THE EFFECT OF AGE ON INSULIN RESISTANCE AND SECRETION : A REVIEW

: Aging is associated with an increased incidence of hypertension, noninsulin-dependent diabetes mellitus, and coronary heart disease. Because these conditions often cluster in the same individuals, there has been speculation that a common mechanism is responsible for all of these pathological states. Both epidemiological and clinical research has shown that insulin resistance and/or hyperinsulinemia are associated with glucose intolerance, dyslipidemia (high plasma triglyceride and low high-density lipoprotein-cholesterol levels), and higher systolic and diastolic blood pressures. Therefore, insulin resistance and hyperinsulinemia have been proposed as the causal link among the elements of the cluster mentioned above, now most commonly referred to as the insulin resistance syndrome, syndrome X, or the metabolic syndrome. The elderly are more glucose intolerant and insulin-resistant, but it remains controversial whether this decrease in function is an inevitable consequence of "biological aging" or the result of what might be referred to as environmental or lifestyle variables: increased obesity, a detrimental pattern of fat distribution, or physical inactivity that usually accompany age. All of these modifiable environmental factors have also been shown to result in increases in insulin resistance and hyperinsulinemia and are risk factors for the development of the diseases of the metabolic syndrome. Recent interventional studies that have attempted to reverse these conditions in the elderly have shown improved insulin sensitivity, and glucose tolerance. Insulin secretion, on the other hand, seems to decrease with age even after adjustments for differences in adiposity, fat distribution, and physical activity. This may be responsible for the glucose intolerance in the very old even after improvements have been made in their lifestyle variables.