Limits to sustained energy intake. XXX. Constraint or restraint? Manipulations of food supply show peak food intake in lactation is constrained
Lactating mice increase food intake 4-5 fold, reaching an asymptote in late lactation. A key question is whether this asymptote reflects a physiological constraint, or a maternal investment strategy (a ‘restraint’). We exposed lactating mice to periods of food restriction, hypothesizing that if the limit reflected restraint they would compensate by breaching the asymptote when refeeding. In contrast, if it was a constraint they would by definition be unable to increase their intake on refeeding days. Using isotope methods we found that during food restriction the females shut down milk production impacting offspring growth. During refeeding food intake and milk production rose again, but not significantly above unrestricted controls. Hypothalamic transcriptome profiling showed that following restriction lactating mice did not upregulate transcription of genes in the hunger signaling network, suggesting this may impose the constraint. These data provide strong evidence that asymptotic intake in lactation reflects a physiological/physical constraint, rather than restraint. Because hypothalamic neuropeptide Y (Npy) was upregulated under both states of restriction this suggests the constraint is not imposed by limits in the capacity to upregulate hunger signaling (the saturated neural capacity hypothesis). Understanding the genetic basis of the constraint will be a key future goal and will provide us additional information on the nature of the constraining factors on reproductive output, and their potential links to life history strategies.