Scientists respond to burning questions about food, exercise and weight loss.
Hall, K.D., et al. 2012. Energy balance and its components: Implications for body weight regulation. American Journal of Clinical Nutrition, 95 (4), 989–94.
Energy balance represents the complex interplay between the fuel we consume and the energy we exert, which makes this balance integral to the process of losing weight.
That reality inspired esteemed researchers representing the American Society for Nutrition and the International Life Sciences Institute to create a consensus statement highlighting state-of-the-art research on energy balance (Hall et al. 2012). This column will summarize their consensus and do the following:
- Explain energy balance and energy imbalance in regard to fuel intake and energy expenditure change over time.
- Discuss the interactions between the components of energy balance and how they are regulated.
- Review the accuracy of popular beliefs on energy balance.
- Point out the limitations of current research findings on energy balance.
Our diets derive energy from fat, protein, carbohydrate and to some degree alcohol. As little as ~2%–10% of total food intake becomes waste to be excreted. The rest is biologically broken down (or oxidized) for the body’s energy needs, including growth, cell maintenance, physical activity, pregnancy and lactation, and other cellular life processes (Hall et al. 2012). The caloric yields for fat (9 kilocalories/gram), carbohydrate (4 kcal/g) and protein (4 kcal/g) refer primarily to the amounts of fuel these sources provide to the body’s cells for physiological processes. Some foods, depending on their type of fiber content, are not digestible and thus are not absorbed and used. Also, some types of cooking can inhibit absorption of certain foods, thus limiting how many calories get into the body.
Energy balance components that are commonly of interest in weight management include resting energy expenditure, the thermic effect of food and activity energy expenditure.
Resting energy expenditure, or REE, is the nonexercise energy that the body uses to survive. REE represents two-thirds of the body’s energy expenditure. It varies widely among people, primarily because of body size (the greater the body mass area, the more REE the body needs) and body composition (fat versus muscle). However, some of the variability in REE (~250 kcal/day) has not yet been fully explained (Hall et al. 2012).
Interestingly, the brain, heart, kidneys and liver represent relatively small amounts of body weight but demand significant energy for life, so they contribute meaningfully to the body’s total REE (Hall et al. 2012).
The thermic effect of food, or TEF, signifies the energy expenditure associated with processing and digesting the foods we eat. Protein elicits the highest TEF, followed by carbohydrate and then fat.
Activity energy expenditure, or AEE, is the fuel the body uses for structured exercise and nonexercise movement, such as shopping, moving, doing daily chores and fidgeting. AEE may be the energy balance component that varies most widely among people, as many of us move a lot during our waking hours and get a considerable amount of daily exercise, while others live primarily sedentary lifestyles.
On average, the human body stores 130,000 kcal of fat, primarily in the form of triglycerides. According to Hall et al. (2012), a lean adult may have ~35 billion adipocytes, or fat cells, while an extremely obese individual may have 140 billion adipocytes. In addition, the fat cells in obese people store two to three times more triglycerides.
Carbohydrate is stored in the form of glycogen, which is bound to water in the liver and muscle. Changes in carbohydrate storage often result in meaningful shifts in fluid storage; the more carbohydrate is eaten and stored, the more fluid the body retains.
Fat, however, does not need to bind with water to be stored in the body, and protein needs very little water for storage. Therefore, a person eating a higher percentage of carbohydrate, but not necessarily more kilocalories, will retain more water, thus increasing his or her total body weight, owing to extra water retention.
A positive energy balance happens when we take in more kilocalories than we expend. Over time, a positive energy balance will result in overweight and/or obesity. The interactions of REE, TEF and AEE may be affected if a person is in a positive (weight gain) or negative (weight loss) kilocalorie consumption mode.
Scientists feel the body has either a “set point” or a “settling point” to regulate body weight (Hall et al. 2012). The set point theory suggests that our bodies have a highly organized feedback-control system that tries to optimally regulate food intake and energy output and maintain our weight near a certain level. Most scientists believe that leptin (a hormone secreted by fat cells) feedback to the hypothalamus in the brain has the predominant role in regulating body weight. The settling point hypothesis is very analogous to the set point theory, only it suggests that the body regulates energy intake and output with a less orderly feedback system. Indeed, the body’s mechanism for regulating energy storage is still a matter of intense investigation among scientists and may represent a combination of both theories.
Hall and colleagues note that owing to daily changes in energy intake (meals we eat) and energy output (activity and exercise), humans are regularly in a state of energy imbalance—going back and forth from positive to negative.
the feeling of fullness from a meal, results from sensory receptors in the body noting an enlargement of the gastrointestinal tract and sending a message to the brain that the stomach is full. Specialized intestinal hormones also send “feeling full” messages to the brain.
Between meals, the stomach will start producing and secreting the hormone ghrelin. Ghrelin is referred to as an orexigenic hormone, meaning it stimulates appetite. Rising secretions of grehlin create signals that tell the brain it is time to eat. As Hall et al. (2012) note, many people are highly affected by cognitive stimuli (like advertising) that encourage food intake, regardless of whether the body needs food or energy.
A major step in creating a negative energy balance to lose weight is adding exercise to a person’s daily life. However, Hall et al. (2012) note that with some people, adding extra physical activity at certain times of day leads to less activity during other times.
This scientific knowledge is extremely useful to personal trainers designing exercise programs for weight loss. Physical activity program designs need to include ways to help clients move more throughout the day in addition to doing structured exercise. Hall et al. observe that some studies show people may expend up to 500 calories daily through spontaneous movement.
Energy balance is a complex integration of several bodily mechanisms (see Figure 1). Up to now, long-sustained research on this phenomenon has not been completed. Eventually, when scientists develop new methodologies to study long-term energy balance, many of the unanswered questions about successful weight loss may be answered.