Energy output

Quantifying this side of the equation is far more complex, and involves several important influencing factors: basal metabolic rate (BMR), total physical activity, thermic effect of food and what's known as facultative thermogenesis. Individually, and especially collectively, these factors account for considerable variance in total energy output among individuals and, therefore, influence why certain people tend to stay lean, while others are prone to being over-fat. Ultimately, however, whether someone is lean, obese or somewhere in between, their body weight will abide by the energy balance principle described at the outset.

Basal metabolic rate (BMR). BMR, by definition, represents the energy expended strictly to maintain basic metabolic functions -- that is the absolute minimum energy output associated with keeping us alive, such as breathing, circulation and normal cellular buildup (anabolic) and breakdown (catabolic) processes.

BMR is influenced by age (it starts to slow down after our mid-20s) and amount of lean body mass, especially muscle (metabolically active tissue). Surprisingly, though, BMR does not tend to vary much at any given age and lean body mass.1, 2 However, even a relatively small variance of 100 to 200 kcals per day will have a significant influence on the long-term ability to either maintain a healthy energy balance or successfully create an energy deficit.

Of course, the greater the lean body mass, the greater the BMR. Total lean body mass, in turn, is determined by genetics, sex, age, and amount and type of physical activity. BMR in men is usually higher than in women, primarily because men typically have a greater amount of lean tissue.

Even though the standard deviation for BMR is relatively small, individuals can vary significantly, in either direction, from the mean. Obviously, if one's BMR is well below average (often as a result of low thyroid activity), he/she will be more likely to either be obese or struggle to stay within normal body composition standards. As health and fitness professionals, we need to emphasize to our clients the reality that even though it's entirely possible that their BMR may be naturally sluggish, this situation is the exception to the rule, and there are several concrete ways in which we can increase our energy expenditure on a regular basis.





Figure 1.

Components of energy intake and expenditures in lean and obese individuals.
Energy intake provides the metabolizable energy with appreciable fecal and urine losses. (Adapted from W.P.T. James & P. Trayburn, 1981.)

Aerobic exercise. Ironically, while physical activity is the easiest contributing factor to energy expenditure to conceptualize, it is difficult to consistently quantify with accuracy. As illustrated in Figure 1, we seem to overestimate our total physical activity level. One reason for this, unfortunately, is that many people in the fitness industry tend to portray formal aerobic exercise sessions as greater sources of energy expenditure than they actually are for the average person. This is especially important when working with people just beginning an exercise program who have a specific need and desire to lose weight.

As an example, consider the person who is told that by walking for 20 to 30 minutes three times per week at a light to moderate intensity, he/she will burn enough kcals to effectively lose 20 to 30 pounds in as little as 10 to 15 weeks (two-pounds-per-week rate of weight loss). Even if this person makes a sensible reduction in food intake (say 300 to 400 fewer calories per day), this weight loss simply will not happen within this time frame if all other variables are held constant. The reason is simple: The total extra energy expenditure from this exercise program will amount to no more than approximately 360 to 480 calories per week.

This total is derived by multiplying the approximate minute rate of energy expenditure (kcals/min), in this case five to seven, by the number of minutes exercised, in this case 20 to 30, by the number of sessions per week, in this case three (see "Calculation of energy expenditure"). The resultant best-case scenario, then, becomes a total energy deficit of approximately 3,300 calories per week (400 kcals fewer per day x 7, plus 480 extra calories expended), or less than one pound of fat loss (one pound of fat = approximately 3,500 kcals).

The above example is not intended to suggest that this aerobic exercise program is not beneficial for your client. In fact, for people beginning an exercise program, light- to moderate-intensity exercise several days per week is generally the best recommendation in terms of reducing the risk of cardiopulmonary and musculoskeletal complications, as well as for facilitating long-term adherence to a physically active lifestyle.

What the above aerobic exercise program does not do, however, is burn a tremendous number of kcals. It is our responsibility as health and fitness professionals to carefully and thoroughly convey this important point to our clients, so as not to mistakenly suggest that their exercise program is contributing more to their energy expenditure/weight loss effort than is actually occurring. Large reductions in storage body fat -- liberating large amounts of energy from our reservoir -- requires time, typically measured in months, if not years, depending on the magnitude of fat loss. We are not physiologically designed to safely and effectively lose body fat rapidly.

Of course, as a person progresses with their aerobic exercise program, they will be able to expend more energy both during their workouts and for an extended period after the exercise session has ended, as the individual gradually returns to a pre-exercise metabolic state. As a result, an individual who makes gradual but steady increases in the frequency, duration and intensity of their aerobic exercise sessions will become more successful in achieving greater energy expenditure and, therefore, will make a significant improvement in their body composition.

