How Metabolism Affects Weigh Control
Elevate your resting metabolic rate with exercise and you will be on your way to losing weight.
Compared with any other time in history, people are obsessed with their weight. They feel either too fat or too bony; too apple-shaped or too pear-shaped. In addition, they are constantly bombarded with images of what is appealing: being lean, muscular and proportional.
Many people, though, have a good reason to be concerned about their weight. More than half of U.S. adults age 20 and older are now considered overweight, and nearly one-quarter are clinically obese. There is also the increased risks for heart disease, stroke, diabetes and cancer that are associated with being overweight.
There is hope, however: It lies in controlling your metabolism. By doing so you csn avoid the many risks that are associated with being overweight.
Energy balance and thermodynamics
Human metabolism represents the sum total of all physiological and chemical processes that take place within the body. These processes require energy and are expressed in energy units commonly called calories. There are essentially two factors that determine the metabolic rate of an individual at any given time: the number and size of respiring cells comprising the tissues of the body, and the intensity of metabolism of these cells.6 Together these two factors lay the physiological foundation for the amount of energy (calories) the body uses.
The First Law of Thermodynamics, sometimes called the Law of Conservation of Energy, states that energy can neither be created nor destroyed, only changed in form. The energy balance equation (the mathematical summation of the energy intake and energy expenditure from all sources) quantifies the Law of Conservation of Energy. It describes the source of potential energy as food ingested, and the various uses of that energy.
The three components of energy balance that need to be considered when discussing weight control are caloric intake, caloric stores and caloric expenditure. Theoretically, if the amount of caloric intake equals the amount of caloric expenditure, caloric balance is reached and body weight remains stable. However, when the caloric balance becomes positive (i.e., more food is consumed than calories expended), the energy is neither destroyed nor lost; rather, excess calories are stored as fat. And, even while eating a low-fat diet, it is possible to gain fat weight. Why? Because most of the dietary fat is stored as the body preferentially burns carbs and proteins to meet its energy needs.6
Once weight is gained, the increased fat remains as stored energy until there is a period of negative caloric balance. For a negative energy balance (i.e., energy deficit) to exist and body weight to be reduced, caloric expenditure needs to exceed caloric intake. Regardless of macronutrient content (e.g., high-carb, low-fat diet vs. low carb, high-fat diet), weight loss will only occur if the diet produces an energy deficit, not because of the purported optimal ratios of macronutrients.3 While reductions in caloric intake (i.e., eating less food and reducing fat in the diet) have been shown to reduce body weight and body fat percent, the remainder of this article will focus specifically on the caloric-expenditure side of the energy balance equation for promoting and maintaining weight loss.
Components of energy expenditure
Energy expenditure can be divided into four components: thermal effect of feeding (TEF), thermic effect of activity (TEA), adaptive thermo-genesis (AT) and resting metabolic rate (RMR).4
Thermal effect of feeding. The TEF can be largely accounted for by the energy required for the digestion, absorption, transport, metabolism and storage of ingested food. In fact, the magnitude of TEF is regulated by the caloric content and composition of the meal, and the nutritional state and antecedent diet of the individual. On average, during a mixed diet (i.e., 65 percent carbohydrate, 15 percent protein and 20 percent fat), the TEF accounts for a daily energy expenditure of approximately 10 percent of caloric intake.6 For people who consume large amounts of protein, the TEF may account for up to 20 percent of caloric intake. And the metabolic rate of a high-fat diet actually lowers the TEF.
Regarding training state, it has been reported that TEF is reduced in endurance-trained compared to untrained individuals.5 This suggests that high levels of endurance training may promote an economy of energy use of ingested macronutrients.
Thermal effect of activity. The TEA component of energy expenditure is the most variable, accounting anywhere from 15 to 30 percent of total caloric requirements. The TEA includes all activities above resting levels. These may include what scientists label “spontaneous physical activity,” such as outdoor yard work, walking up a flight of stairs, grooming yourself throughout the day and exercise (purposeful, structured activity) such as weightlifting, running and inline skating. Few, if any, factors appear to affect the TEA, except the amount of work completed. In other words, the harder and longer people move, the more calories they burn. Activities that use the large muscle groups of the body and that are performed continuously with greater intensities expend the greatest amount of calories.
