The increasing risk of glucose intolerance and diabetes associated with aging is well established. However, it is difficult to determine whether changes in glucose metabolism result from biological aging itself or due to various environmental factors that occur during the aging process. Many epidemiologic studies have shown that plasma glucose levels after oral glucose tolerance test rise consecutively for every decade of age, but many of these studies also demonstrated the effects of environmental factors including obesity and exercise. In some studies, the development of insulin resistance and insulin secretion defects due to biological aging itself have also been identified as major etiologic factors of glucose intolerance. However, the rate of diabetes development due to these factors is expected to be very slow and largely preventable by addressing environmental risk factors.