mechanisms associated with energy balance are very complex.

Energy Intake

Energy intake is the consumption of food and beverages whose macronutrients are absorbed into the body. Energy from macronutrients is transformed in the cells into a form of energy that the body can use. The energy that is stored and used by the body is measured in calories (5). Energy intake is a complex process and can be affected by biological, psychological, social, and developmental processes (1, 6, 7).

Energy Expenditure

Energy expenditure is defined as the use of energy by the body. Energy expenditure is usually divided into three general categories: 1) basal metabolism, 2) thermic effect of food, and 3) activity thermogenesis (i.e., energy expenditure from physical activity) (1, 8). The total amount of the energy expended by these mechanisms is often referred to as total energy expenditure.

Basal Metabolism

Basal metabolic rate (BMR) is a measurement indicating the amount of energy required to sustain life for a given period. For example, it takes a certain amount of energy to breathe, circulate blood, and regulate body temperature. This amount of energy or the BMR is measured when an individual is at rest, at thermoneutrality (i.e., ambient temperature is held constant such that metabolism is not affected by heat or cold stress), and in a post-absorptive state (8–12 hours after last food ingestion) (5). Determining BMR is a sophisticated and difficult process, so resting metabolic rate (RMR) is often used clinically as an estimate of BMR. It has been estimated that RMR accounts for up to 60–75% of total daily energy expenditure among sedentary adults (8). One can generally estimate that RMR is ∼1 kcal/kg of body weight/hour.

Thermic Effect of Food

It takes energy to process food once it is consumed. The thermic effect of food is the energy expended for digestion, absorption, and storage of food in the body. These processes account for approximately 10% of an individual’s total daily energy expenditure, which equals intake when in balance (8).

Activity Energy Expenditure

Activity energy expenditure, or activity thermogenesis, is the energy expenditure associated with any voluntary or involuntary body movement. That is, activation of skeletal muscles requires energy. Activity thermogenesis is often conceptualized as exercise-related activity (e.g., running on a treadmill) and non-exercise activity (e.g., activities of daily living such as vacuuming). Activity thermogenesis has been estimated to account for 15–50% (8) of an individual’s total daily energy expenditure, and this range is determined by a person’s occupation, leisure time physical activity, household tasks, and self-care activities (9, 10).

Difference between Activity and Exercise

Exercise and physical activity are often thought to be the same construct. However, there is a small but important distinction. Exercise is a subset of physical activity and implies planned, structured, and repetitive bodily movement with the primary goal of improving or maintaining physical fitness (11). For example, riding a stationary bike or walking on a treadmill with the goal of improving fitness would be considered exercise. Physical activity, on the other hand, is any body movement that results in skeletal muscle contraction and noticeable increases in energy expenditure (12). Non-exercise activity thermogenesis (NEAT) has been used to describe these non-exercise body movements. NEAT includes activities such as occupational tasks, standing, walking, talking, toe-tapping, playing an instrument, and shopping (8, 13). NEAT activities are often overlooked as significant and measurable sources of energy expenditure. Indeed, it has been estimated that NEAT can contribute 15–50% of total daily energy expenditure (9, 10, 14). Data suggest that energy expenditure through exercise-related activities (e.g., structured sports) is negligible compared to the energy expenditure associated with NEAT (8) (see Table 21-1). Blair and colleagues (15) described a lifestyle approach to physical activity as opposed to structured exercise, and studies have demonstrated that moderate-intensity lifestyle activities (e.g., walking, gardening, house cleaning) provide the same weight loss and health benefits as vigorous exercise (16, 17).

Table 21-1 Kcal Expended Per Hour Across Activities For A 90 Kg Person

Ainsworth BE, Haskell WL et al. Compendium of physical activities: an update of activity codes and MET intensities. Med Sci Sports Exerc 2000; 32 (Suppl 9): S498–S516.

For example, Andersen and colleagues studied 40 women with an average BMI of 32.9. All the women participated in weekly weight-loss behavior modification classes and followed a self-selected low-calorie diet for 16 weeks (18). The women were randomized into two groups: 1) structured aerobic exercise three times a week; or 2) lifestyle classes in which the women discussed how to increase daily physical activity. At week 16, both groups had lost comparable amounts of weight (7.9 and 8.3 kg respectively). In addition, the lifestyle physical activity intervention was as effective as structured exercise in improving blood pressure and lipids for up to one year. Thus, it appears that a program of diet plus lifestyle physical activity may be a reasonable alternative to structured exercise for individuals who are overweight or obese.

Effect of Physical Activity on Weight

Prevention of Weight Gain

It has been estimated that the average American gains 0.45–0.90 kg of weight each year, and this current rate of gain could be prevented if 100 more kcal were expended each day or if 100 kcal fewer were consumed (19). Indeed, epidemiological studies have demonstrated that modest increases in physical activity may prevent weight gain. For example, DiPietro and collegues (20) followed more than 3,000 men and women for an average of 5.2 years. The authors categorized participants’ physical activity levels into low, moderate, or high. They found that a modest amount of activity (i.e., approximately 30 minutes of walking five days/week) was associated with the prevention of weight gain. Similarly, two studies examining samples from the CARDIA Study (21) and National Health and Nutrition Survey (22) found that weight gain was negatively associated with relatively small changes in physical activity, including leisure-time activities, among men and women over a 10-year period.

