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Which fats should be in infant formula?

baby's face at mother's breast

Written by: Lauren Milligan Newmark, Ph.D. | Issue # 5 | 08 2012

  • A mother’s diet affects her milk fatty acid composition
  • Tsimane women produce milk with more DHA and fewer fatty acids from processed foods than American mothers
  • A mother’s body composition, independent of diet, may also affect milk fat composition

Human milk fat is made up of over 150 different types of fatty acids. While the mammary gland is able to synthesize many of these fatty acids, others must be supplied by fats in the mother’s diet. As human mothers are not consuming identical diets, it is not surprising that human milk fatty acid profiles vary widely among populations. The variation in the types of fat in human milk has long been a concern of nutritionists, pediatricians, and more recently, anthropologists. Of special interest is the amount of DHA (docosahexaeonic acid), an omega-3 fatty acid implicated in neurodevelopment. Breastfed infants receive a wide range of DHA (between 0.06% to 1.4% of total fatty acids)1 and the concentration of DHA in formula is approximately 0.3% (based on milk from women consuming a Western diet). But what is the optimal concentration of DHA for human infants?

A new paper by Martin et al.2 addresses this question using an evolutionary perspective. They argue the extreme variation in the types of dietary strategies we see today (e.g., veganism, vegetarians, the Western diet) is novel. For the majority of human evolution, diets lacked domesticated meats, dairy, processed foods, and hydrogenated oils. Human milk fatty acid synthesis and infant postnatal patterns of growth and development evolved during a time when mothers consumed pre-agricultural diets, so the best model for human milk may come from human populations consuming a traditional diet.

Martin et al. compared milk samples from US women and the Tsimane of Bolivia, forager-horticulturalists who consume a largely plant-based diet while also eating wild game and fresh-water fish (both sources of omega-3 fatty acids, including DHA). Compared to milk from US women, Tsimane milk has significantly higher proportions of DHA – four times more, to be exact (0.69% vs. 0.16% of total fatty acids). Other significant differences include higher proportions of linoleic acid in US milk (a fatty acid found in vegetable oils and processed foods) and higher amounts of arachidonic acid in Tsimane milk (a long-chain polyunsaturated omega-6 fatty acid found in animal protein). Although not necessarily the optimal values for infant growth and neurodevelopment, Martin et al. propose that the proportions of various fatty acids in milk from Tsimane women are a better model for human milk fatty acid composition than those provided by women on a Western diet or other diets far removed from traditional dietary practices.

There are other novel aspects of Westernized populations that may also influence milk fatty acid composition. Take, for example, the current obesity epidemic. It is unlikely that pre-agricultural females attained a body mass index (BMI) greater than 25 (the general cut-off between normal and overweight), so it seems plausible that milk synthesis would be affected by such dramatic changes in body composition. In a recent study, Mäkelä et al.3 found higher proportions of saturated fats, lower proportions of unsaturated omega-3 fats (especially DHA), and a lower ratio of unsaturated to saturated fats in the milk of overweight as compared to normal weight women (categories based on pre-pregnancy measures of BMI). Importantly, these differences were not explained by dietary differences between the groups; after adjusting for estimated omega-3 fat consumption, normal weight mothers still produced higher mean values of DHA. The authors do not provide an explanation for this finding, but do indicate a previous study found the opposite pattern: higher DHA (and other polyunsaturated fatty acids) in milk from overweight women. Although contradictory, the results of both studies suggest body composition influences milk fatty acid composition in some way, even when controlling for dietary fat intake. Hopefully, further research on this topic will identify just what this influence is, and more importantly, how it may affect infant growth and development.

Taken together, these studies demonstrate that modern dietary practices have modified milk fatty acid composition. Beyond just being an interesting factoid, this finding has direct application to human infant formula – after all, the current model for formula fatty acid supplementation is milk from women consuming Western diets. The more we understand the impact of our cultural practices on milk synthesis, the better able we are to develop formulas that optimize milk fatty acid composition.

References

1. Brenna, JT. Varamini, B. Jensen, RG, Diersen-Schade DA, Boettcher, JA, Arterburn LM. (2007) Docosahexaenoic acid and arachidonic acid concentrations in human breast milk worldwide. Am J
Clin Nutr 85: 1457–1464.

2. Martin MA, Lassek WD, Gaulin SJC, Evans RW, Woo JG, Geraghty ST, Davidson BS et al. (2012) Fatty acid composition in the mature milk of Bolivian forager-horticulturalists: controlled comparisons with a US sample. Matern Child Nutr. 8:404-418.

3. Mäkelä J, Linderborg K, Niinikoski H, Yang B, Lagstrom H. (2012) Breast milk fatty acid composition differs between overweight and normal weight women: the STEPS Study. Eur J Nutr. [Epub ahead of print].

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