Limits to sustained energy intake. IV. Effect of variation in food quality on lactating mice Mus musculus.

Observations were made on 30 MF1 mice with their litters. The animals were fed either normal pelleted mouse food (SDS BP Nutrition Ltd) containing 13.4 kJ x g(-1) digestible energy or a specially formulated diet that provided 25% less digestible energy (9.75 kJ x g(-1)) but equivalent amounts of protein and essential minerals and vitamins per gram as the normal diet. Half the animals were switched to the low-energy diet during early pregnancy and half after parturition. The food intake of the two groups increased enormously following parturition, reaching an asymptote over the last few days of lactation. In both groups, the asymptotic food intake exceeded that previously observed across 71 litters of this strain of mice fed the normal diet throughout pregnancy and lactation; the intake of the group fed the low-energy diet from early lactation significantly exceeded that of the mice switched to the low-energy diet after parturition. The increased intakes of the experimental groups were, however, insufficient to offset the lower digestible energy content of the food during lactation. The body mass of the mothers at the end of lactation did not differ between the two experimental groups and the controls. Offspring mass at weaning was inversely related to litter size, but also did not differ between the three groups; pup mortality did not differ between the experimental and control groups. Behavioural observations showed that during both the dark and light phases the general activity of the mother declined enormously from early pregnancy to late lactation. In the dark phase, the time spent in general activity was replaced by time spent both feeding and resting (suckling young), but in the light phase it was replaced only by feeding. At peak lactation, the mice fed for 30-50% of the dark phase and for 30-40% of the light phase. The data indicate that a previously observed asymptote in food intake during peak lactation at 23 g x day(-1) is unlikely to be a limit mediated centrally by the alimentary tract. A higher central limit may exist, at 26.9 g x day(-1), but this is unlikely to reflect the time available for feeding. The data are consistent with limits on sustainable daily energy intake being mediated by the performance of the mammary glands. Animals appeared to accommodate the demands for milk production within a constrained total energy budget by compensating their behaviour, most notably by reductions in the time spent in 'general activity'.