A gradual and persistent physiologic increase in body weight of 3-5 kg per decade occurs between the third to the fifth decade. The thrifty genotype theory explains weight gain in large populations, the thrifty phenotype theory explains weight gain in subjects with intrauterine growth retardation. The young hunter theory explains the physiologic age-related weight gain. We believe this is nature's method of preservation by default. According to the young hunter theory, in the past food providers needed an appropriate muscular apparatus to cope with continual hunting expeditions to ensure maximal survival. At the end of the chronological 'hunting' age, there was a gradual redirection of metabolic processes toward energy conservation in anticipation of aging. According to our hypothesis, muscle loss allows for the full expression of hyperinsulinemia and insulin resistance, which allows the fuel previously directed to the muscle to be deposited as adipose tissue. Thus, weight gain is an adaptive process engineered to compensate for adult muscle mass loss, guaranteeing survival and longevity beyond the age of hunting.
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