Adaptive regulation of sugar and amino acid transport by vertebrate intestine.

The adaptive regulation of sugar and amino acid transport by vertebrate intestine constitutes a neglected area. We review the patterns, signals, and mechanisms involved in adaptation. Mechanisms include changes in mucosal mass, specific transport systems, and the sodium gradient. Signals include the transported solutes themselves, hormones, and pancreaticobiliary secretions. The pattern of adaptation is examined for effects of dietary solutes, starvation, hyperphagia, dietary bulk, diabetes, intestinal position, intestinal resection, time of day, season of the year, hibernation, gestation, lactation, and aging and for differences among species. These observed patterns are compared with the patterns predicted by a simple teleologically deduced hypothesis: increased metabolic requirements should be met by increased absorption achieved through increased mucosal mass, while nutritionally essential solutes and nonessential solutes used as calorie sources should, respectively, repress and induce their own transport. We conclude with a summary of major unsolved questions in this area.