Passive and active carbohydrate absorption by the ageing gut

We investigated the effects of advanced age, hospitalization and poor nutrition on passive and active carbohydrate absorption using the probe molecules lactulose and mannitol (passive absorption), and 3-O-methylglucose (active absorption). We studied five groups of subjects; healthy controls aged 20-30 years, 40-50 years and over 65 years, respectively, together with long-stay patients and elderly in-patients being investigated for malnutrition. Each subject undertook two separate studies ingesting a drink containing 5 g lactulose, 2 g mannitol and 2.5 mg 3-O-methylglucose (3-O-MG), on one occasion in isotonic solution and in the second study in hypertonic solution, following overnight fast. Urinary recovery of all three probe molecules declined significantly with age (P less than 0.02) and was reduced in malnourished elderly subjects compared to healthy elderly controls (P less than 0.05). Correction of urinary recoveries for renal function on the basis of creatinine clearance abolished significant differences between groups. Thus passive absorption of carbohydrate is not impaired with advanced age in healthy elderly subjects or long-stay hospital patients. The ratio of the percentage recovery of 3-O-MG to the percentage recovery of mannitol was significantly reduced in the healthy elderly subjects compared to middle-aged and young controls in the hypertonic study; similar changes did not occur in the long-stay and malnourished elderly patients, interpretation of this finding is thus difficult. If confirmed, this impairment would suggest a possible defect in active sugar transport in the elderly.