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| Controversies in Metabolism Paige Kinucan and Len Kravitz, Ph.D. Introduction Most personal trainers and fitness professionals would agree that resistance training is essential to a well-rounded exercise program for healthy active individuals. The fitness, medical and musculoskeletal benefits of resistance training are certain, but speculation exists as to whether resistance exercise can truly accelerate a slowing metabolism or provide substantial success to a weight loss intervention. As well, much misinformation also exists with diet, age and resting metabolic rate. This column addresses and attempts to clarify many of the common controversies and myths associated with metabolism. Metabolism 101: Basics and Terms Human metabolism represents the sum total of the living cells’ energy producing and energy utilizing reactions. Energy balance can be thought of as the combined processes of caloric intake, caloric storing, and caloric expenditure. Total daily energy expenditure (TDEE) is the term used to describe how much energy is used (or how many calories are burned) by an individual during a 24-hour period. TDEE is made up of three primary components: resting metabolism rate (RMR), the thermic effect of physical activity (TEPA), and the thermic effect of feeding (TEF). RMR, which accounts for 60-75% of all calorie-burning processes is the amount of energy required to keep homeostatic processes (the regulation of organ systems and body temperature) performing efficiently. The second component of energy expenditure, TEPA, accounts for 15-30% of daily calorie burn depending on an individual’s activity level, which includes structured exercise as well as non-structured activity such as shivering and fidgeting. Most recently, this non-structured movement has been coined the name NEAT, for non-exercise activity thermogenesis (Levine et al., 2005). The final component, TEF, is the energy required for the digestion, absorption, transport, metabolism and storage of consumed food. It accounts for approximately 10% of daily calorie burn. Muscles’ Key Contribution to RMR: Protein Synthesis Skeletal muscle composes up to 40% of the adult human body weight and is influenced by genetics, physical activity, nutrition, hormones, disease and trauma (Rasmussen and Phillips, 2003). In addition, muscle contains 50-75% of all proteins in the human body, making it the central tissue for amino acid metabolism. Resistance training promotes hypertrophy (a net gain in muscle mass) over an extended period of time when protein synthesis (growth) has exceeded protein breakdown. In order for muscles to develop in size, protein synthesis must exceed protein degradation (catabolism). Protein synthesis and protein breakdown account for approximately 20% of RMR (Rasmussen and Phillips, 2003). Protein synthesis is additionally stimulated by a high amino acid supply, which is regulated by anabolic hormones (growth hormone, insulin-like growth factors, and testosterone). What is the Metabolic Rate of Muscle Tissue? Although muscle is the largest tissue in the entire body, its estimated metabolic rate is much less than has been advertised in the consumer media and suggested by many ill-informed fitness product advertisers. Please note that the complex scientific estimations of energy expenditure of body tissues are derived by taking measurements of oxygen concentrations across arteriovenous cell membranes in conjunction with the measurement of blood flow (Elia 1992). In fact, scientific estimation of the metabolic rate of muscle is about 10 to 15 kcal/kg per day, which is approximately 4.5 to 7.0 kcal/lb per day (Elia, 1992). Muscle tissue contributes approximately 20% to TDEE versus 5% for fat tissue (for individuals with about 20% body fat). It is fascinating to note that the combined energy expenditure of the heart, lungs, kidneys, brain and liver represent approximately 80% of the TDEE (Elia 1992). These organs have a metabolic rate that is 15-40 times greater than their equivalent weight of muscle and 50-100 times greater than fat tissue (Elia 1992). How Much Can Resistance Exercise Programs Really Effect Metabolism? In a recent comprehensive research review, Donnelly and colleagues (2003) note that the majority of peer-reviewed resistance training studies (lasting from 8 to 52 weeks) show increases of 2.2 to 4.5 lbs of muscle mass. Therefore, the 4.5 lbs of muscle mass would increase the resting metabolic rate by about 50 kilocalories per day. Although not near as much as is promoted, this small change does help to close the “energy gap” between energy intake and energy expenditure. Therefore it is appropriate to share (but not over tout) with students and clients that more muscle creates a higher demand for energy, since muscle will need to maintain itself at rest and during exercise. Perhaps one of the most meaningful benefits of resistance training during a reduced-calorie intake intervention is that it helps to prevent the loss of fat-free mass (muscle) (Donnelly et al., 2003). What Effect Will Diet Have on RMR? All foods contain calories, which can be thought of as energy units. To lose weight, an individual must burn more calories than he or she consumes, and to gain weight, an individual must eat more calories than he or she burns. Very low calorie diets often fail because not enough calories are being consumed to fuel physical