Aerobic exercises such as running, cycling and stair climbing burn many more calories during the exercise bout when compared to resistance training. A typical 30-minute aerobic exercise session may burn between 300 to 500 calories. For the same period of time, a traditional weightlifting workout may only burn about half that. However, research has shown that caloric expenditure may be higher after resistance training when compared to aerobic exercise.1
Adaptive thermo-genesis. Another component of energy expenditure, ATis still not completely understood. Adaptive thermogenesis is manifested predominantly as a change in metabolism as the result of adaptations to environmental stresses. Examples include adaptations to changes in temperature (e.g., non-shivering thermogenesis), emotional stress (e.g., increased thyroid production) and food intake (e.g., changes in catecholamines, insulin and protein synthesis, and degradation).4 AT appears to account for no more than about 10 to 15 percent of total daily energy expenditure.
Resting metabolic rate. RMR represents the largest portion of total energy expenditure (approximately 60 to 75 percent). It includes the costs of maintaining homeostasis of the systems of the body (e.g., digestive, cardiovascular and endocrine systems) and body temperature, and constitutes the energy required to maintain electrolyte gradients, central nervous system function and other chemical reactions at rest. Simply put, it represents the amount of calories the body requires to simply exist.
A closer look at RMR
Shedding unwanted body fat is not as difficult as you may think. To lose weight, you need to create a caloric deficit. This can be achieved by elevating their RMR, since the higher your RMR, the more calories they will burn at rest. But how is this done?
It is still not known what regulates RMR; however, some researchers have linked it with thyroid hormones,4 and more recently, catecholamine stimulation of B-adrenergic receptors.10 Even so, there are multitudes of determinants, both genetic and environmental, which may contribute to the variability in RMR among individuals. Resting metabolic rate has been shown to be influenced by factors such as fat-free mass, age, gender, drugs, diet and genetics.
Fat-free mass. Fat-free mass (everything but fat) has been shown to be the main determinant of RMR. During resting conditions, tissues such as the heart, liver, kidneys and other internal organs are more metabolically active than skeletal muscle, while muscle tissue is more metabolically active than fat. Changes in the proportion of these tissues in the body will, therefore, have an effect on RMR. Many studies have concluded that resistance-trained individuals have an elevated RMR of approximately 15 percent higher compared to sedentary individuals.1,6,7 In fact, it has been estimated that for every 1 pound of muscle people add, they may burn anywhere from 50 to90 more calories at rest.1 So, if you were to add 10 pounds of muscle in 10 weeks, your RMR would be elevated to somewhere between 500 to 900 extra calories per day.
Age. Data from both longitudinal and cross-sectional studies indicate that RMR decreases with age. Because infants have a large proportion of metabolically active tissue, and are growing rapidly, their RMR is quite high. However, as full growth and maturation are achieved, RMR declines. The drop in RMR from ages six to 18 is approximately 25 percent, or 2 percent per year. After this age, the decline in RMR slows to about 2 to3 percent per decade, and is thought to be due largely to the loss of fat-free mass that accompanies aging.5 This loss of metabolically active muscle tissue is attributed partially to physical inactivity, and can be stopped and even reversed if older adults engage in a regular exercise program that includes both resistance training and aerobic exercise.
Gender. It is well-established that men have a larger volume of skeletal muscle mass than women. Since greater muscle mass has been associated with a higher RMR, on average, men will have a greater absolute RMR than women. However, when RMR is adjusted per pound of muscle, the variations in RMR between men and women are significantly reduced.7,10
Drugs. Two commonly consumed drugs that increase RMR levels are caffeine (a natural stimulant) and nicotine. This may be one reason why some individuals who quit smoking gain weight.
Diet. The thermal effect of feeding (TEF) increases RMR around 6 to16 percent for perhaps as long as eight to10 hours after eating. Other possible fluctuations in RMR may occur from overfeeding, underfeeding and weight cycling. Some researchers have found that, by overfeeding subjects during a 24-hour period, the metabolic intensity of the tissue may be enhanced for up to 14 hours, and thereby reflected by increases in RMR.6 The opposite is true for underfeeding. That is, the metabolic intensity of the tissues becomes depressed. However, unlike overfeeding, underfeeding appears to require several days before decreases are observed in RMR.9 It has also been theorized that weight cycling (repeated bouts of weight loss and regain) slows down RMR.
Genetics. After accounting for the influences of gender, age and body composition, there is evidence of a significant genetic component (perhaps up to 40 percent) in determining individual variations in RMR. Aside from where you may fall genetically, there may be ways to “rev up” your metabolism (see Seven Tips for Losing Weight).
1. Dolezal, B.A., andJ.A. Potteiger. Concurrent resistance and endurance training influence basal metabolic rate in non-dieting individuals. Journal of Applied Physiology 85(2): 695-700, 1998.
2. Dolezal, B.A., L.A.Potteiger, D.J.Jacobsen and S.H. Benedict. Muscle damage and resting metabolic rate after acute resistance exercise with an eccentric overload. Medicine and Science in Sports and Exercise (in press), 1999.
