Enriched milk has new nutrient

by Polly Stanley

ows' milk, already a source of so many good nutrients, is getting an added boost. Guelph researchers have discovered how to enrich milk with docosahexaenoic acid (DHA), an essential nutrient missing in many people's diets.

Prof. Brian McBride and graduate student Tom Wright of the Department of Animal and Poultry Science developed a special feed supplement for dairy herds and teamed up with Prof. Bruce Holub, Human Biology and Nutritional Sciences, to determine its effects on milk fat.

The milk these herds produce after being fed the supplement will be DHA-enriched.

"This is a very practical discovery," says McBride. "It has the potential to significantly benefit human nutrition."

Despite the emphasis on reducing dietary fat intake in Canada, our bodies do need some fat. In particular, DHA -- an omega-3 polyunsaturated fatty acid (PUFA) -- is needed in the eye and brain for optimal visual performance and mental functioning.

And there's growing interest in the cardiovascular benefits of eating DHA-rich diets. Recent studies show that as DHA consumption increases, the risks of cardiovascular disease go down, possibly because of DHA's antiarrhythmic effect.

But food sources of DHA are limited. Fish and fish oils are the primary source. There's a small amount in eggs and some meats, but plant foods contain no DHA. Alpha-linolenic acid (LNA), another omega-3 PUFA, is found in some plant oils. The body can convert LNA to DHA only to a limited degree.

"The body can transform about four per cent of LNA to DHA," says Holub, "but there's evidence that babies have an even lower conversion rate."

And that's a concern because infants have high needs for DHA. Infants are in a stage of active learning, information processing and intellectual development, all of which require DHA.

Breast-fed babies are usually not at risk because human milk naturally contains DHA; the level will depend on the mother's diet. But this isn't the case for infants on commercial formula or for babies and young children on cows' milk.

Cows' milk contains zero to trace amounts of DHA. Cows get some fatty acids in their diet, but those they ingest are from plant sources, which don't include DHA.

Fatty acids in plants are often polyunsaturated. The bovine digestive system changes fatty-acid structure from polyunsaturated to saturated through a process called biohydrogenation. Because LNA undergoes biohydrogenation, there is limited opportunity for conversion of LNA to DHA. As a result, cows' milk is devoid of DHA.

The customized feed developed by the researchers provides for the natural enrichment of DHA in cows' milk at levels that parallel those found in nursing mothers.

How does it work? Holub suggests that DHA in the feed is less susceptible to biohydrogenation than LNA and other PUFAs are. McBride and Wright also think that something about the formulation they've created inhibits biohydrogenation. That means LNA could pass through the gut unaltered and be converted in part to DHA. DHA -- included in the feed supplement -- remains unchanged as well.

This research was sponsored by the Natural Sciences and Engineering Research Council and the Ontario Ministry of Agriculture, Food and Rural Affairs.