Originally appeared in the UC Berkeley Wellness Letter, May 2002
Do flaxseed muffins fight breast cancer and prostate cancer? Should we all be eating flaxseeds and using flaxseed oil on our salads? Some people would say yes, and it's true that recent research on the potential health benefits of flax has been promising. But it pays to delve deeper.
The flax plant, an ancient crop, yields the fiber from which linen is woven, as well as seeds and oil. The oil, also called linseed oil, has many industrial uses—it is an important ingredient in paints, varnishes, and linoleum, for example. Flaxseed oil also comes in an edible form, sold mostly at health-food stores. Like olive, canola, and most other plant oils, it is highly unsaturated and heart-healthy. And flaxseeds have yet another very interesting component—lignans—which may have anti-cancer properties.
Plant hormones: cancer protection?
Lignans are a type of fiber, and at the same time a type of phytoestrogen—a chemical similar to the human hormone estrogen. Flaxseeds are the richest source of lignans. When you eat lignans, bacteria in the digestive tract convert them into estrogen-like substances called enterodiol and enterolactone, which are thought to have anti-tumor effects. Lignans and other flaxseed components may also have antioxidant properties—that is, they may reduce the activity of cell-damaging free radicals. (Flaxseed oil lacks lignans, but some processors add them to their oil.)
Recently small studies of cancer patients who consumed flaxseeds have produced some encouraging results. In one study men with prostate cancer who ate an ounce of ground flaxseeds (almost three tablespoons) a day as part of a very-low-fat diet were able to slow the progress of their cancers between the time they were diagnosed and the time of surgery. A similar study of women awaiting surgery for breast cancer found that those who ate a flaxseed muffin daily (with about four tablespoons of ground flaxseeds per muffin) had a slower tumor growth rate. Studies of animals, too, suggest some anti-cancer benefit from flaxseed. But it's always difficult to know whether it's the lignans that help, or some other element in the flaxseeds. And not all studies have yielded positive results.
It is still too early to say that flax can prevent or cure cancer and to recommended it for that purpose. It's important to remember that plant estrogens, like human hormones, are not always benign. At high doses—and no one knows how much is too much—lignans might turn into cancer promoters. Indeed, some animal studies have found that high doses of plant estrogens can cause cancer cells to proliferate. We have no idea where that line—between enough and too much—might be drawn. All we can do is wait for further developments.
The heart-healthy side of flax
Besides lignans, flaxseeds and their oil are also the best food sources of an essential fatty acid, alpha-linolenic acid. "Essential" means we must consume it, because our bodies cannot manufacture it. Essential fatty acids are important for cell membranes, blood pressure regulation, and other functions. Alpha-linolenic acid is an omega-3, similar to some of the fatty acids in fish oil. Like aspirin, omega-3s may reduce blood clotting, thus lessening the chance of a fatal heart attack. Flaxseeds and their oil may also lower total blood cholesterol, as well as LDL ("bad") cholesterol. But that should come as no big surprise, since any highly unsaturated oil will do that, particularly if substituted for saturated fats. The fiber in flaxseeds may also help against cholesterol, since it is soluble (similar to that in oats).
Several population studies have linked a high intake of alpha-linolenic acid with a reduced risk of heart disease and/or death from heart disease. And a French study, as we reported in 1999, found that a diet relatively rich in alpha-linolenic acid greatly reduced the risk of second heart attacks. (The alpha-linolenic acid in that study did not come from flaxseeds, but from canola-oil margarine.) Besides flaxseeds and canola oil, alpha-linolenic acid is also found in soybean oil and walnuts.
Good food, no magic bullet
All plant foods, including flax, have good things to offer. Garlic, onions, tomatoes, broccoli, legumes, and whole grains all have a range of beneficial chemicals. If you want to add flaxseeds to your diet, that's a good idea. But if cheeseburgers are your main source of calories, adding flaxseeds won't help much.
Flaxseeds have a pleasant, nutty flavor and taste good sprinkled on salads, cooked vegetables, or cereals. The oil is quite tasty, too, though expensive. Here are some flax tips:
• Grind the seeds or else chew them very well—whole seeds simply pass through the body. Grinding the seeds just before using them best preserves flavor and nutrition, but pre-ground seeds are more convenient. Keep them refrigerated. There are no nutritional differences between brown and yellow seeds. • Combine flaxseed flour with wheat flour for breads, quickbreads, and pancakes. • Ready-made flaxseed breads, muffins, cereals, and breakfast bars can be found in many stores. • The oil spoils quickly; it comes in dark bottles to extend its shelf life. Keep it refrigerated, and pay attention to the expiration date. "Cold-pressed" flaxseed oil is more expensive but no better than other kinds. • Flaxseed oil cannot be used for frying or sautéing. • Pregnant or lactating women should not eat lots of flax. • A few people may have allergic reactions to flaxseeds. • Pass up flaxseed supplements—eat the foods instead.