3. Gillette, C.A., R.C. Bullough and C.L. Melby. Postexercise energy expenditure in response to acute aerobic or resistive exercise. International Journal of Sports Nutrition 4(4):347-360, 1994.
4. Horton, E.S. Introduction and overview of the assessment and regulation of energy balance in
humans. American Journal of Clinical Nutrition 38:972-977, 1983.
5. Horton, T.J., and C.A. Geissler. Effect of habitual exercise on daily energy expenditure and metabolic rate during standardized activity. American Journal of Clinical Nutrition 59:13-19, 1994.
6. Mole, P.A. Impact of energy intake and exercise on resting metabolic rate. Sports Medicine 10:72-87, 1990.
7. Poehlman, E.T. A review: Exercise and its influence on resting energy metabolism in man. Medicine and Science in Sports and Exercise21(5):515-525, 1989.
8. Sale, J.M., L.J. McCarger, S.M.Crawford and J.E. Taunton. Effects of exercise modality on metabolic rate and body composition. Clinical Journal of Sports Medicine 5(2):100-107, 1995.
9. Sjodin, A.M., A.H.Forslund, K.R.Westerterp, A.B.Andersson, J.M.Forslund and L.M.Hambraeus. The influence of physical activity on BMR. Medicine and Science in Sports and Exercise28(1):85-91, 1996.
10. Wickelgren, I. Obesity: How big a problem? Science 280:1364-1367, 1998.
Seven Tips for Losing Weight
Weight gain is the result of energy intake exceeding energy expenditure. While a reduction in caloric intake plays an important role in weight loss, caloric expenditure, especially resting metabolic rate (RMR), can play a significant role in dropping weight. The following seven principles can help your members and clients to lose those unwanted pounds.
Strength training. Resistance training, which increases muscle mass, is the most important, controllable factor that can increase RMR. Remember, muscle is one of the most metabolically active tissues in the body. More muscle translates into a higher RMR. Unlike aerobic exercise, where post-exercise elevations in RMR are only temporary, an habitual inclusion of resistance training will have positive, long-term benefits on RMR.
Staying active. People now sit more and move less. It has been hypothesized (although not scientifically tested and proven) that, on average, RMR has slowly fallen in inactive people, and hence, weight gain has become more prevalent.
Eating enough. Hypocaloric diets (low in calories), over time, will decrease RMR. This is likely due to low protein consumption that may diminish muscle mass.2 However, research has revealed that people on hypocaloric diets that are high in protein, and who engaged in resistance training, were found to preserve muscle mass and prevent reductions in RMR.1
Using caffeine. Caffeine, a stimulant commonly found in foods (e.g., chocolate) and drinks (e.g., soda, coffee, tea) has been shown to elicit a significant but temporary rise in RMR (possibly up to 12 percent).
Using thermodynamics. Exposure to cold temperatures may trigger the secretion of hormones and muscular shivering, which may increase heat production by 40 percent.7 RMR may also be temporarily increased after exercise in hot conditions because of the sweating response and greater cardiovascular demands.
Performing negatives. Not only does metabolism increase during exercise, but depending on the intensity, duration and type of exercise, it keeps RMR elevated during the recovery period, too. This process, which researchers have labeled excess post-exercise oxygen consumption (EPOC), reflects the additional caloric cost of the exercise above and beyond the calories expended during the activity itself.
A recent study examined the influence of eccentrically biased leg press exercises on RMR for up to 72 hours post-exercise in 18 resistance-trained and untrained men.9 Subjects were required to perform eight sets at their 6RM on a leg press machine. To maximize the eccentric component (i.e., muscle lengthens with an overload), they were instructed to lower the resistance platform slowly and deliberately so that the exercise was four seconds in duration. Results from this study indicate that RMR remained elevated (12 percent at 24 hours, and 8.5 percent at 48 hours) so that an additional 450 calories were burned in the two days following the protocol. The researchers speculated that the tissue damage and subsequent stimulus for repair and tissue hypertrophy resulting from the resistance exercise may have been sufficient enough to contribute to this increased total energy during recovery. The take-home message is this: Performing negatives during the last set of each exercise may contribute to an even greater caloric expenditure than previously thought.
Futile cycling. Intense, low-rep training will promote ‘futile cycling’ (a term coined by scientists referring to the wasteful energy pathways found in post-exercise RMR elevations). In other words, the numerous metabolic reactions that take place in the body release energy as heat instead of converting it into potential energy and storing it as fat.10 In return, this will burn more calories.