|Making Sense of Calorie-burning Claims.
By Robert A. Robergs, Ph.D., and Len Kravitz, Ph.D.
If people answered honestly to the question, 'What are the reasons why you exercise?', a frequent answer would be to burn calories. In fact, according to U.S. Department of Health and Human Services (1992), 26% of U.S. adults between the age of 20 through 74 are overweight, which clearly demonstrates the impact of this national concern. When combined with the facts that reducing body fat can reverse several disease processes (eg. type II diabetes, heart disease, etc.), that exercise adds to total caloric expenditure, and that exercise also maximizes body fat loss and the maintenance or increase of muscle mass, participation in exercise is a very consequential and rewarding strategy to lose body fat and improve your health.
The suitability of exercise as a means to burn calories has been recognized by the fitness industry. There are many types of exercise modalities that are marketed with the claim of 'burning more calories,' and the consumer is left to wonder just what it is that determines the number of calories burned during exercise. This situation is the fundamental reason for writing this article.
You the fitness professional should be aware of what determines how many calories your body burns during exercise, why your body obeys certain rules that dictate the magnitude of caloric expenditure, and what are the best types of exercises that increase caloric expenditure. With this knowledge you can effectively educate your clients to more realistic goals that may be accomplished with your exercise prescription. In addition, you can better explain to your clients the truth about many of the advertising claims that suggest a particular exercise modality is best for caloric expenditure and weight loss.
We will begin with a brief discussion on the relationship between aerobic exercise, caloric expenditure, and exercise intensity. We will then present data from a laboratory case study we conducted to compare cardiorespiratory responses and caloric expenditure during cycle ergometry, arm ergometry, and combined leg and arm ergometry. The results presented will be combined with the published research on this topic to clearly illustrate the interrelation between exercise intensity, lower and upper body exercise, and caloric expenditure.
What determines caloric expenditure during exercise?
At rest, your body expends energy to maintain the functions of cells that are essential for life. The continual pumping of blood by the heart demands energy, as does the continual ventilation (movement of air into and out) of the lungs. In addition, maintaining a life supporting environment within and around cells requires a constant breakdown of certain energy releasing molecules. This energy is also used to form the molecules necessary for repairing cells, storing energy (glycogen and triglycerides), fighting infection, and processing nutrients obtained from digestion. These energy demanding functions combine to form the body's basal metabolic rate, which can vary from approximately 800 to 1500 Kcals depending upon body size and total caloric intake (ingested quantity of food).
Adenosine triphosphate (ATP) is the main molecule the body uses as a means to use chemical energy to perform cellular work. Exercise adds to the caloric expenditure of the body, as muscle contraction involves the need to repeatedly form and breakdown ATP. The energy released from the breakdown of ATP fuels the contraction of skeletal muscle, thereby adding to the energy demands of the body and raising caloric expenditure. Research has shown that during exercise the increase in caloric expenditure is almost entirely due to the contraction of skeletal muscle; the balance is due to an increase in the energy demands of the heart and the muscles used during ventilation.
How is caloric expenditure measured?
Caloric expenditure can be measured directly, which requires the measurement of the heat released by the body, or indirectly be measuring ventilation and the exchange of oxygen and carbon dioxide by the body. These methods are termed direct calorimetry and indirect calorimetry, respectively, and the research and validation of these methods date back to the late 1890's (Lusk, 1928). For numerous methodological reasons, the method of indirect calorimetry is the most suitable and accurate to evaluate caloric expenditure during exercise.
When a person expends calories, the body uses oxygen and produces carbon dioxide. Not all bodily reactions consume oxygen and produce carbon dioxide, but without the reactions that do, the remaining reactions of the cells would eventually stop, and the cells would die. This fact is important, as it means that quantifying the consumption of oxygen and production of carbon dioxide is an indirect means to measure the calories that are released and used by the reactions of the body. All that we need to know is the relationship between oxygen consumption and caloric expenditure. Fortunately, scientists who studied calorimetry during the first two decades of the 19th century determined this relationship for us (Lusk, 1928).
