"It is well known that genes control the nutrient levels in cow's milk," says Wake Forest's Richard Weinberg, MD, in a news release. "But until now, no one has considered how genes might affect human milk."
One particular gene variation may enhance breast milk levels of a vital nutrient that babies need for brain and eye development, and another might affect the fat content (and calories) of breast milk, say the researchers.
The findings were presented in Chicago at the Digestive Disease Week 2005 conference.
More DHA With Gene Variant
Weinberg's study included 111 lactating women. They drank a high-fat milkshake loaded with an omega-3 fatty acid called DHA. Babies need DHA to build healthy brains and eyes, and breast milk may be their main source of it, say the researchers.
After drinking the milkshake, the women gave samples of blood and breast milk every hour for 12 hours. The researchers noted DHA levels in the breast milk samples. Women with the 347S variant of a gene called ApoA4, which is involved in dietary fat absorption, had 40% more DHA in their breast milk.
"These women were more successful at getting the DHA they had just eaten into their bloodstreams and then into their breast milk," says Weinberg.
The 347S gene variant is found in about a third of the U.S. population, says the news release.
Gene Variant Affects Fat in Breast Milk
The researchers also found that women with another variant of the same gene had 40%-75% less total fat in their breast milk than other women.
That gene variant, called E4, is even rarer than 347S. The E4 gene variant is found in about 20% of the U.S. population, and it's associated with an increased risk of heart disease and Alzheimer's disease, says the news release.
The finding about the E4 variant was "unexpected," says Weinberg, a professor of internal medicine (gastroenterology), physiology, and pharmacology at Wake Forest's medical school. The E4 variant "could affect the total amount of calories that a mother can provide to her infant in her milk," he says.
The Tip of the Iceberg?
It's too soon to base dietary recommendations for pregnant and nursing women on the findings; more research is needed first, says Weinberg.
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, says the news release.
Dietary Sources of DHA
However, cautions have been raised about mercury levels in some fish. The U.S. government says that while fish have health benefits, high mercury levels make certain fish not appropriate for people including young children and women who are pregnant, nursing, or planning to conceive.
The FDA and the Environmental Protection Agency offer these guidelines for those women:
- Do not eat shark, swordfish, king mackerel, or tilefish because they contain high levels of mercury.
- Eat up to 12 ounces (two average meals) a week of a variety of fish and shellfish that are lower in mercury.
- Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.
- Another commonly eaten fish, albacore ("white") tuna, has more mercury than canned light tuna. So when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week.
- Check local advisories about the safety of fish caught by family and friends in local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don't eat any other fish during that week.
The same recommendations -- with smaller portions -- apply to young children.