Basal metabolism refers to the energy required by the body for the functioning of the vital organs, including the heart, lungs, liver, kidneys, nervous system, muscles and skin. Basal metabolism does not include energy required by the digestive system and is measured after fasting for at least 12 hours. It was reported in the Journal of Clinical Investigation that between 50 percent and 70 percent of the total energy expended per day is utilized for the functioning of the vital organs.
Basal Metabolic Rate
Cells require oxygen to function. In a lab, the basal metabolic rate is calculated by measuring the rate of oxygen consumption in a fasted person who is lying down in a thermo-neutral environment, so that maintaining body temperature does not affect energy expenditure. The amount of oxygen consumed in liters per minute is multiplied by five to calculate the amount of kilocalories burned per minute. A food calorie found on nutrition labels equals one kilocalorie.
Estimating Basal Metabolic Rate
Several Internet sources, including Discovery Health, are available that estimate basal metabolic rate with the inputs of gender, weight, height and age. The calculations use the Harris-Benedict equation, which does not account for differences in lean body mass. A human study published in the American Journal of Clinical Nutrition concluded that the Harris-Benedict equation is 86 percent accurate for predicting resting metabolic rate in healthy individuals.
Hypothalamus
The hypothalamus, located in the middle of the base of the brain, regulates basal metabolism. It consists of several sets of neurons that communicate between areas in the brain and the hormone-secreting pituitary gland. The hypothalamus secretes thyrotropin-releasing hormone, or TRH, which stimulates the pituitary gland to release thyrotropin, which triggers the thyroid gland to secrete thyroid hormone.
Thyroid Hormone
Thyroid hormone is the primary hormone involved in regulating basal metabolism. It targets all cells of the body and increases their basal metabolic rate. Described in the journal Obesity Facts, thyroid hormone decreases the efficiency of cellular metabolism, which increases the amount of energy lost as heat. This effect of thyroid hormone increases the amount of energy that is needed to perform basal metabolic processes.
Exercise
J. Cunningham reports in the American Journal of Clinical Nutrition that lean body mass, or muscle mass, positively correlates with basal metabolic rate. Muscle tissue expends more energy at rest than other tissues, so increasing muscle mass increases the amount of energy utilized for basal metabolism. In general, because exercise increases lean body mass, it also increases basal metabolism. There is much evidence that says a combination of aerobic exercise and resistance training increases basal metabolism more than either alone and that the increase is most profound in the first 48 hours after exercise. Some controversy revolves around whether exercise increases basal metabolic rate in the long run. A study published in the International Journal of Obesity and Related Metabolic Disorders reported that endurance training causes several physiological changes in muscles, including an increase in the efficiency of metabolism. This increase in metabolic efficiency actually causes a decrease in basal metabolic rate. These results may explain some of the controversy on the influence of habitual exercise on basal metabolic rate.
References
- Pubmed.gov: Journal of Clinical Investigation: Determinants of 24-hour Energy Expenditure in Man
- American Journal of Clinical Nutrition: The Harris Benedict Equation Re-evaluated
- Intelihealth.com: Burning Calories with Exercise
- Discovery Health: Basal Metabolic Rate
- Colorado State University: Mechanism of Action and Physiologic Effects of Thyroid Hormones
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