by Sally Colby
Consumers receive mixed messages about consuming fat from animal products, often from sources they deem credible. Dr. Jana Kraft, associate professor in the department of animal and veterinary sciences at the University of Vermont, conducts research on dairy fat and fatty acids in the context of human health and disease prevention, especially type II diabetes.
“In the United States, milkfat content of whole milk is 3.25%,” said Kraft. (The exception is California, where the whole milk standard is 3.5%.) “Most consumers don’t know how much fat is in whole milk.” Kraft said that in surveying her students, many believed whole milk ranged in fat from 5% to 50%, and even 100%. When she asked students why they thought whole milk was so fatty, students said the term “whole” implied fat.
“This 3.25% translates to eight grams of total fat in one serving (one cup) of whole milk,” said Kraft. “Saturated fatty acids are the predominant fatty acid class in dairy fat, accounting for about 70% of total fatty acids. These are not static – it depends on the season and how animals are fed.” Monounsaturated fatty acids account for 25% of total fat and polyunsaturated fatty acids account for about 5% of total fat.
Kraft said students in her animal nutrition class ask why saturated fatty acid levels in whole milk are so high, while the rumen of the dairy cow receives feedstocks that contain a relatively high level of unsaturated fatty acids. “Only a very small amount of that is finding its way into the milk,” said Kraft. “The answer is the bacteria in the rumen.”
Through the process of biohydrogenation, bacteria in the rumen convert unsaturated fatty acids in feed, specifically polyunsaturated fatty acids, to saturated fatty acids. “They do this because they protect themselves,” said Kraft, referring to the bacteria. “They protect themselves because polyunsaturated fatty acids are actually harmful to them. In an environment where there are a lot of unsaturated fatty acids, they can’t thrive – they can’t grow or reproduce.”
Kraft explained that ruminants have adapted a protective mechanism that converts unsaturated fatty acids to saturated fatty acids. “There is actually a relatively high amount of saturated fatty acids in whole milk, or in dairy products in general,” she said. “Dairy products are the major contributor to saturated fatty acid intake in our diet.” Kraft said the association of dietary fatty acids with cardiovascular and metabolic diseases is the issue.
“That’s the reason a lot of health authorities have been recommending limiting intake of saturated fatty acids,” said Kraft. “This hits dairy fat in dairy products hard.” Although dietary guidelines from the USDA, the American Diabetes Association and the American Heart Association all recommend two to three servings of dairy as part of a healthy diet, they also recommend choosing fat-free and low-fat dairy products. “Whole dairy products have not made it into these recommendations,” said Kraft. “What’s more striking is they have never made it into the recommendations in the United States.”
Looking further to determine the origin of recommendations, in 1977 dietary guidelines recommended consuming fat-free and low-fat dairy. Kraft said this thinking is based on the 70-plus-year-old diet-heart hypothesis. “It’s based on research conducted by Dr. Ancel Keys, who found that saturated fatty acids intake is linked to cardiovascular disease,” she said. “This has been implemented into dietary guidelines. All the way through the last 40 years, that hasn’t been changed.”
New dietary guidelines for Americans will be coming out by the end of this year, and Kraft said she’s curious to see what will be recommended because there’s emerging evidence showing that scientists from 70 years ago may not have been correct in their findings.
“There’s more evidence now showing that saturated fatty acids may actually not be linked to coronary heart disease,” said Kraft, explaining that detailed statistical studies pooling results from independent clinical trials showed saturated fatty acids are not likely linked to coronary heart disease. “Moreover, in recent years there have been several epidemiological studies and clinical trials that have shown that saturated fats derived from dairy might be either neutral or protective.”
Kraft said saturated fats are not bad after all. Milk fat is the most complex fat mixture of all edible fats, and is composed of more than 400 fatty acids. While many of the 400 fatty acids in milk are minor, their biological activity impacts human health.
In comparison, Kraft pointed out that sunflower oil, soybean oil and peanut oil have between 25 and 30 fatty acids, and olive oil has between 18 and 20 different fatty acids. Dairy fat has fatty acids that only occur in dairy products and nowhere else. “They’re coming from the rumen,” said Kraft. “The rumen of the dairy cow has a very dense and complex community of microbes. Rumen microbes have a synergistic relationship with their host, the dairy cow. The cow is the landlady that provides housing and warmth, and also food. The rumen microbes are the tenants, and they are digesting the otherwise indigestible plant matter for the cow and transform it into energy and other components the cow can actually use.”
Bacteria and protozoa comprise the majority of rumen microbes. “They also make many unique bioactive fatty acids,” she said. “These fatty acids are leaving the rumen, are absorbed in the gastrointestinal tract into the bloodstream where they travel to the tissues and end up in meat. They also travel to the mammary gland where they are incorporated into milk.”
There’s been major discussion around trans fatty acids, and in January, the FDA mandated that industrially produced trans fatty acids must be removed from the food chain. “Trans fatty acids from dairy will be the sole source of trans fatty acids in our diets,” said Kraft, “but trans fatty acids from ruminants are very different from any industrially-produced trans fats like Crisco. They’re naturally occurring, produced by bacteria in the rumen. In general, they occur in very low amounts in dairy fats as opposed to what you’d find in Crisco.” Kraft added that many consumers are aware of and often seek CLA, or conjugated linoleic acid, which is a trans fat.
What’s the bottom line on dairy fat? Kraft said the various classes of fatty acids have different health properties, and those properties are beneficial.
Fatty acids found in dairy products may reduce the risk for cardiovascular disease and type II diabetes, reduce the risk for inflammatory diseases, improve insulin sensitivity, lower blood lipid levels and inhibit tumor growth.
“When we are consuming dairy products,” said Kraft, “we eat all of the fatty acids in concert, and they likely work in concert.”