Body composition and weight are the sum of numerous factors that regulate and influence the “intake” and “expenditure” sides of the energy balance equation. Although diet and physical activity (PA) are recognized as key players in energy balance, many nutrition educators may not fully understand how they are physiologically linked and that their impact on body weight is highly interrelated, complementary, and synergistic [1
]. The role of diet and PA for weight management and obesity prevention is not as simple as ‘eating less’ or ‘exercising more’. Weight management is no longer a ‘diet vs. PA’ or ‘diet and PA’ issue, but an understanding of the synergy and interrelated nature of these two factors [1
]. Unfortunately, the public is often confused by what they hear and read in the media about weight management [5
]. Although there is much research to the contrary [7
], it is frequently stated that PA does not promote weight loss since obesity has increased while PA has remained constant in the United States [8
]. Still others debate whether obesity is “due to lack of exercise” or ‘low energy flux’ [9
] or due to a ‘bad diet’ [8
]. Malhotra et al. [8
] clearly lays the blame for obesity on the ‘junk food industry’s public relations machinery’ and states that one ‘cannot outrun a bad diet’. These mixed and sometimes incorrect messages are confusing for the public. They also convey the message that only one side of the energy balance equation is important for weight management and obesity prevention. Thus, it is imperative that nutrition educators understand and utilize a dynamic energy balance approach when discussing energy balance and weight issues with the public.
Diet can affect energy balance and health beyond just providing energy. For example, daily energy expenditure is influenced by total energy intake (e.g., kcals or kJ consumed), dietary macronutrient composition (percentage of energy from protein, fat, carbohydrate and alcohol) [5
], the energy density of the diet (kcals or kJ per g of food) [10
], and the timing of food intake [14
]. These dietary factors can also alter the thermic effect of food (see Table 1
) and the type of substrates stored or used for fuel during PA [5
Similarly, PA or exercise (see Table 1
) affects energy balance beyond simply expending energy. Depending on the type, intensity and duration of PA, the amount energy expended, and the type of fuel used, can vary dramatically (e.g., 30 min of running expends more energy than 30 min of walking). PA that increases muscle mass, such as strength training, can also increase resting metabolic rate (RMR) and total daily energy expenditure. Additionally, research shows that PA alters appetite and appetite-regulating hormones (i.e., inducing appetite suppression or promoting hunger), which could ultimately alter total energy intake [24
]. Regular and frequent PA also increases energy flux, which is defined as the rate of energy conversion after absorption from food into body tissues for use in metabolism or its conversion into energy stores (Table 1
]. A higher level of energy flux improves the body’s ability to match energy intake with expenditure and, thus, can make weight management easier [28
]. Finally, appropriate PA improves muscle mass and strength [21
], and can increase or maintain bone mass [31
]. Together, these factors improve overall body composition and health, which increases an individual’s ability to maintain an active lifestyle and reduces the risk of obesity and chronic disease [7
This paper reviews key concepts nutrition educators need to know about how PA or exercise impact dynamic energy balance, weight management and obesity prevention, appetite regulation, energy flux, and overall health. It also provides examples of how nutrition educators can practically integrate these concepts into obesity prevention programming or discussions of weight management.
2. Dynamic vs. Static Energy Balance
Currently, many nutrition and health educators use the classic “static or linear energy balance” approach when discussing weight management or weight loss with the public or their clients (see Table 1
and Figure 1
]. This approach states that a ‘change in energy stores = energy intake − energy expenditure’ and assumes that by simply changing either side of the energy balance equation weight is gained or lost (e.g., increasing or decreasing 3500 kcal (7700 kJ) will result in a one pound (454 g) weight gain or loss) [18
]. This approach does not consider individual differences and the numerous factors that change as energy intake or expenditure is altered [34
]. Energy balance is a ‘dynamic’, non-linear process rather than a ‘static’ or linear process [10
]. This means that altering one component of the energy balance equation (i.e., reducing energy intake or increasing energy expenditure) can affect numerous biological and behavioral factors on both sides of the equation in unpredictable and unintended ways [10
Swinburn and Ravussin provide a classic example to illustrate the fallacy of the static energy balance equation under conditions when body weight is changing [40
]. Using a 165-pound (75 kg) man they demonstrated how body weight would change if this individual consumed an extra 100 kcal/day (~420 kJ/day) for 40 years [40
]. The static energy balance equation would calculate the amount of extra energy consumed to equal ~1.5 million kcals (~6.3 million kJ) with an estimated weight gain of 417 pounds (~190 kg) over the 40-year period. Yet, intuitively nutrition professionals know this probably would not happen. The static or linear energy balance equation does not take into account the increase in energy expenditure that would occur as body weight is gained. As body weight increases, RMR and total energy expenditure also increase due to the greater energy cost of maintaining and moving a larger body. Eventually this individual would achieve energy balance and become stable at a higher body weight. How much body weight is actually gained depends on a number of factors including the energy surplus and macronutrient composition of energy consumed [41
], current body composition, type and amount of PA engaged in, and overall energy expenditure. Figure 2
provides examples of the numerous ways diet (energy intake) and PA (energy expenditure) interact to affect our ability to maintain body weight.
