An Experimental Test for a Ceiling on Sustained Metabolic Rate in Lactating Mice

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Abstract
Time-averaged sustained metabolic rates (SusMRs) of humans and wild animals have been observed not to exceed about seven times basal metabolic rate (BMR), which suggests a possible ceiling on sustained metabolic scope. We tested experimentally for such a ceiling by subtracting or adding pups to vary the litter size of lactating mother mice between five and 26 pups. Mothers could regularly wean 14pups but not more (natural litter size is 8-10). Although food intake at peak lactation increased to as much as 3.4 times virgin values and increased with litter size, digestive efficiency remained constant The mass of the small intestine and of other gut compartments increased up to severalfold at peak lactation, and as a consequence so did the intestine's brush-border uptake capacities for glucose and for proline. Time-averaged sustained metabolic rate at peak lactation reached 7.2 times BMR; this ratio is evidently close to a ceiling on sustained metabolic scope. Estimates of intestinal nutrient uptake capacities exceeded nutrient intakes by only a modest safety margin of reserve capacity. Intestinal hypertrophy during lactation tended to preserve those safety margins and thus to maintain digestive effciency. Conversely, postlactational intestinal atrophy and the modest size of the safety margins tended to avoid waste of biosynthetic energy.