soybean powder were absorbed equally well.81 This may have implications for older individuals who follow a macrobiotic diet. Studies of protein turnover indicate that rapidly absorbed proteins such as whey protein resulted in greater net protein gain in older subjects compared with younger individuals, although the effect disappeared if fat and carbohydrate were added to the test meal.82 It is not clear whether this indicates a true impairment in protein absorption or the effects of fat and carbohydrate on small bowel motility. Aging may alter visceral chemosensitivity or hormone responsiveness, because an enhanced satiating effect of carbohydrates delivered into the small intestine by infusion has been reported in older men.83 Mucosal regeneration is actually increased in aging. Crypt cell proliferation rates in all segments of the small intestine were found to be greater in aged rats compared with their youthful counterparts.84 These studies suggest that aged animals have increased cell production as a result of a greater number of crypt cells undergoing cell division. Minor effects of aging on small bowel motility have been described,85 despite reports that several neuronal subtypes (substance P, vasoactive intestinal polypeptide, somatostatin, and nitric oxide containing) are decreased in aged animal models.86, 87, 88 Studies in cats do not show significant effects of age on small bowel transit of radiolabeled lipids. In human studies, small bowel transit time measured by breath hydrogen testing was shorter in elderly men but not women.89 The pathophysiology of enteric neuronal loss and efficacy of treatment of aging-associated neuropathy have been the focus of several reviews.15, 90, 91 It is of interest that caloric restriction to 70% of normal intake prevents ileal myenteric neuronal loss in Sprague–Dawley rats up to 30 months of age.92 This seems to be mirrored in other studies that show a beneficial effect of calorie restriction on pathophysiologic degenerative changes in aging. It is not clear whether any benefit to neuronal function occurs in human subjects as a result of inadvertent weight loss attributed to impairment in food intake, and recommendations do not currently promote this degree of deliberate weight loss in healthy older adults. Bacterial overgrowth in small bowel Minor alterations in small bowel motility, in conjunction with antibiotic use or concurrent illness, may predispose older individuals to bacterial overgrowth. Bacterial overgrowth in the small bowel is much more prevalent in older community-dwelling adults than in younger subjects. Parlesak et al93 found that the prevalence of bacterial overgrowth as measured by breath hydrogen testing was 15.6% in 294 adults aged 61 years or older, as compared with 5.9% of subjects aged 24–59 years. Bacterial overgrowth was associated with significantly lower body weight, lower plasma albumin concentration, and increased prevalence of diarrhea. It is difficult to determine the causal relationship in studies that simply assess prevalence; however, treatment of bacterial overgrowth has been shown to improve diarrhea and promote weight gain. Effect of Aging on Colonic Function Aging is associated with a number of diverse effects on the large intestine, including alterations in mucosal cell growth, differentiation, metabolism, and immunity. Disorders commonly observed in the elderly include colon cancer, diverticulosis, and altered bowel habits leading to