Supplementation with fish oil as a source of n-3 fatty acids does not downregulate mammary lipogenesis in lactating mice.

The very long chain n-3 (ω-3) polyunsaturated fatty acids (VLCn3PUFAs) are potent regulators of hepatic lipid synthesis, but their effect on lipid synthesis in the lactating mammary gland is less well investigated. The objective of the present study was to examine effects of fish oil (FO) supplementation on mammary lipogenesis and the expression of lipogenic genes in mammary and hepatic tissues of lactating mice. Beginning on day 6 of lactation and continuing for 7 d, female C57BL/6J mice (n = 8/diet) were fed 1 of 3 dietary treatments: a 5%-fat diet containing mainly saturated fatty acids (FAs) (low-fat control) or 2 10%-fat diets, 1 enriched with FO as a source of VLCn3PUFAs and the other enriched with a safflower/palm oil mixture (high-fat control) as a source of oleic acid. Mammary lipogenic capacity, measured by (14)C-glucose incorporation into FAs by mammary explants, was similar among treatments, and there were no treatment effects on the proportion of de novo synthesized FAs in milk fat or on litter weight gain, a proxy for milk energy secretion. Also, there were no treatment effects on mammary mRNA abundance for key lipogenic enzymes and proteins involved in the regulation of milk lipid synthesis. In contrast, there was a treatment effect on hepatic lipogenesis, with FO resulting in a decrease of ~50% in hepatic lipid content and a similar downregulation of lipogenic gene expression compared with the 2 control diets. Overall, there were tissue-specific differences in dietary VLCn3PUFA effects on lipid synthesis with no observed effects for mammary lipogenic variables but marked reductions occurring in hepatic lipogenesis.