Age-related insulin patterns in normal glucose tolerance

Insulin and aging

Aging is characterized by a progressive loss of physiological function leading to increase in the vulnerability to death. This deterioration process occurs in all living organisms and is the primary risk factor for pathological conditions including obesity, type 2 diabetes mellitus, Alzheimer's disease and cardiovascular diseases. Most of the age-related diseases have been associated with impairment of action of an important hormone, namely insulin. It is well-known that this hormone is a critical mediator of metabolism, growth, proliferation and differentiation. Insulin action depends on two processes that determine its circulating levels, insulin secretion and clearance, and insulin sensitivity in its target tissues. Aging has deleterious effects on these three mechanisms, impairing insulin action, thereby increasing the risk for diseases and death. Thus, improving insulin action may be an important strategy to have a healthier and longer life.

Mechanisms of insulin resistance in aging

We have studied 17 elderly and 27 non-elderly, nonobese subjects (mean age 69+/-1 and 37+/-2 yr, respectively) to assess the mechanisms responsible for the abnormal carbohydrate tolerance associated with aging. Serum glucose and insulin levels were significantly elevated in the elderly subjects compared with the nonelderly subjects during a 75-g oral glucose tolerance test, suggesting an insulin resistant state. Peripheral insulin sensitivity was assessed in both groups using the euglycemic glucose clamp technique during an insulin infusion rate of 40 mU/m(2) per min. Similar steady-state serum insulin levels led to a peripheral glucose disposal rate of 151+/-17 mg/m(2) per min in the elderly compared with a value of 247+/-12 mg/m(2) per min in the nonelderly, thus documenting the presence of insulin resistance in the elderly subjects. Insulin binding to isolated adipocytes and monocytes was similar in the elderly and nonelderly groups (2.34+/-0.33 vs. 2.62+/-0.24% and 5.04+/-1.10 vs. 5.12+/-1.07%), respectively. Thus, insulin resistance in the presence of normal insulin binding suggests the presence of a postreceptor defect in insulin action. This was confirmed by performing additional euglycemic clamp studies using infusion rates of 15 and 1,200 mU/m(2) per min to assess the contours of the dose-response relationship. These studies revealed a 39 and 25% decrease in the glucose disposal rate in the elderly subjects, respectively. The results confirm the presence of a postreceptor defect as well as a rightward shift in the dose-response curve. Insulin's ability to suppress hepatic glucose output was less in the elderly subjects during the 15 mU/m(2) per min insulin infusion (77+/-5 vs. 89+/-4% suppression), but hepatic glucose output was fully and equally suppressed in both groups during the 40 and 1,200 mU/m(2) per min infusion. Finally, a significant inverse relationship was observed between the degree of glucose intolerance in the individual elderly subjects, as reflected by the 2-h serum glucose level during the oral glucose tolerance test, and the degree of peripheral insulin resistance as assessed by the glucose disposal rate during the 40 mU/m(2) per min insulin infusion (r = 0.59, P < 0.01).We conclude that carbohydrate intolerance develops as part of the aging process. This carbohydrate intolerance appears to be the consequence of peripheral insulin resistance caused by a postreceptor defect in target tissue insulin action, which causes both a decrease in the maximal rate of peripheral glucose disposal and a rightward shift in the insulin action dose-response curve. In elderly subjects, the severity of the abnormality in carbohydrate tolerance is directly correlated to the degree of peripheral insulin resistance.

Glucose tolerance in the elderly: the role of insulin and its receptor

Oral glucose tolerance tests were performed in young and elderly subjects with minimal risk factors for diabetes mellitus. Compared to the normal glucose tolerance in the young there was a 45% rate of impaired tolerance in the elderly. Fasting insulin levels were significantly lower in the elderly but post-glucose insulin responses in the first hour were similar in young and elderly subjects. Peripheral insulin action was assessed in terms of the 125 monoiodoinsulin binding to specific insulin receptor sites on circulating lymphocytes in the young, the elderly and a group of age and sex matched obese maturity-onset diabetics. Specific insulin binding was not significantly different in the elderly than in the young but was significantly lower in the diabetics than the young and the elderly. The results suggest that neither defective insulin binding are major causative factors in the reduced glucose tolerance of the elderly.

Effects of age on various aspects of glucose and insulin metabolism

Age is known to be associated with the development of glucose intolerance. In this review an effort has been made to differentiate between the effects of age per se on glucose tolerance, as distinguished from those of such age-related variables as obesity, diet, development of frank diabetes, etc. At the same time, an attempt was made to evaluate the evidence implicating abnormalities of insulin secretion and/or insulin action in the development of glucose intolerance with age. It is concluded that the questions being asked are far from simple, and that available data do not provide unequivocal answers.

The effect of aging on carbohydrate metabolism: a review of the English literature and a practical approach to the diagnosis of diabetes mellitus in the elderly

There seems little doubt that the disposal of a glucose load is progressively impaired during aging. The mechanism(s) for this alteration remains unclear. Five possibilities have been raised: (1) poor diet, (2) physical inactivity, (3) decreased lean body mass in which to store the carbohydrate load, (4) decreased insulin secretion, and (5) insulin antagonism. Although poor diet and physical inactivity may contribute to some of the abnormal glucose tolerance tests of the older population, these two factors do not provide a full explanation. Diminished lean body mass may play some role but there is almost certainly an additional effect due to aging. A few papers have suggested that glucose-induced insulin secretion may be impaired as the population ages, but the bulk of studies in this area conclude that normal or increased amounts of insulin are released by the pancreatic beta-cell during aging. If abnormalities of insulin secretion exist, either in degree or timing, they are subtle and would not seem sufficient to account for the great number of older subjects who manifest impaired glucose tolerance. The evidence for insulin antagonism seems the strongest but the data are certainly not conclusive. In actuality, the aging effect on carbohydrate metabolism may be heterogeneous in nature. Either some or all of these five factors may contribute to the aging effect to varying degrees in individual subjects. Alternatively, the glucose intolerance of aging may represent a heterogeneous group of disorders. In any event, until better methods to identify possible subgroups of these subjects and/or a marker for diabetes mellitus independent of glucose concentration become available, this problem will remain difficult to resolve. Based on the currently available data, it seems prudent to diagnose diabetes mellitus only if fasting hyperglycemia is present.

Insulin and glucagon relationship during aging in man

To determine if the glucoregulatory hormones, insulin and glucagon, are altered with aging inman, 44 healthy volunteers, 22-81 yr of age, were evaluted by measurement of basal levels of glucose, insulin, and glucagon in relationship to their fat mass. In addition, the secretory capacities of the alpha and beta cells were assessed by measurement of the amounts of glucagon and insulin released after intravenous administration of glucose and arginine, respectively. Although no significant differences in weight could be distinguished longitudinally, the percentage adiposity was found to increase with age. Basal concentrations of glucose, glucagon, and insulin were not appreciably altered as a function of advancing years. After intravenous glucose, the glucose disappearance rate (Kg) was significantly slower in the elderly in comparison with the young, yet no differences in glucose-induced release were found. Similarly, insulin responses after arginine infusion between young and old were indistinguishable. The release of glucagon in response to arginine infusion was not perceptibly altered during aging. Thus, despite a decline in Kg with advanging age in this healthy population, gross changes in insulin and glucagon release were not apparent. We infer from bese data that decreased carbohydrate tolerance accompanying aging in some populations may be due to factors other than abnormalities in secretion of insulin and glucagon.