LIMITS TO DIETARY NUTRIENT INTAKE AND INTESTINAL NUTRIENT-UPTAKE IN LACTATING MICE

In a previous study of lactating mice we experimentally, varied litter size up to 26 pups. We found that mothers' food intake at peak lactation increased with litter size up to a maximum weanable litter size (14 pups). That intake corresponded to a sustained metabolic scope (ratio of sustained to basal metabolic rate) of 7.2. Did that food intake represent a ceiling on intake and sustained metabolic rate, or would intake have increased even further if lactation had been extended (so that the mother mouse was still providing all the nourishment for her growing pups) beyond 15 d? To answer this question, we prolonged lactation to 24 d, using a cage with food at the top of a ladder that only the mother could climb. Pup and litter mass increased 39%-44%, but the mother's intestinal mass and uptake capacities for glucose and proline did not increase beyond their values at day 15. Food intake for mothers of either eight or 14 pups at day 24 was the same as reported for mothers of 14 pups at day 15. Thus, the observed food intake appears to represent a ceiling for lactating mice (under our experimental conditions). It remains to be determined whether that ceiling is imposed by the intestine itself or something else (e.g., by milk production) and whether it applies to other types of demand.