Most fats take a lot of flack for the risks they can pose to health -- especially when consumed in the large quantities typical of most Western diets. One exception: an unusual variant of a common polyunsaturate that is essential to health.
Because animals are unable to make linoleic acid, they must derive it from plants in their diet. Some animals seem to convert this fat into the variant known as conjugated linoleic acid, or CLA. A host of studies over the past 12 years have established that at least in laboratory animals, this variant offers some protection against breast cancer (SN: 3/19/94, p. 182) and other malignancies -- apparently through its role as a potent antioxidant (SN: 2/11/89, p. 87) and as a chemical that can rewire intercellular communications so that cells respond differently to growth cues (SN: 2/15/92, p. 105).
The earliest studies isolated CLA from hamburger (SN: 12/22&29/84, p. 390). In later investigations, it turned up in Cheese Whiz, and, to a lesser extent, other cheeses. The findings pointed to milk as a good source of the apparently beneficial nutrient.
Now, scientists at the Agriculture Departments U.S. Dairy Forage Research Center in Madison, Wis., have observed that to really enrich a cows milk with CLA, farmers might want to send Bossie out to pasture.
Normally, CLA concentrations hover around 3 to 5 milligrams per gram of milkfat, notes Larry D. Satter, director of the forage center. To probe the role of diet in CLA's formation, his team gave Holsteins a range of feeds.
Much to their big surprise, Satter says, "The highest level of CLA in milk was obtained with cows just eating pasture -- nothing else." While there was some variability between animals, in general, those eating nothing but grass produced five times as much CLA in their milk as those deriving much of their calories from the normal, high-protein rations in the dairy barn.
Adding soybean oil to the diet in concentrations equivalent to 3.5 percent of calories achieved a similar fivefold increase in CLA. The scientists tinkered with other forms of soybeans or soy products, but none performed nearly as well.
Satter is the first to acknowledge they dont know why these dietary manipulations so dramatically affect CLA production. But they have some theories. It could be that grass or the soybean oil promotes in a cows forestomach the growth of bacteria that exhibit a particular propensity for changing dietary linoleic acid into CLA. Or, these foods may provide the raw material for a particular type of digestive reaction that spurs CLA production. Its even possible, he says, that the residence time of food in the cows digestive tract plays some role. His team has noticed that fresh grass moves through the animal faster than feed that has been enriched with grains, silage, or hay.
In addition to any anticancer benefits, CLA also seems to dramatically reduce the deposition of fat. Livestock eating feed supplemented with this fat tend to lay down more lean tissue. This has led Satters team to speculate how this tendency might play out in terms of a young cow. If a cow lays down too much fat in her mammary tissue around the time of puberty, it can have lifelong consequences, reducing her milk productivity, Satter observes. As a result, he told Science News Online, "Were now wondering if pasturing a heifer in the month or so preceding puberty might reduce this fat deposition in the mammary gland" boosting her productivity.
"Its a far-out idea," he acknowledges, "but one for which theres some rationale."
Today, most nutritionists caution dairy lovers to restrict their diet to no-fat, or at least low-fat, milks and cheese. Since CLA doesnt show up in skim products, the question remains: Will any health benefits associated with boosting the CLA in whole milk offset the risks posed by dairy goods other fats?
Then again, theres no reason why CLA cant be separated from milk. Michael W. Pariza, the University of Wisconsin microbiologist who first uncovered CLAs anticancer potential, argues that this unusual fat may one day be isolated as a beneficial food additive. Indeed, he notes, this "remarkably nontoxic" fat might be harnessed to protect foods from the natural, oxidative reactions that promote rancidity.
Dhiman, T.R., L.D. Satter, et al. 1997. Conjugated linoleic acid (CLA) content of milk from cows offered diets rich in linoleic and linolenic acid. Journal of Dairy Science 80(Supplement 1):184.
Dhiman, T.R., G.R. Anand, L.D. Satter, et al. 1996. Conjugated linoleic acid content of milk from cows fed different diets. Journal of Dairy Science 79(Supplement 1):137.
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Larry D. Satter
U.S. Dairy Forage Research Center
USDA-Agricultural Research Service
1925 Linden Drive, W.
University of Wisconsin
Madison, WI 53706-1108
This week's Food for Thought is prepared by Janet Raloff, senior editor of Science News.