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What Is Metabolism, and Can You Really "Rev" It?

Metabolism is affected by many variables, some of which can be modified to help clients lose or maintain weight.

Most clients are familiar with the terms "metabolic rate" and "metabolism," and generally use these words to mean how quickly the body consumes fuel, or "burns" calories. Weight control is the primary concern of most people asking these questions. They are usually seeking advice about how to get the most calorie-expending power from their workouts so that they can lose weight, or eat more without gaining weight. People who gain weight easily, or have difficulty losing weight, often attribute their problems to a slower-than-average metabolism, and hope for tricks to rev up the rpms on the metabolic machinery of their bodies.

What is metabolic rate?

Metabolic rate is the energy expenditure required to sustain metabolism. Metabolism refers to the entire collection of biochemical processes that occur in our bodies, many of which require energy. Daily energy expenditure refers to the total amount of energy used in a 24-hour period, and is commonly measured in calories. Basal and resting metabolic rate refer to the energy required just to stay alive in a resting state. (Basal metabolic rate, or BMR, is measured while you are lying down; resting metabolic rate, or RMR, is taken in a seated position.) RMR consumes more than half of the calories required in a 24-hour period. The processes of digestion and absorption consume about 10 to 15 percent of daily caloric requirements. Physical activity uses up the remainder.

What factors affect resting metabolic rate?

Many factors affect resting metabolic rate, so anything that affects those factors will affect the idle of the metabolic engine. One of the most significant factors is size, especially the amount of metabolically active tissue. Muscle is one of the most metabolically active tissues, so a large, muscular person will have a higher RMR than a small, fat one. This is why increasing muscle mass will increase RMR, and why big people can eat more than small people without gaining weight.

All sorts of chemicals affect RMR. Several hormones, including the catecholamines (adrenaline and noradrenaline) and the thyroid hormones, help to regulate metabolic processes. The levels of these vary with genetic profile, age, stress level and many other factors. RMR slows as people age, due partly to loss of muscle tissue, but partly to the aging process, which leads to changes in many physiological parameters, including hormone levels.

Many drugs, such as nicotine, some diet pills and stimulants such as speed, raise metabolic rate. But fitness professionals should use a client's interest in metabolic rate to encourage healthful lifestyle changes only! A little caffeine appears to be safe, and will also increase metabolic rate slightly for several hours. So will spicy foods. Dietary manipulations to increase RMR include eating several small meals at regular intervals, rather than one large meal, and consuming a high protein diet. The cumulative effect of dietary manipulations is relatively small, but may be helpful for some people, especially if dietary changes also reduce hunger and food cravings, so fewer calories are consumed per day.

Do small changes really make a difference?

Even a small number looks significant if you multiply it by a large number. For example, if your resting metabolic rate increases by 5 calories per hour, you will expend an extra 120 calories per day, or an extra 43,800 calories per year! Unfortunately, these numbers are purely theoretical, because, by the time you account for changes in hunger, appetite and satiety, and the various hormones and mental processes regulating these sensations - not to mention Thanksgiving and the holidays - little weight loss occurs. Small changes will only make a difference if they are part of a larger program of lifestyle modification that includes eating less and exercising more.
References
Farshchi, J.R., M..A. Taylor and I.A. MacDonald. Beneficial metabolic effects of regular meal frequency on dietary thermogenesis, insulin sensitivity and fasting lipid profiles in healthy obese women. American Journal of Clinical Nutrition 81: 16-24, 2005.
Lejeune, M.P.G.M., K.R. Westerterp, T.C.M. Adam, et al. Ghrelin and glucagon-like peptide 1 concentrations, 24-h satiety, and energy and substrate metabolism during a high-protein diet and measured in a respiration chamber. American Journal of Clinical Nutrition 83:89-94, 2006.
Taheri, S., L. Lin, D. Austin, et al. Short sleep duration is associated with reduced leptin, elevated ghrelin and increased body mass index. PLoS Med 1 (3): e62 doi:10.1371/journal.pmed.0010062, 2004. Accessed 10/01/07.
Westerterp-Plantenga, M., K. Diepvens, A.M.C.P. Joosen, et al. Metabolic effects of spices, teas, and caffeine. Physiology & Behavior 89:85-91, 2006.
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