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Your Metabolism: Facts and Fables David C. Nieman, DrPH, FACSM Appalachian State University, NCRC, Human Performance Lab [email protected] Resting Metabolic Rate (RMR) Description The basal metabolic rate (BMR) represents the energy needed to support the basic cost of living. The BMR is typically measured in the morning after an overnight fast and lying down for 30 minutes. RMR is any time of day. RMR Variance WOMEN: slightly less than 1 kcal/min (1,150 to 1,440 kcal/day) MEN: somewhat more than 1 kcal/min (1,580 to 1,870 kcal/day). One kcal/min equals the heat released by a burning candle or by a 75-watt light bulb. About 70-80% of RMR variance between individuals is due to differences in body composition, with the greatest RMR found in those with the largest fat-free mass. Daily Energy Expenditure The largest number of calories expended by most people (except for athletes during heavy training) is from the resting metabolic rate (RMR). Am J Clin Nutr 55:242S-s45S, 1992. Activity 23% RMR 67% TEF 10% Energy Expenditure and Body Mass Change The resting metabolic rate is directly related to body weight (higher in the obese, lower in the lean), and parallels changes in weight. N Engl J Med 332:621-628, 1995 -300 10% weight loss Back to initial weight -25 500 10% weight gain -400 -200 0 200 400 Energy Expenditure Change (kcal/day) 600 RMR Measurement The accurate measurement of BMR and RMR requires sophisticated methods that are typically conducted in research laboratories. All of these methods are costly and cumbersome to conduct, require highly skilled technicians, and are impractical for most clinical and community settings. RMR Estimation Through Equations BMR prediction equations have been developed using easily and accurately measurable variables such as age, height, and body weight. The Food and Nutrition Board of the Institute of Medicine recently developed these equations for estimating BMR in adults: Men: BMR (kcal/day) = 293 - (3.8 x age) + (456.4 x height) + (10.12 x weight) Women: BMR (kcal/day) = 247 - (2.67 x age) + (401.5 x height) + (8.6 x weight) [Age is in years, height is in meters, and weight is in kilograms]. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, D.C.: The National Academies Press, 2002. BMR Equations (IM) for Boys and Girls Normal Weight Boys (BMI 85%, ages 3-18)* BEE = 68 – (43.3 x age) + (712 x height) + (19.2 x weight) Overweight/Obese Boys (BMI > 85%, ages 3-18)* BEE = 419.9 – (33.5 x age) + (418.9 x height) + (16.7 x weight) Normal Weight Girls (BMI 85%, ages 3-18)* BEE = 189 – (17.6 x age) + (625 x height) + (7.9 x weight) Overweight/Obese Girls (BMI > 85%, ages 3-18)* BEE = 515.8 – (26.8 x age) + (347 x height) + (12.4 x weight) *Height in meters, weight in kilograms, age in years. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, D.C.: The National Academies Press, 2002. Equation Example 40 yr old woman, 65 inches, 140 pounds Women: 65 inches x 0.0254 = 1.65 meters 140 pounds x 0.4536 = 63.5 kg BMR (kcal/day) = 247 - (2.67 x age) + (401.5 x height) + (8.6 x weight) BMR (kcal/day) = 247 - (2.67 x 40 yr) + (401.5 x 1.65 m) + (8.6 x 63.5 kg) = 1,349 Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, D.C.: The National Academies Press, 2002. Limitations of Equations Unfortunately, only 50% to 75% of the variability in BMR is explained by these equations, and they tend to misclassify obese children and adults, critically ill patients, and individuals with eating disorders. Ethnicity affects BMR and RMR. Thus it is highly recommended that BMR and RMR be measured. RMR of Three Obese Women Mean of 2 Douglas bag tests, >4 h postprandial, 4:00-5:00 pm Data from Nieman’s Human Performance Laboratory, ASU Comparison of Measured Vs. Estimated RMR Douglas bag Age 46, BMI 36.7 1617 1269 Age 42, BMI 42.1 1549 +348 1689 +140 -506 1594 Age 47, BMI 33.4 1000 IM Equation 1200 1400 1600 2100 1800 RMR (kcal/day) 2000 2200 FFM Equation: RMR = 418 + 20.3 FFM Challenge---accurately measuring fat-free mass (kg) Arciero PJ, Goran MI, Poehlman ET. Resting metabolic rate is lower in women than in men. J Appl Physiol 75:2514-2520, 1993. Human calorimeter, starting in the late1800s (direct calorimetry) Indirect calorimetry, closed-circuit spirometry, starting early 1900s Indirect calorimetry, open-circuit spirometry, computerized metabolic carts, 1980s -to present Challenge has been finding an instrument to measure oxygen consumption accurately in non-laboratory settings. First portable spirometer, open circuit, to measure oxygen consumption during exercise, Germany, 1940s Cosmed produced a portable, computerized spirometer, open circuit, starting in the 1990s. Cosmed: “FitMate” can measure both resting and exercise energy expenditure ($10,000; www.fitmate.net). Cannon Village 600 Laureate Way ASU-NCRC Human Performance Lab (Rm 1201) UNC-Chapel Hill, Nutrition Research Institute, Metabolic Chamber How to Use the RMR in Weight Loss/Maintenance Programs RMR x 0.85 = 1.0-1.5 pounds weight loss per week for obese clients during first 12 weeks. RMR x 1.0 = 0.5-0.75 pounds weight loss per week. RMR x 1.3 = 0-0.5 pounds weight loss per week. RMR x 1.4-1.6 = weight maintenance for most people (low activity). RMR x 1.6-1.9 = weight maintenance for moderately active people. RMR x 1.9-2.5 = weight maintenance for very active people (athletes). Sources: 1. 