Passive and Active Carbohydrate Absorption by the Ageing Gut

Abstract

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.

Small intestinal permeability in older adults

“…In line with our results, all three studies concluded that the tightness of the small intestine barrier remains intact with advancing age (Saweirs et al 1985; Beaumont et al 1987; Saltzman et al 1995). All three investigations were limited by one or more factors.…”

“…As Saweirs group used hospital patients, no firm conclusion can be drawn from their results for healthy older volunteers. Two years later in 1987, Beaumont and colleagues issued their study that included eight older healthy people undergoing the lactulose and mannitol test (Beaumont et al 1987). In 1995, Saltzman et al investigated another 17 older healthy people and compared the results to 39 younger adults also by means of the lactulose and mannitol test.…”

“…So far, only two small investigations have evaluated the intestinal permeability in healthy older people (Beaumont et al 1987; Saltzman et al 1995). These two investigations included 26 volunteers, but this number is too low to draw firm conclusions.…”

Ageing and the gut

“…Some small studies have demonstrated no significant change to the permeability of lactose and mannitol associated with ageing (21)(22)(23) . Further studies are needed to explore changes in intestinal permeability with ageing as these are likely to affect how the mucous membrane interacts with the luminal contents including the small-and large-intestinal microbiome to cause disease or inflammation.…”

The Intestinal Epithelial Barrier: Does It Become Impaired with Age?

“…In humans, studies of intestinal permeability were performed measuring the intestinal absorption of different-sized molecules such as lactulose, mannitol, polyethylene glycol and EDTA. Although the overall uptake of lactulose and mannitol decreased slightly with age, no differences were found when the renal function of the probands was taken into account [24,25] . Particularly the lactulose:mannitol ratio (LTM) did not differ between young and old subjects, indicating that intestinal permeability to these sugars does not change significantly with age in humans [26] .…”