Limits to sustained energy intake. XXVIII. Beneficial effects of high dietary fat on lactation performance in mice.

Maximal animal performance may be limited by the ability of an animal to dissipate heat: the heat dissipation limitation (HDL) theory. Because the incidental heat produced during digestion [specific dynamic action (SDA)] varies among diets, the HDL theory predicts that lactating female mice consuming diets with lower SDA should have increased reproductive performance. Dietary fat has a lower SDA than dietary carbohydrate. Female mice were fed low (LF), medium (MF) or high (HF) fat diets (10, 45 and 60% energy from fat, respectively) from days 4-18 of lactation. HF- and MF-fed mice weaned significantly heavier litters than LF mice. This was because they not only consumed more energy [metabolisable energy intake ( E mei ); HF: 306.5±25.0, MF: 340.5±13.5 kJ day-1 ] at peak lactation, but also delivered more milk energy to their pups [milk energy output ( E milk ); HF: 203.2±49.9, MF 229.3±42.2 kJ day-1 ] than the LF-fed mice ( E mei =266.7±4.5, E milk =164.60±30.59 kJ day-1 ). A mathematical model based on the predictions from the HDL theory showed that this effect was greater than predicted from differences in SDA between the diets. Fatty acid profiles of the diets, milk and pups showed significant correlations between the profiles. Besides reduced SDA, HF- and MF-fed mice were probably able to directly transfer absorbed dietary fat into milk, reducing the heat production of lactogenesis and enabling them to perform better than expected from the HDL model. In summary, HF and MF diets had beneficial effects on reproductive performance compared with the LF diet because they enabled mice to generate milk more efficiently with less incidental heat production.