[13C]linoleic acid oxidation and transfer into milk in stunted lactating women with contrasting body mass indexes

Background: The fat concentration of human milk is associated with maternal adiposity, but there is no clear understanding of the mechanisms controlling milk fat concentration. Objective: We evaluated the effect of postpartum body mass index (BMI; in kg/m(2)) on the metabolic distribution of an oral dose of [C-13]linoleic acid in lactating women. Design: Ten lactating women stratified by BMI (either < 22.5 or > 23.5) at 5 mo postpartum received orally 2.5 mg [C-13]linoleic acid/kg body wt. Exhaled air, milk, and plasma samples were collected in relation to tracer administration. Linoleic acid was determined by gas chromatography. Dietary intake, serum, milk composition, [C-13]linoleic acid enrichment in milk and plasma, and exhaled (CO2)-C-13 (by isotope ratio mass spectrometry) were assessed. Results: Women with a higher BMI exhaled more (CO2)-C-13, than did women with a lower BMI (22.8 +/- 9.4% compared with 8.6 +/- 3.5% of dose, P < 0.03). Cumulated 72-h transfer of [C-13]linoleic acid to milk was not significantly different between groups (14.8 +/- 6.5% compared with 17.7 +/- 6.7% of dose). Within the first 9 h after dose administration, 51.6 +/- 4.9% of the total isotope transfer into milk had passed in women with a higher BMI, but only 24.0 +/- 15.3% had passed in those with a lower BMI (P = 0.02). Conclusions: Women with a lower BMI, who were reputed as having less body fat, oxidized and secreted into milk less dietary linoleic acid within 12 h after tracer administration than did women with a higher BMI. In both groups, a large proportion of [C-13]linoleic was retained in the maternal compartment, most likely fat tissue, in a slow turnover pool, and released slowly in later hours.