The role of diet in the development of cancer has been the focus of much scientific research during the past decade. Researchers now know that high intakes of certain naturally occurring dietary chemicals increase our risk of cancer. Such chemicals include benzo(a)pyrene (found in charcoal-broiled meat), aflatoxin (found in peanuts), and certain hydrazines (found in edible mushrooms). But scientists studying the chemical makeup of the human diet have also identified some naturally occurring anticancer chemicals. One looks especially promising.
Conjugate Bliss?
Among the more potent naturally occurring anticarcinogens is conjugated linoleic acid (CLA), a fatty acid found mainly in milk fat. The compound was first isolated in 1983 (from ground beef), by a research team led by Michael W. Pariza, Ph.D., of the University of Wisconsin. During the past few years, researchers have not only confirmed CLA's anticarcinogenicity in experimental animals it is the only fatty acid that has been shown unequivocally to inhibit cancer growth in such animals but have also explored ways of increasing the amount of CLA in our diet. However and here's the rub while CLA's anticarcinogenicity has been demonstrated in rats and mice and in in vitro ("test-tube") studies of human cancer cells, whether it can significantly protect humans is wide-open to question. Exactly how CLA inhibits cancer is the subject of ongoing scientific investigation.
Whence CLA?
The term "conjugated linoleic acid" refers to a group of several variants of linoleic acid (also called octadecadienoic acid), an essential fatty acid. Variants of linoleic acid differ from one another in the type and arrangement of their chemical bonds. The specific structure of CLA's chemical bonds is crucial to the compound's ability to fight cancer; at high levels, linoleic acid, whose chemical structure is slightly different, increases cancer growth in lab animals. Researchers are excited about CLA because it is anticarcinogenic at much lower dosages than are many other naturally occurring anticarcinogens. It is effective in animals at dietary levels as low as .05 percent. According to researchers at the October 1996 Cornell Nutrition Conference, the normal human intake of CLA from dairy products provides about one third of the level of CLA shown to afford cancer protection in experimental animal models.
Most dietary substances protective against cancer originate in plants, but CLA is found almost exclusively in animal products. Dairy products are the main source of CLA for humans. Meat, particularly beef, is another major source. The meat of ruminants cows, sheep, and other animals that chew the cud contains more CLA than nonruminant meats, such as turkey, chicken, and pork.
Because CLA is a fatty acid, its concentrations in food are generally measured against the fat in the food: milligrams (mg) of CLA per gram (g) of fat. According to one study that used this measure, lamb, beef, and veal in descending order of concentration contain more CLA than other meats. The study also showed that seafood is very low in CLA and that plant oils contain much less CLA than animal fats.
Grazin' in the Grass
Microorganisms that live in the rumen (the first stomach compartment) of cows help create the CLA found in dairy products by secreting enzymes that contribute to the breakdown of food. CLA forms during the digestion of dietary linoleic acid. CLA is absorbed from the rumen and ends up in the cow's milk.
The concentration of CLA in milk fat varies substantially. Dr. Dale Bauman of Cornell University examined milk from various herds of New York cows and found that CLA levels ranged from 2.4 to 18 mg CLA/g fat more than a sevenfold variation. A survey of milk from Canadian creameries uncovered a similar range of CLA levels. This wide variation in cows' CLA levels results partly from differences in diet and dairy-farm management.
Evidence indicates that when cows are allowed to graze in pastures, their milk contains more CLA than when they are fed grain concentrates. This probably explains the finding of one study that in summer cow's milk contained double the CLA it contained in winter. (Researchers found no seasonal variation when they measured CLA from dairy cows given the same diet throughout the year.) CLA levels in milk also increase when cows are fed supplements (corn oil, for example) that contain high levels of unsaturated fatty acids.
CLA seems to be a stable component of milk fat. Some investigators have found that neither processing (heating, canning, etc.) nor storage has much effect on the CLA levels of dairy products. In contrast, studies of CLA levels in meat suggest that cooking and processing meat can increase its concentration of CLA. One study showed a nearly fivefold increase in the CLA content of ground beef after grilling.
The Downside
Even if researchers demonstrate that CLA is anticarcinogenic in humans, the compound would have limited public-health utility, since it is abundant only in fatty foods. It is the fat in milk that contains CLA. The making of "fat free," low-fat, and "reduced fat" dairy products entails losses of the compound. For example, "fat free" cottage cheese and "fat free" ice cream contain very little CLA.
Increasing consumption of fatty foods to increase CLA intake is ill-advised not only because researchers have not demonstrated CLA's effectiveness against cancer in humans, but also because high fat intakes may have adverse health effects, such as increasing one's risk of heart disease and certain types of cancer.
One approach to increasing CLA's availability would be to boost the proportion of CLA in milk fat, which would increase the CLA content of products made from the milk.
As mentioned above, changing cow's diets as by increasing their intake of unsaturated fatty acids might increase the CLA content of their milk. Another possibility for increasing the CLA content of milk involves genetic engineering: Potentially, DNA technology could enhance the production of enzymes by ruminal bacteria. For example, in a lab, scientists could insert a gene that would boost bacterial production of the CLA-synthesizing enzyme into the DNA of bacteria native to the bovine rumen. They could then feed the altered bacteria to a cow. In the cow's rumen, the bacteria might become a stable colony, producing offspring that continue the production of CLA-synthesizing enzymes at high levels. For the rest of its lifetime, the cow might produce milk with a high CLA content.
However, many questions about CLA remain unanswered. The short- and long-term human health consequences of ingesting CLA are unknown. "The application of CLA as an anticarcinogen in humans is intriguing but far from proven," says Dr. Pariza.
Other Possible Uses
CLA may have various physiological effects. Researchers have found that CLA can suppress atherosclerosis in rabbits. Dr. Pariza and others have suggested that CLA has antioxidative effects. Antioxidants may help prevent chronic diseases such as heart disease by inhibiting free radicals (disruptive atoms, and groups of atoms, that arise from normal metabolism).
CLA research may interest not only health professionals but also meat producers. According to the February 1996 issue of Inform, a publication of the American Oil Chemists Society, mice, rats, chickens, and rabbits on diets that included CLA ate less overall and showed an increase in body protein and a drop in body fat. Thus, supplementing animals' diets with CLA might result in leaner meat and lower feed costs.
The Bottom Line
Conjugated linoleic acid (CLA) ranks among the most potent natural anticarcinogens for animals ever identified, and researchers are investigating ways of increasing its concentration in dairy products. But the carcinogenicity or anticarcinogenicity of a given substance in laboratory animals does not necessarily indicate either in humans. How indeed, whether CLA can improve human health remains uncertain.
Selanna Reiner holds a B.A. degree in biology from Oberlin College.
Molecular Structure of CLA
Like all other fatty acids, conjugated linoleic acid is basically a chain of carbon atoms. The position and types of bonds distinguish CLA from other fatty acids.
C
carbon atom
-C-C-
single bond (-)
-C=C C C=C- -C=C-
Detail of Linoleic Acid
double bond (=)
-C=C C=C-
Detail of Conjugated Linoleic Acid
In part of the molecule, a double bond is followed by two single bonds and then another double bond. In part of the molecule, a double bond is followed by one single bond and then another double bond. This alternation of double and single bonds is called "conjugation," hence the term "conjugated linoleic acid."
