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Glucose tolerance is known to decrease with advancing age. This decline begins in the third or fourth decade of life and is progressive throughout the entire adult life span. The primary cause of this age-related impairment in glucose metabolism results from tissue unresponsiveness to insulin. The plasma insulin response to glucose shows little change with age. Several factors, including decreased physical activity and decreased lean body mass, probably contribute to the insulin resistance. The aging process per se, however, appears to have its own deleterious effect on tissue sensitivity to insulin.
Ralph A. DeFronzo (Wed,) studied this question.