activity, and this underfeeding can diminish metabolic processes. These intense energy restrictive diets are not only tough to maintain, but actually trigger the body to suppress its RMR by as much as 20% (Hill. 2004). Biological processes adapt as if the body were in a state of famine (which was a valid threat to our ancestors), so it increases metabolic efficiency by burning less calories to do the same work than an equally matched effort would burn in an adequately-fueled individual (Benardot and Thompson, 1999). Bernadot and Thompson add that underfeeding may also interfere with the body’s ability to synthesize muscle because of a lower production of insulin-like growth factor (IGF-1) and the body’s decrease in power producing capacity. As well, eating carbohydrates provides a “protein-sparing effect”; a person who eats too few (like a diet low in carbohydrates) would need to use some protein for energy, rather than for building and repairing muscle from resistance exercise. Low-carbohydrate diets are not recommended for individuals who train to enhance muscular fitness levels (Benardot and Thompson, 1999). How Will Age Effect Resting Metabolic Rate? The metabolic rate per kilogram body weight in young children (&Mac178; 6 years) is about two times greater than that in adults (Elia, 1992). Increasing age equates to decreasing RMR because organs, which account for a large percentage of RMR, are a larger proportion of a young person’s body in comparison to an adult. Therefore, infants and children have a very high RMR because they are growing rapidly and a great part of their body weight is metabolically active tissue (heart, lungs, brain, liver, kidneys). A 25% drop in RMR between the ages of 6 and 18 is expected as more adult proportions are reached, and then an additional drop of 2-3% in RMR each decade is predictable. This downward progression of RMR in later life can be attributed to the loss of fat-free mass due to physical inactivity. Fortunately, this trend can be minimized with regularity in resistance training exercise throughout the aging cycle. You’re Invited to Take the Metabolism Quiz To conclude this article you are invited to test your knowledge on five common metabolism questions. Answer True or False? 1. Exercising first thing in the morning increases all-day energy expenditure and metabolism more than a workout later in the day would. 2. Spicy foods elevate metabolism. 3. The fitter a person is, the faster their metabolism. 4. “Yo-yo dieting” will permanently diminish your metabolism. 5. Certain medications can slow metabolism. Answers 1. False: Regardless if it is in the morning, afternoon, or evening, the same amount of calories will be expended by the body according to the intensity and duration of the workout. 2. True: Foods like peppers and chili do elevate body temperature slightly, which will increase the metabolism by small increments. Still, this elevation is not enough to make a difference in the body’s ability to expend enough calories to aid in weight loss. 3. True and False: As a person becomes fitter and gains muscle mass, it is correct that he or she burns more calories at rest. However, some exercise enthusiasts have insufficient energy intakes, which do not meet the demands of their exercise programs. This imbalance may eventually reduce resting metabolic rate. 4. False: Diets that cause people to lose weight and then put it on again repeatedly are referred to as “yo-yo” diets. Research does not indicate that this will permanently slow down any component of metabolism. 5. True: There are certain prescription drugs (like depression medication) that have been shown to lower metabolism. Biography Paige Kinucan is currently earning her bachelor’s degrees in exercise science and professional writing at the University of New Mexico in Albuquerque. She is a Regent’s Scholar and an honors student interested in endurance training and nutrition’s influence on performance. Len Kravitz, Ph.D., is the Program Coordinator of Exercise Science and Researcher at the University of University of New Mexico where he recently won the "Outstanding Teacher of the Year Award (2004). Len was honored with the 1999 Canadian Fitness Professionals “International Presenter of the Year” award and the first person to win the IDEA “Fitness Instructor of the Year” award. References Benardot, D., Thompson, W.R. “Energy from Food for Physical Activity: Enough and on Time.” ACSM’s Health and Fitness Journal. 1999, July/August; 3(4):14-18. Donnelly, J.E., Jakicic, J.M., Pronk, N., Smith, B.K., Kirk, E.P., Jacobsen, D.J., Washburn, R. “Is Resistance Training Effective for Weight Management?” Evidence-Based Preventive Medicine. 2003; 1(1): 21-29. Elia, M. “Organ and Tissue Contribution to Metabolic Weight.” Energy Metabolism: Tissue Determinants and Cellular Corollaries. Kinney, J.M., Tucker, H.N., eds. Raven Press, Ltd. 1999. New York: 61-79. Hill, A.J. “Does Dieting Make You Fat?” British Journal of Nutrition. 2004; 92(1). Levine, J.A., Lanningham-Foster, L.M., McCrady, S.K., Krizan, A.C., Olson, L.R., Kane, P.H., Jensen, M.D., Clark, M.M. “Interindividual variation in posture allocation: Possible role in human obesity. Science. 2005; 307; 584-586. Rasmussen, R.B., Phillips, S.M. “Contractile and Nutritional Regulation of Human Muscle Growth.” Exercise Sport Science Review. 2003; 31(3):127-131 |
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