Their study on more than 100 mothers found that those with a specific genetic variant present in about one-third of the US population had 40 per cent more DHA in their breast milk than women who had the more common version of the gene.
"It is well known that genes control the nutrient levels in cow's milk," said author Richard B. Weinberg. "But until now, no one has considered how genes might affect human breast milk. This is the first study to demonstrate a genetic effect on human lactation."
The research, presented on Tuesday at this year's Digestive Disease Week in Chicago, adds to the emerging knowledge on genes and diet that is expected to lead to an increasingly personalised approach to nutrition.
The team from the Wake Forest University School of Medicine looked at how much docosahexaenoic acid (DHA) gets from a mother's diet into her breast milk. DHA is essential for healthy brain and eye development and is now added to several brands of infant formula because of research showing its beneficial effects on babies.
The researchers gave 111 women a meal with an added dose of DHA and then asked them to pump their breast milk hourly for 12 hours. They analyzed the amount of DHA and other fats in their blood and breast milk, and determined which women carried variants of ApoA4, a gene involved in dietary fat absorption.
"These women were more successful at getting the DHA they had just eaten into their bloodstreams and then into their breast milk," said Weinberg. "Until now, we've assumed that women consuming an equal amount of DHA in their diets would have the same amount in their breast milk."
The researchers also observed a significant impact of the E4 variant of ApoE, a gene that regulates fat metabolism in the bloodstream. The E4 variant, which is present in about 20 per cent of the US population, is associated with an increased risk for both heart disease and Alzheimer's disease.
They found that mothers who carried either one or two copies of the E4 variant had 40 to 75 per cent less total fat in their breast milk compared to women who did not have the variant.
"This unexpected finding suggests that the E4 variant could affect the total amount of calories that a mother can provide to her infant in her milk," said Weinberg.
Weinberg cautioned that much more research is needed before these findings can be translated into nutritional recommendations for pregnant and nursing women. He predicted, however, that in the future, similar genetic testing may help identify women who need to modify their diet or take supplements to maximize the nutritional value of their breast milk.
An expanded study is planned to examine the clinical impact of additional genes and their effects on prenatal storage of DHA and other omega-3 fatty acids in body fat.