The number of calories released from the consumption of oxygen during cellular metabolism differs slightly when carbohydrate, fat, or protein are the nutrient source. However, as the body predominantly uses carbohydrate and fat as the nutrient sources, or "energy substrates," we only need to focus on these substrates. The energy released from carbohydrate and fat within the body approximates 4.0 and 9.0 Kcal/gm of substrate. Thus, fat is a more dense source of energy than carbohydrate. However, remember that we are not concerned with the amount of energy available from a given amount of energy substrate, but with how much energy is available relative to oxygen consumption. For pure carbohydrate and fat catabolism (breakdown), these caloric amounts are actually 5.05 and 4.73 Kcal/Liter O2, respectively (Table 1). Therefore, the difference in caloric expenditure between pure carbohydrate and fat catabolism, of an average healthy person exercising for 30 min at a VO2 (oxygen consumption) of 1.5 L/min, would amount to 14.4 Kcals (227.25 Kcals carbohydrates to 212.85 Kcals fat). This is a small difference, but indicates that for accurate calculations of caloric expenditure during exercise, there is a need to know how much carbohydrate and fat are being used as energy substrates.
The contribution of carbohydrate and fat to energy metabolism (the process of chemical changes to provide energy) can be determined from the ratio between carbon dioxide production and oxygen consumption. This is referred to as the RER, or respiratory exchange ratio of carbon dioxide to oxygen consumption. The metabolic basis for this relationship lies in that there is greater carbon dioxide production from carbohydrate catabolism compared to fat catabolism. Thus, the lower the carbon dioxide production relative to oxygen consumption, the greater the contribution of fat catabolism to caloric expenditure (Table 1).
We are now armed with the academic knowledge needed to understand exercise intensity. In this article, our focus is exercise intensity and caloric expenditure. We will not elaborate on how one can maximize either carbohydrate or fat catabolism during exercise. We will save that topic for another article in another issue.
What are valid methods for estimating exercise intensity ?
From the information thus far presented, it should be clear that the best measure of the change in metabolism during exercise is oxygen consumption. As one completes the transition from rest to exercise, there is often an exponential increase to a plateau in oxygen consumption until a steady rate is attained, termed steady state (Figure 1). For low intensity exercise, steady state is attained in approximately 3 min. However, if the intensity is too high, or the duration of exercise at this intensity is too short, steady state is not attained. Research has shown that steady state VO2 increases in a linear manner with increases in the work or power performed during exercise (McArdle et al., 1991).
Much research has been completed using cycling as the exercise mode. This is because a measure of work and power is easily obtained from cycling using suitable stationary cycles that allow quantification of work and power. These special cycles are termed cycle ergometers, and ergometers such as these are also made for arm exercise, and are termed arm ergometers. The ability to quantify power or work while exercising is important, as it enables a scientifically valid way of changing the intensity of exercise, and therefore, to evaluate how the function of the body changes during known changes in intensity. It is because of this scientific precision that we used an arm and leg ergometer for the case study experiment presented in this article (See photos 1 & 2).
If we want to measure the change in physiological variables (such as heart rate, carbon dioxide production, ventilation, blood pressure, etc.) during exercise, a useful way to compare them to exercise intensity is to graph the response relative to oxygen consumption. This is especially important for exercises that do not have a means to quantify work or power, such as walking or running on level ground, aerobics, stair climbing, etc. Throughout a large portion of aerobic exercise, a linear relationship exists between heart rate intensity of the exercise and the oxygen consumption (and therefore caloric expenditure). Therefore if the heart rate is known, oxygen consumption can often be reasonably estimated. Although this technique is practical, other factors such as environmental temperature, body position, food intake, muscle groups exercised, and the nature of the exercise (continuous vs. stop and go) can all influence the heart rate. For instance, heart rates in aerobic dance appear to be higher than comparable oxygen consumption values on a treadmill due to the vigorous involvement of the arms. The rating of perceived exertion (or RPE) is also a measure of self perceived exertion during exercise which has also been shown to be a useful marker of exercise intensity (Table 2).