Anaerobic exercise. The traditional message conveyed to our clients has been that weight training is done to improve muscular strength and endurance, with little or no mention of the activity's contribution to energy expenditure. "Lift weights to get stronger, do aerobic exercise to burn calories" has long been a common prescription.

The implication that weight training somehow does not make a significant contribution to increasing energy expenditure is misleading and unfortunate. In fact, many people attempting to improve their body composition tend to shy away from weight training so as not to add weight onto the scale. While a quality weight-training program will most likely result in at least a modest increase in lean tissue, body fat is typically reduced due to the significant increase in energy expenditure-calories burned -- both during the exercise and on a chronic basis as a result of enhancing the metabolic activity of existing and new lean tissue.

Consequently, weight training, as well as all anaerobic physical activity, definitely increases energy expenditure and, therefore, favorably influences an individual's ability to maintain a desirable energy balance, or create an energy deficit to improve body composition.

Activities of daily living (ADL). The other category of physical activity that needs to be identified and modified, if necessary, is total informal or incidental daily activity, also referred to as activities of daily living (ADL). As important as formal bouts of both aerobic and anaerobic exercise are for expending energy, the total number of kcals burned during daily activity: 1) varies tremendously among individuals and 2) has the potential to add significantly to our total daily energy expenditure.

Activities of daily living are often influenced heavily by habits that tend to categorize people based upon relative physical movement. While some people consistently avoid any extraneous exertion, others seem to be constantly on the move. If you observe most real-life situations, this behavior pattern is usually evident. For those who are always moving -- fidgeting is a term often used -- their total daily energy expenditure is increased proportionate to how much they engage in spontaneous movement. Even if this extra expenditure is relatively small, say for example 100 kcals per day, it would obviously provide a long-term benefit to either maintaining a desirable energy balance or contributing to an energy deficit.

Thermogenesis

Specific dynamic action (SDA). Specific dynamic action represents the heat production (thermogenesis) and, therefore, energy expended in the process of digesting and assimilating our food. At least three important factors determine not only the quantity, but also the quality of our SDA: the composition of the food, the amount of food intake at one time and the frequency of our eating.

In terms of composition, the three foodstuffs -- protein, carbohydrate and fat -- elicit varying degrees of thermogenesis. Protein has the highest SDA, followed fairly closely by carbohydrate and, finally, by fat with a significantly lower value. This is another reason why a high-fat diet is unfavorable. The body does not expend very much energy, especially relative to the other foodstuffs, in the digestive processing of fat.

The amount of food eaten at one time also tends to affect SDA. Generally speaking, when the quantity of food ingested at one sitting exceeds a certain threshold, thermogenesis will be suppressed to some extent, especially if it contains moderate to high fat levels. This threshold varies from one person to the next and is dependent upon several factors, in particular those that determine total energy needs (e.g., body mass, age, physical activity levels).

Frequency of food intake typically is strongly inversely related to the size of our meals and snacks -- the more often we eat, the fewer number of calories we tend to consume per meal. Again, the more frequently we eat, the more energy we will expend -- the greater the thermogenesis -- in digesting our food. The major caveat to this habit, however, is that while most registered dietitians and nutritionists recommend eating several -- usually four to six -- meals/snacks per day, this strategy will only be healthful and helpful if the composition (quality) and total caloric intake (quantity) is appropriate.

Facultative thermogenesis. Finally, after having accounted for all tangible variables that appear to influence energy intake and output, we are faced with an observational paradox: How can any two (or more) people's body composition vary so much when seemingly everything that would affect their energy balance relationship are so similar? This puzzling situation strongly suggests there must be at least one other important factor. This other apparent factor, termed facultative thermogenesis,2 refers to heat production (energy expenditure) not accounted for by BMR, the specific dynamic action of food or physical activity. When food is ingested, the calories not required for metabolism are either stored as fat or dissipated as heat. Apparently, we tend to vary significantly in our efficiency for utilizing our energy intake.

In this context, efficiency is generally regarded as a detriment, since those who are more efficient at storing unneeded energy do so in adipose tissue, whereas those who are inefficient tend to lose a significant amount of heat energy and are, therefore, less prone to storing excess body fat (see Figure 1). Regardless of the exact mechanism, for those who are apparently efficient at storing body fat, successful lifestyle modification of energy intake and expenditure becomes all the more important.

Guidelines/recommendations to our clients

The following guidelines and recommendations should be emphasized and thoroughly explained during a consultation encompassing fitness assessment results, exercise program and/or dietary concerns:

1. General conceptual understanding of energy balance principle

* Clearly explain energy balance using the three possible outcomes.

