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Metabolic age is calculated by comparing one's basal metabolic rate to the average of one's chronological age group. Basal metabolic rate is the amount of energy consumed per unit of time when all environmental factors are considered neutral, the digestive system is in a post-absorptive state (meaning that the digestive system is inactive, which requires about twelve hours of fasting in humans), and the energy expenditure is only sufficient to support normal functioning of the vital organs, the heart, lungs, nervous system, kidneys, liver, intestine, sex organs, muscles, and skin. Formulas for estimating basal metabolic rate take into account age, weight, height, activity level, body fat mass, and lean body mass.
All the components in the body require various levels of energy to be maintained. Body fat requires much less energy than lean muscle, as lean muscle is much more metabolically active and therefore requires more energy expenditure to remain in homeostasis. If comparing two individuals, with all variables being equal, the person with more lean muscle mass will have a higher basal metabolic rate, and therefore, a lower metabolic age in comparison to those with the identical chronological age.