2. Powell LA, Nieman DC, Melby C, et al. Assessment of body composition change in a community-based weight management program. J Am Coll Nutr 20:26-31, 2001. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, D.C.: The National Academies Press, 2002. Misconceptions About Exercise and Metabolism Does one bout of exercise increase RMR for a long time afterwards, burning a significant number of extra calories? Can exercise counter the dietinduced decrease in RMR? Do physically fit compared to unfit people have higher RMRs, helping them to stay lean? For most fitness enthusiasts, the answer to these questions is “no”. Misconception #1: Exercise Causes the Resting Metabolic Rate to Stay Elevated for a Long Time after the Bout, Burning Extra Calories The truth is that the energy expended after aerobic exercise is small unless a great amount of high intensity exercise is engaged in. Moderate intensity exercise for 30 min increases the RMR for only 20-30 min, burning 10-20 extra calories. Thus when the obese individual walks for 20-30 min, about 10 extra calories will be burned afterwards, hardly enough to be meaningful. Energy Expenditure (kcal/min) 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 Sitting Standing Rest Rest 5-min 10-min 15-min 1-min 2-min 3-min 4-min 5-min 10-min 15-min 20-min 25-min 30-min Exercise Exercise Exercise Post-Ex Post-Ex Post-Ex Post-Ex Post-Ex Post-Ex Post-Ex Post-Ex Post-Ex Post-Ex RMR back to pre-exercise level in 5 minutes following 15 min calisthenics using the PowerPlate (whole body vibration exercise). Influence of 47-min vigorous cycling on post-exercise RMR 14 Exercise Day Exercise (47 min) 519±57.8 kcal (P<0.001) 12 Rest Day Post-exercise (18.7 h) 223±82.4 kcal (P<0.001) 8 6 Sleep (8 h) 45.0±50.2 kcal (P=0.025) 4 2 Br Dr Lu Di LO WU 8 am 6 am 4 am 2 am 12 am Time 10 pm 8 pm 6 pm 4 pm 2 pm 12 pm 10 am 0 8 am kcal / min 10 2.6 Exercise Day Rest Day 2.4 2.0 1.8 1.6 1.4 1.2 LO WU Time 7 am 6 am 5 am 4 am 3 am 2 am 1 am 12 am 11 pm 1.0 10 pm kcal / min 2.2 RMR 100 Fat Mass 90 75 80 Percentage Fat-Free Mass 60 50 40 50 25 20 10 0 -20 -40 -5 -15 1200-1500 400-800 -25 Fasting Energy Intake (Kcal/day) During caloric restriction (dieting), the resting metabolic rate drops 5-30%, and the fat-free mass drops 10-50% (with degree of decrease depending on the severity of the energy decrease). Misconception #2: Exercise counters the dietinduced decrease in RMR. Most studies have found that exercise during dieting does not counter the decrease in RMR and fat-free mass. 65 obese, 8 wk, formula diet at 70% RMR; aerobics 3/wk, leg/arm cycling; weights 3x/wk, 3x6, 8 stations. Am J Clin Nutr 66:557-563, 1997. 0 Change in RMR (kcal) -20 -40 Diet Alone Aerobics & Diet Weights & Diet -60 -80 -100 -88 -120 -127 -140 -160 -148 Percent Decrease in RMR 0 -2 Diet Alone Aerobics & Diet Weights & Diet -4 Diet & Aerobics/Weights -6 -8 -9 -10 -10.3 -10.6 -12 90 day study of 69 obese females, all on 520 kcal/day formula. Aerobics = 4 d/wk, 20 min progressing to 60 min/session; weights = 4 d/wk, 2-3 sets, 6-8 reps. Am J Clin Nutr 54:56-61, 1991. -10.3 Misconception #3: Fitness Accelerates the RMR Fitness and RMR Most studies have concluded that there is little difference in the RMRs of aerobically fit and unfit people. (Am J Physiol 260:E257-261, 1991; Am J Clin Nutr 61:473-481, 1995). Even highly trained endurance athletes who have rested for two days have RMRs that are about the same of sedentary individuals of the same weight and body composition. During periods of heavy training, athletes eat more, increasing overall energy turnover and RMR. But when athletes don’t exercise, they also don’t eat as much, and RMR returns to normal. Weight Training and RMR Weight training can add fat-free mass, and RMR increases 7-10 kcal/day for each pound added. However, most fitness enthusiasts who take up weight training only add 4-5 pounds of fat-free mass, increasing RMR by 28-50 kcal/day, hardly an impressive gain. Bodybuilders and strength athletes, however, can put on high amounts of fat-free mass, significantly increasing RMR. Practical Conclusions To sum up, individuals adopting a moderate exercise program to lose weight should not count on a revved up metabolism to burn a significant amount of extra calories beyond that linked to the exercise itself. Vigorous and prolonged exercise does increase the RMR for 19 hours.