To help operationalize the dynamic energy balance model and make it usable for nutrition educators and health professionals, two mathematical models of dynamic energy balance have been developed to better predict body weight changes in response to changes in energy intake and/or energy expenditure over a given time period [38
]. One model has been developed by Hall et al. [38
] at the National Institutes of Health (NIH) [43
] and a second model has been developed by Thomas et al. [42
] at the Pennington Biomedical Research Center (PBMC) [44
]. These models simulate how alterations in energy deficit or excess, which result from adaptations of total energy intake, fuel selection, and energy expenditure, will affect body weight. Both prediction models were developed using weight change results from weight loss studies with overweight or obese adults and thus not be completely applicable to youth, athletes, or individuals who are not overweight. (See Manore [19
] for a case study of how these tools can be used with an active individual to predict weight change over a given time period and Webb [45
] for how these changes can be incorporated into general weight loss counseling).
4. Role of Physical Activity in Appetite Regulation
Physical activity can also alter appetite, which has the potential to alter total energy intake and, thus, body weight. The effect of PA on appetite and the desire to eat are influenced by the type and intensity of the PA, the environmental temperature, and the characteristics of the exerciser. Thus, the ability of PA to create negative energy balance relies not only on its direct ability to increase energy expenditure but also indirectly on its potential to modulate appetite and/or energy intake. Increases in energy intake that match or exceed the energy cost of an exercise bout (or increased PA) negate body weight loss and can even result in weight gain. A recent review of studies evaluating exercise and weight loss found that “dieters” frequently lost only a third as much weight as was expected, given their energy expenditure during workouts [63
]. This phenomenon is more common in women, and is partially explained by compensatory behaviors—including increases in energy intake—that counter exercise energy expenditure and negate body weight loss [63
]. Compensatory behavior is driven by increased hunger due to exercise or an increased desire to eat (see Table 1
). Thus, the energy deficit from a three-mile (~4.8 kilometers) run could easily be reversed by consumption of a 300 kcal (1255 kJ) cookie (or two). Exercise-induced alterations in appetite and their potential effect on energy balance and body weight are summarized below.
4.1. Effect of Type and Intensity of Physical Activity on Appetite and Energy Intake
Evolving research suggests that both the type and intensity of PA or exercise influences post-exercise alterations in appetite. Most studies suggest that higher-intensity exercise is more likely to suppress hunger or food intake during the post-exercise period [65
] than is moderate or light PA. This appetite-suppressing effect seems to last 15–60 min following exercise but can potentially delay the next meal or snack. Lower-intensity PA does not seem to have this same effect. Similarly, the type of PA is important. Research shows that activities such as running, jumping rope, or high-intensity exercise interval workouts suppress appetite [70
], while swimming and walking [71
] are more likely to stimulate appetite and/or food intake. Running seems to have a stronger dampening effect on appetite than does strength training [75
]. Overall the types of PA that have the greatest dampening impact on appetite (and favoring negative energy balance) include those that are more intense or which could be considered to jar the gut, such as running.
4.2. Effect of Environmental Temperature on Appetite and Energy Intake
The environmental temperature during or following PA can also impact appetite. Cold environments promote hunger and/or food intake whereas hot environments blunt hunger. Recent research has shown that exercising for 45 minutes in cold water (20 °C or 68 °F) promoted an average 44% higher post-exercise (1 h) food intake compared to exercise in neutral (32 °C or 89 °F) conditions [76
]. Differences in environmental or body core temperature [77
] could be another reason why swimming seems to promote hunger compared to other types of PA.