The evaluation of exercise intensity by RPE is important in your classes, where often times there are no heart rate monitors, or gas analyzers and computers beside you and your students to monitor exercise exertion.
Will your fitness level effect the number of calories you burn?
Yes, as you do endurance training your body adapts in many physiological mechanisms. One positive adaptation is a lower submaximal heart rate intensity during your aerobic workouts at a given oxygen consumption. Fit individuals will often challenge themselves by exercising harder, elevating their heart rate intensity and thus burn more calories because they are also then elevating their submaximal oxygen consumption.
Can certain types of exercise burn more calories than others?
Based on the fundamental principles of indirect calorimetry, to burn more calories during exercise you need to increase oxygen consumption. The issue of exercise and caloric expenditure is as simple as that. Nevertheless, many people have been told, or developed an understanding that caloric expenditure can be different for certain types of exercise. You may recall certain advertising slogans that claim, "This exercise will burn more calories than running, or cycling alone," etc. The majority of these claims are based on how certain exercises use more muscle, and therefore will increase oxygen consumption and burn more calories. It sounds attractive doesn't it? If I use more muscle, I will use more oxygen and burn more calories! However, lets apply some straight forward scientific logic to the results of published research findings, and see how certain exercises stack up to the issue of caloric expenditure.
The increase in oxygen consumption when arm ergometry is added to cycle ergometry is illustrated in Figure 2. This exercise combination increases the muscle mass exercised. This data appears to support the rationale that increasing muscle mass increases oxygen and energy expenditure. However, as they say in the classics, " not all is as it seems!"
Figure 3 presents data of the increase in heart rate relative to oxygen consumption for our single case subject experiment (with a healthy, physically active female) while performing either maximal exercise tests in cycling, cycling and arm ergometry combined, or just arm ergometry. The subject attained the highest or peak VO2 from cycling, and for any given submaximal oxygen consumption the heart rate response was highest for arm ergometry, and lowest for cycling alone. The higher heart rates also corresponded to higher perceptions of exertion (RPE) for a given oxygen consumption during arm and combined arm and leg ergometry. In other words, it is more difficult to exercise when arm exercise is combined with lower body exercise (Toner et al., 1990). Therefore, adding the arms to the exercise made the exercise feel harder due to increase in upper body muscle mass, but the actual oxygen consumption for a given heart rate was less than the legs only work.
A better comparison of the body's responses to different types of exercise occur when the exercises are compared at the same intensity. After all, it is obvious that adding arm ergometry will increase oxygen consumption, as it adds additional exercise, which increases intensity. In addition, we know that an increase in intensity will increase oxygen consumption and energy expenditure. However, if the addition of arm exercise makes the exercise too intense, then the completion of lower body exercise alone will enable exercise to be performed for longer at a given exercise intensity, and therefore burn more total calories (Loftin et al., 1988). In fact, this is what was illustrated in Figure 3. For a given heart rate, there is a higher oxygen consumption during cycle ergometry, than for arm or combined arm and cycle ergometry. This fact is made more clear in Figure 4.
Why is oxygen consumption and caloric expenditure lower during exercise involving the upper body?
Exercise involving the upper body musculature is generally complicated by the relatively small muscle mass. The smaller muscle mass is less effective than a large muscle mass in inducing the return of blood flow to the heart, reducing the volume of blood pumped by the heart each beat, and therefore causing an increased heart rate. In addition, for a given intensity, contraction of the upper body musculature provides greater resistance to blood flow than for lower body exercise, resulting in a greater increase in blood pressure. These factors result in a relatively lower maximal ability to consume oxygen during arm ergometry.
