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[¹³C]Linoleic acid oxidation and transfer into milk in stunted lactating women with contrasting body mass indexes^1,2,3

¹ From the Unidad de Investigación en Nutrición, Hospital de Pediatría, CMN, Instituto Mexicano del Seguro Social, Mexico City, and the Klinderklinik and Kinderpoliklinik, Dr von Haunersches Kinderspital, Ludwig-Maximilians University, Munich, Germany.

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/m²) on the metabolic distribution of an oral dose of [¹³C]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 [¹³C]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, [¹³C]linoleic acid enrichment in milk and plasma, and exhaled ¹³CO₂ (by isotope ratio mass spectrometry) were assessed.

Results: Women with a higher BMI exhaled more ¹³CO₂ 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 [¹³C]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 [¹³C]linoleic was retained in the maternal compartment, most likely fat tissue, in a slow turnover pool, and released slowly in later hours.

Key Words: [¹³C]Linoleic acid • human milk • lactation • milk fat • isotope ratio mass spectrometry • body mass index • BMI • body fat • stunted • Mexico

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