The aging gut.

A spectrum of changes occurs in the function of the gastrointestinal tract throughout the life of the animal. With aging, there is a decline towards newborn levels of the villus surface area and brush border membrane markers, but the absorption of some nutrients continues to increase (such as glucose and vitamin), whereas the absorption of some nutrients falls (such as cholesterol) and fatty acids). Clearly there is a distinction between age-related alterations in intestinal form and function, and some of the enzyme- or carrier-mediated changes are substrate specific. With aging, the structure and brush border membrane composition of the intestine tends to regress towards newborn levels. It remains to be clarified whether these changes are a benefit to the animal, whether they represent a degeneration of normal function, and whether these changes are the cause or the effect of various age-related alterations in metabolic and physiological function.