K. Hammond et J. Diamond, LIMITS TO DIETARY NUTRIENT INTAKE AND INTESTINAL NUTRIENT-UPTAKE IN LACTATING MICE, Physiological zoology, 67(1), 1994, pp. 282-303
In a previous study of lactating mice we experimentally, varied litter
size up to 26 pups. We found that mothers' food intake at peak lactat
ion increased with litter size up to a maximum weanable litter size (1
4 pups). That intake corresponded to a sustained metabolic scope (rati
o of sustained to basal metabolic rate) of 7.2. Did that food intake r
epresent a ceiling on intake and sustained metabolic rate, or would in
take have increased even further if lactation had been extended (so th
at the mother mouse was still providing all the nourishment for her gr
owing pups) beyond 15 d? To answer this question, we prolonged lactati
on to 24 d, using a cage with food at the top of a ladder that only th
e mother could climb. Pup and litter mass increased 39%-44%, but the m
other's intestinal mass and uptake capacities for glucose and proline
did not increase beyond their values at day 15. Food intake for mother
s of either eight or 14 pups at day 24 was the same as reported for mo
thers of 14 pups at day 15. Thus, the observed food intake appears to
represent a ceiling for lactating mice (under our experimental conditi
ons). It remains to be determined whether that ceiling is imposed by t
he intestine itself or something else (e.g., by milk production) and w
hether it applies to other types of demand.