Currently, there is a lack of prospective scientific data on which to base definitive recommendations for the amount of physical activity needed to prevent weight gain. Nonetheless, it appears that 15–30 minutes of daily moderate-intensity physical activity may prevent weight gain for many individuals (23). This translates into adding about 1–1.5 miles of walking or ∼2,000–3,000 steps per day (24). It also seems likely that some individuals, probably due to genetic factors, may need more activity, perhaps much more, to prevent weight gain (25).

Initial Weight Loss

A large body of scientific evidence suggests that greater weight loss is achieved through calorie restriction or calorie restriction plus physical activity than through physical activity alone (2, 26, 27). In fact, the amount of weight loss achieved with only physical activity rarely has clinical significance, and the amount of activity required to be effective is much more than most can achieve and sustain (28). For example, Jakicic and Otto (29) reported that a 90 kg individual would have to engage in 1 h and 23 min–2 h and 45 min of brisk walking per day to expend 500–1,000 kcal/day. This energy expenditure would result in 0.5–1 kg of weight loss per week. On the other hand, an individual could reduce calorie intake by 500–1,000 kcal/day to see a similar reduction in weight. At least in the short term, and in initially sedentary and unfit individuals, it is more feasible to create a significant daily energy deficit by calorie restriction than by exercise.

Weight Loss Maintenance

Although adding physical activity to a low-calorie diet may not produce substantially more weight loss than diet alone, physical activity is important in that it may help prevent weight regain, prevent loss of lean muscle mass, and improve health independent of weight loss. Several studies (2–4) including data from the National Weight Control Registry (NWCR) demonstrate that the duration and intensity level of physical activity is associated with weight loss maintenance. The NWCR is an ongoing longitudinal registry including individuals who have lost 13.5 kg or more and have maintained this loss for a minimum of one year (30). It appears that long-term weight loss maintainers not only eat fewer calories but also report at least one hour of moderate-intensity physical activity each day (26). In addition, decreases in physical activity are associated with weight regain in this sample (2, 31).

Resistance Training

Physical activity in the form of resistance training (e.g., weightlifting) also has been considered in the context of weight loss and weight loss maintenance. In theory, the accumulation of fat-free mass (i.e., muscle) through resistance training would result in weight loss by increasing resting metabolic rate. However, in practice this may not be the case. Resistance training does preserve lean muscle mass among individuals on low-calorie diets, but it does not appear that resistance training maintains or increases resting metabolic rate among dieters (32, 33). These findings, however, do not negate the importance of resistance training in the context of overall health. Preserving or increasing lean muscle mass increases overall body strength, which may increase the likelihood of sustained cardiovascular physical activity. This in turn may result in increased energy expenditure leading to weight loss or weight loss maintenance. Resistance training also may prevent loss of bone density associated with normal aging or calorie-deficit-induced weight loss.

Measurement of Physical Activity

An important consideration when evaluating the role of physical activity in weight management is the accurate assessment of physical activity habits. There are three general types of physical activity assessment methods: 1) criterion methods; 2) objective measures; and 3) subjective/self-report. Although criterion methods are considered the gold standard for measuring physical activity, they are often expensive, inaccessible, and cumbersome to use. As a result, they rarely are used in clinical settings.

Doubly labeled water and indirect calorimetry are examples of criterion methods for measuring energy expenditure (34). The doubly labeled water method measures total energy expenditure and is commonly used to validate energy intake measurement. In this method, the subject drinks water made with the non-radioactive heavy isotopes, deuterium, and oxygen 18. The initial concentration of the “labeled” water in the total body water is determined with a urine, saliva, or blood sample a few hours after administration. The elimination rates of deuterium and O-18 in the subject are determined with follow-up samples of the body fluid. The technique measures a subject’s carbon dioxide production between the two measurement times by considering that deuterium is lost in only the water, while the heavy oxygen is lost in both H₂O and CO₂. This allows the O₂ loss through water to be used to estimate O₂ loss through CO₂ production. With simplifying assumptions about the dietary mix of carbohydrates, fats, and proteins, and about the energy produced when oxygen is used and CO₂ is produced in metabolism, total energy expenditure or metabolic rate can be determined.

This method is objective and involves minimal interference with subjects’ daily lives. The method does not require the participant to log or report his or her behavior. However, doubly labeled water cannot distinguish between physical activity energy expenditure, basal metabolism, or diet-induced energy expenditure (34).

Indirect calorimetry is a method for determining energy expenditure from oxygen consumption and carbon dioxide production. This is typically done by measuring the volume of expired air from the lungs and analyzing it for oxygen and carbon dioxide content. The amount of oxygen consumed can be used to determine the amount of fuel that is used in metabolic processes. This information can then be used to calculate the energy expended.

