We examined the size of the "safety margin," if any, by which the small intestine's daily capacities to absorb nutrients exceed prevailing daily intakes of those nutrients. This safety margin, also known as reserve capacity, is widely assumed to be enormously large. As a test, we suddenly transferred mice from an ambient temperature of 22 to 6 degrees C and measured food intake, apparent digestive efficiency, intestinal morphometrics, and intestinal brush-border uptake capacities for D-glucose and L-proline over the next 28 days. Food intake jumped 68% within the first 12 h and rose in 2 days to a new plateau level 2.5 times the previous intake. Nevertheless, apparent digestive efficiency remained unchanged, even within the first 12 h, and intestinal transit times also remained unchanged, implying the existence of at least some safety margin. Masses of the small and large intestine, liver, kidneys, and spleen nevertheless increased within 4 days by 16-20%. Glucose and proline uptakes per milligram intestine increased by approximately 5%, so that the intestine's summed uptake capacities for these solutes increased by 24-26%. The animal's intestinal adaptation expressed in these increased uptake capacities implies that safety margins at the new plateau value of food intake would otherwise have been dangerously narrow. Comparison of calculated summed uptake capacities with measured dietary intakes suggests that safety margins are approximately 220-300% in mice at 22 degrees C, only 27-50% in mice at 6 degrees C before intestinal adaptation, but 60-88% in mice at 6 degrees C after intestinal adaptation.