4.3. Factors that Drive Hunger and Desire to Eat after Exercise
Alterations in key appetite regulating hormones including the hunger hormone ghrelin and the satiety hormones peptide YY (PYY), glucagon-like peptide-1 (GLP-1), and leptin are thought to partially drive appetite changes during and after PA [79
]. However, the changes in appetite, and subsequent food intake after PA, are not driven by appetite-regulating hormones alone, since these hormones are influenced by a variety of factors (e.g., exercise intensity, body composition, thirst, sex, energy restriction). In addition, these physiological regulators of appetite can be overridden by eating. For example, after exercising, some individuals can easily “eat back” the energy burned during exercise with an energy-dense snack or calorie-containing beverage, thereby countering the energy cost of the previous bout of PA.
4.4. Differences Between Men and Women
Studies consistently suggest that women are more prone to compensatory behaviors, or “eating back” energy expended during exercise by increasing energy intake, thereby negating body weight loss [63
], although such compensatory behaviors do occur in both sexes. It is not known what factors drive this sex difference or whether increased health-related fitness, through regular engagement in PA, dampens compensatory behavior. Recognizing that women and men can unconsciously or consciously engage in compensating energy intake, which can be driven by hunger or food rewards, is important for education about exercise-associated body weight loss.
5. Energy Flux: Putting It All Together
After food is digested and absorbed, energy flux refers to the rate of energy conversion for either energy expenditure or transformation to storage (see Table 1
]. Thus, energy flux represents the amount of energy moving through the body each day (e.g., higher energy expenditure requires a higher level of energy intake to maintain body weight and body systems). Maintaining a high energy flux (e.g., maintaining a higher level of PA and matching energy intake) could be key to successful weight maintenance, preventing excess weight gain, or maintaining weight loss in the following ways:
Maintains overall higher energy expenditure by maintaining muscle mass, thermic effect of food, and a higher RMR (e.g., in high energy flux), in addition to the energy expended in PA. A person in high energy flux will expend more energy in PA and need to eat more food to cover their energy needs.
Heightens sensitivity to appetite control through its impact on appetite-regulatory hormones and food preferences. Thus, the desire to overconsume food is dampened and the total energy intake modified.
Allows for more appropriate energy intake or volume of food consumed, thus, reducing the probability of overeating. Sedentary individuals (e.g., in low energy flux) can have daily energy needs that are so low that it is easy to consume more food (e.g., kcals) than needed in our current obesogenic environment.
Energy flux plays an important role in helping with weight maintenance, preventing excess weight gain, and maintaining weight loss after “the diet is over” [29
]. However, for weight loss to occur either diet, exercise, or both need to be altered in such a way as to sustain a ‘negative energy balance’ or a ‘larger gap between intake and expenditure’ over an extended period of time. Research shows that maintenance of fat free mass (FFM) is better if both diet and exercise are included in a weight loss program compared to diet alone [81
]. For most sedentary, overweight, or obese individuals, key minimum changes in diet and exercise need to occur for significant weight loss to be realized. First, research shows that PA needs to increase to at least 250 min/day to achieve clinically significant weight loss [61
]. Second, energy intake needs to decrease [81
], but not so dramatically as to increase the loss of FFM or suppress metabolic rate [61
]. Typical recommendations for diet are to decrease energy intake to a level that produces weight loss, but allows for adequate PA, is above the energy cost of RMR [61
], and includes low energy-dense foods and dietary food choices that can be sustained [82
Understanding the dynamics of energy balance and the synergistic and interrelated role that diet and PA play in weight management is important for the development and implementation of effective obesity prevention programs. If obesity prevention efforts only focus on diet and nutrition, or place a limited emphasis on PA, these efforts will likely fail. How an individual’s body weight changes respond to changes made in diet and PA will depend on numerous factors such as health-related fitness level, body composition (i.e., relative muscle mass vs. fat mass), metabolic rate, regulatory hormones, appetite, and the level of energy flux. Understanding dynamic energy balance will help nutrition educators increase their knowledge and comfort level with PA so they can build the skills required to explain how diet and PA work synergistically to help consumers achieve and maintain a healthy body weight and composition. This in turn will help consumers understand that PA does more to influence body weight and composition than just burning calories, and that PA must be done in combination with healthy dietary approaches to achieve effective weight management and obesity prevention.