Objective measures of physical activity include pedometers, accelerometers, and heart rate monitors. The pedometer is a small motion sensor worn on the waist that measures steps taken while walking or running. Pedometers are inexpensive and have acceptable validity and reliability (34). Accelerometers, also motion sensors, provide an estimate of overall physical activity and are sensitive to all body movement (35, 36). One drawback is that accelerometers are significantly more expensive than pedometers due to their advanced technology. Accelerometers range from $200 to $450, as compared with $10–20 for pedometers (34).

Subjective measures of physical activity include diaries, interviews, and questionnaires. These methods are inexpensive, except perhaps for interviews, and are used frequently in epidemiological and intervention studies. One commonly used interviewer-assisted questionnaire is the 7-Day Physical Activity Recall (7-Day PAR) (37). The 7-Day PAR has been validated in both community-based surveys and experimental studies (37). This questionnaire estimates activity types (e.g., walking vs. stair climbing) and converts these types into energy expenditure. Weekly and daily energy expenditures also can be determined with this measure. Commonly used self-report measures include the 11-item Physician-Based Assessment and Counseling for Exercise (PACE), the Paffenbarger physical activity index (38), and the Short Questionnaire to Assess Health-Enhancing Physical Activity (SQUASH).

Since physical activity levels frequently change with age, several measures have been developed specifically for older adults such as the Community Health Activities Model Program for Seniors (CHAMPS) (39), the Physical Activity Scale for the Elderly (PASE) (40), and the Yale Physical Activity Survey (YPAS) (41). These questionnaires may be more sensitive to the activity levels of an older, obese population compared to a healthy and younger population.

Methods and Theories to Initiate and Maintain Physical Activity Among Overweight and Obese

The goal of any physical activity intervention is to help individuals not only to initiate activity but also to maintain it over time. Several theories and strategies have been developed and applied to help achieve these goals. One that has been applied to physical activity interventions is the Transtheoretical Model (TTM), which explains the stages and processes associated with health behavior change (i.e., decreasing an unhealthy behavior or initiating a healthier behavior). The theory posits that individuals progress through five stages when changing any health-related behavior. The five stages of this model are: 1) pre-contemplation, 2) contemplation, 3) preparation, 4) action, and 5) maintenance.

Using physical activity as an example, in the pre-contemplation stage individuals have no intention to become physically active. They may have no awareness that physical activity is healthy or that their inactivity may have negative health consequences. In the contemplation stage individuals are aware that change is needed and are considering becoming physically active. During the preparation stage individuals have started to make small changes and may have increased their physical activity levels, but not on a regular basis. The action stage describes the period when individuals have taken action; that is, they are regularly engaging in physical activity. The last stage, maintenance, refers to the period when physical activity has been continued for six months or more (42). TTM assumes that individuals move through the five stages in a non-linear fashion and may move back from a later stage of the model into an earlier stage. This may help explain why some individuals begin a physical activity program but then stop. For example, someone in the action stage may experience a life event (e.g., birth of a child) that moves them back into the pre-contemplation stage.

Strategies used to augment the transtheoretical model include the use of cognitive techniques (e.g., setting realistic goals) and behavioral techniques (e.g., placing reminders to exercise at work or home) (43, 44). Social Cognitive Theory (SCT) incorporates these general strategies and is based on the interaction of psychosocial, environmental, and behavioral factors that can influence behavior change (45). Physical activity interventions based on SCT often target increasing people’s confidence (i.e., their self-efficacy) in their ability to initiate and maintain physical activity (46–49). Self-efficacy for physical activity is enhanced by encouraging individuals to set realistic and easily attainable goals (46–47, 49–51) as well as focusing on progress and reinforcement of successes (48–50). Techniques to increase self-efficacy include modeling (50), self-regulation strategies (51–53), and developing social support (46, 47, 49). SCT behavioral interventions appear to present a realistic approach for promoting physical activity among individuals initiating or maintaining weight loss (46, 48–50, 54, 55).

Other models used to explain health behavior change include the health belief model and theory of planned action. The health belief model has been associated with exercise adherence (56, 57) and posits that individuals will engage in physical activity if they perceive their sedentary lifestyle to be a health threat and if they believe that activity will be helpful in reducing that threat. The theory of planned action proposes that health behaviors are not only a function of health beliefs but are also associated with other factors such as a person’s attitude toward a health behavior (e.g., physical activity), perception of social norms, self-efficacy, and locus of control (58, 59). For example, the belief that physical activity helps maintain weight loss is a positive attitude toward becoming physically active. The individual’s belief that he or she can successfully become physically active (self-efficacy) and can assume control of lifestyle and health (internal locus of control) moderates the likelihood of health behavior change. In addition, people who believe that others support them and want them to change behavior are more likely to become physically active.

As these theories suggest, changing physical activity behavior is a complex process influenced by many factors. Interventions based on several behavior-change theories and strategies have been developed and successfully implemented (17, 48, 60). However, these interventions have been implemented under the constraints of clinical research. Translating research findings to practice, making them available to the general population, is an important next step. This was done in the Active Living Every Day program (61) (Box 21-1) and Healthy Eating Everyday program (69). This behavioral program translates the findings of several research studies aimed at increasing physical activity behavior. It is available to the general public in book and Internet form and includes a highly structured series of lessons with support tools (17, 61–71).