As exercise intensity increases, the body must attempt to provide increased blood flow to the contracting muscle. Normally for lower body exercise, this is tolerated. However, the addition of upper body exercise to lower body exercise can provide a demand that exceeds the body's ability to distribute and pump blood to the working muscle. In short, blood can't be pumped fast enough to adequately perfuse (spread through) both the lower body and upper body musculature. This reasoning explains the lower maximal oxygen consumption for combined arm and leg ergometry compared to leg ergometry alone, even though maximal heart rate is identical for the two tests. This explanation is illustrated in Figure 5, where the extraction of oxygen multiplied by the blood flow to skeletal muscle is larger for leg exercise alone, than the combined oxygen extraction and blood flow to the upper body and leg muscles combined. However, for individuals who are genetically gifted with a large heart, large blood volume, and muscles that can use oxygen at higher rates, the cardiovascular limitations of combined upper and lower body exercise are less severe. This fact explains the known large values for VO2 max reported in the literature for elite cross-country skiers and rowers.
So, is lower body exercise better than combined upper/lower body exercise?
Better for what? The goal of this article was to explain the relationship between exercise and caloric expenditure during upper body, lower body and combined upper/lower body exercise using physiological research and a case study demonstration. If life-time commitment to physical activity for health-related fitness is the goal for yourself and your students, our recommendation is to choose an aerobic activity (lower body only or upper/lower body) that you enjoy and will be able to adhere to. A secondary goal of this article was to help you as professionals explain to your clients why certain advertising claims may be misleading or untrue. Hopefully we have provided the information for you to do that now. It should be mentioned that the long-term physiological benefits of regular upper/lower body exercise have not been fully elucidated in research findings. Therefore, it's not a matter of one type of exercise being better than another. Rather, it is the clarification of the body's distinctive adaptations to the imposed demands of these different exercise regimes that's important.
Recommendations and Applications
From this article we would like to offer the following recommendations and applications:
1. Although equipment accessibility and availability is always a consideration, when helping clients to choose aerobic modalities attempt to find exercise activities that they enjoy and will want to continue, regardless of the lower body vs. upper/lower body issue.
2. Many individuals find upper/lower body exercise more challenging, thereby meeting an important criteria for their personal workout demands. The muscular fitness benefits of upper/lower body exercise have yet to be fully realized.
3. Because of the complex physiological demands of the body, at a given heart rate intensity, lower body only exercise will result in slightly greater caloric expenditure when compared to upper/lower body exercise.
4. When upper body exercise is combined with lower body exercise, the increased heart rate and RPE response does not necessarily reflect a significant increase in caloric expenditure.
5. With the number of well-equipped university, hospital and private sector physiology laboratories available throughout the nation, advertisers should be able to support their claims with valid, independent research. As health and fitness professionals, and potential owners of their products, you have every right to ask them to share this data with you.
Borg G.A.V. Psycholphysical bases of perceived exertion. Med. Sci. Sports Exerc. 14:377-381, 1982.
Lofton M., Bioleau, R.A., Massey, B.H., & Lohman, T.G. Effect of arm training on central and peripheral circulatory function. Med. Sci. Sports Exerc. 20:136-141, 1988.
Lusk, G. The elements of the science of nutrition. 4th ed, W.B. Saunders Company, Philadelphia, 1928.
McArdle, W.D., Katch, F.I., & Katch, V.L. Exercise physiology: Energy, nutrition, and human performance. 3rd ed, Lea & Febiger, Philadelphia, 1991.
Toner, M.M., Glickman, E.L., & McArdle, W.D. Cardiovascular adjustments to exercise distributed between the upper and lower body. Med. Sci. Sports. Exerc. 22:773-778, 1990.
U.S. Department of Health and Human Services. Health United States 1992 and Healthy People 2000 Review (DHHS [PHS] Publication No. 93-1232). Washington, DC: U.S. Government Printing Office, 1992.