* Quantify the amount -- approximately 3,500 calories -- required to either gain or lose a pound of storage body fat.

* Stress the achievement of a sensible energy deficit of no more than approximately 500 to 1,000 kcals per day (one to two pounds per week) through a combination of sensible adjustments in energy intake -- fewer of the wrong foods (especially excess fat and simple sugars) and more of the right foods (increase complex carbohydrates and fiber when existing levels are too low) -- and moderate increase in energy expenditure through both formal and informal physical activity (see below).

2. Energy intake factors

* Emphasize the tendency to underestimate total caloric intake.

* Conduct a thorough body composition assessment, including: 1) A reliable estimated measurement of current percent body fat level with an accepted methodology (e.g., hydrostatic weighing, skin-fold measurements) conducted by highly trained personnel (e.g., ACSM-certified exercise test technologists or exercise specialists) followed by a thorough counseling session. 2) A waist-to-hip-ratio measurement and calculation of body mass index (BMI), followed by appropriate counseling on norms and risk factor awareness and modification relevant to obesity.

* Strongly suggest a well-designed dietary inventory evaluation to closely quantify typical caloric intake.

* As a natural follow-up, strongly suggest meeting with a registered dietitian or nutritionist. If your clients are serious about body composition improvement (i.e., weight reduction), they must have a strong working knowledge of quantifying and qualifying their food intake.

* Make it clear that severe caloric restriction, short-term and especially long-term, is absolutely the worst choice to make.

* Suggest that your clients have a valid laboratory assessment of their BMR, especially if their typical energy intake and energy expenditure patterns suggest they should not be having difficulty maintaining a healthy energy balance/body composition.

* Instruct your clients to weigh themselves once per week only, on the same scale, at the same time of day, with little or no clothing. Far too many people weigh themselves too often and tend to become easily discouraged with any day-to-day stagnation or increase in weight that is predominantly caused by fluid level changes.

* Refer your clients, with their consent, for professional psychological/behavioral counseling if you feel this will help them improve eating and/or physical activity behavior.

3. Energy output factors

* Be certain your clients understand that beginning and/or low-level aerobic exercise programs do not burn as many kcals as most people have been lead to believe. Illustrate this point mathematically (see "Calculation of energy expenditure") to drive home this point. Within the context of this discussion, however, also emphasize how beneficial low-level aerobic exercise programs are compared to a sedentary lifestyle. Reassure clients that as they gradually progress with their program, they will steadily increase their energy expenditure during and after their exercise sessions.

* Educate your clients on the metabolic benefits of anaerobic training.

* Encourage your clients to be active throughout the day. Promote the importance of informal exercise, such as taking a flight of stairs versus the elevator, parking farther from the store entrance and getting up from the couch, as a means by which to increase their total energy expenditure.

Above all, help your clients keep everything in perspective. We are all different. Some people will always tend to struggle with attaining or maintaining a healthy body composition, while others seem to be able to stay lean with little or no effort. Remember, the ultimate goal is to make sensible, steady improvement in body composition that can realistically be maintained for the rest of a healthier life. Understanding and applying the principle of energy balance will greatly facilitate this endeavor.



REFERENCES

1. Brooks, G., & T. Fahey. Exercise Physiology, Human Bioenergetics and its Applications. NY: Macmillan Press, 1985, pp. 528-534.

2. James, W.P.T., & P. Trayburn. Thermogenesis and obesity. British Medical Bulletin 37: 43-48, 1981.

3. Lichtman, et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. New England Journal of Medicine 327: 1893-1898, 1992.

Jim Merrill, Ph.D., is an exercise specialist for the Cardiovascular Care Center, Presbyterian Intercommunity Hospital, Whittier, Calif.



Calculation of energy expenditure during aerobic exercise

A person of fair to average fitness level will have a maximal work capacity of approximately 8 to 12 METS (VO2max of 28 to 42 ml/kg/min) and will consequently only be able to sustain a comfortable exercise intensity-talk test of approximately 5 to 7 METS. Given that 1 MET represents approximately 1 kcal/min of energy expenditure (proportionately more for heavier people), even 30 minutes of 7 MET intensity exercise will total approximately 150 to 210 kcals (5 x 30 to 7 x 30) expended minus the number of calories that would have been burned at 1 MET (rest) during those 30 minutes -- approximately 30 -- yielding a net increase in energy expenditure of approximately 120 to 180 kcals for the exercise session.

It is important to note that at this moderate exercise intensity, excess post-exercise energy expenditure is quite limited and, therefore, does not contribute significantly to total energy expenditure.

Medical experts now see obesity as a chronic condition that is remarkably resistant to treatment. Obesity rates in American adults and children continue to climb, with no reversal of this trend in sight.