Variation of Intestinal Activity with Age: A Review **

From the day of birth to the time of death, nearly all of the body's nutritional requirements are met by absorption from the intestinal lumen. However, the morphology and function of this absorbing membrane are not constant ; they vary with age. The origin of such changes, and their relationship to all of the events that we associate with biologic aging, is as yet uncertain. Studies in this field are of more than passing interest, for they probably hold clues as to how the alterations can be modified. The present discussion of the variation of intestinal activity with age introduces recent experimental findings from this laboratory, as well as a compilation of literature data. Although a comprehensive picture cannot be drawn, some generalizations begin to emerge which may serve as a baseline for later comparisons. Particular mention is made of the effects on the intestine of modalities which have been shown to alter the aging process (particularly dietary changes and ionizing radiation). Intestinal alterations are conveniently described as a function of the chronological age of the animal. This useful yardstick must not hide our lack of comprehension of the underlying mechanisms. For example, altered flora, dietary changes, and increased weight are all possible participants in intestinal changes shown by a weaning animal. Several experimental tools have recently become practical and might be used with advantage to separate some of these variables. Thus, pathogen-free and germ-free animals are available and may be employed to determine the role of bacteria in gut changes.' Initial studies have been made on overfed (and, hence, "weight aged") animals,2 and these could be followed by investigations of corresponding intestinal alterations. Conversely, animals given balanced but calorically limited diets (a technique introduced by McCay)`' and kept "weight young" will be useful subjects for studies on the relationship of weight to the stimuli initiating gut alterations.