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That Old Flax Magic—Real or Imaginary?

Some people believe that eating flaxseeds protects against breast and prostate cancer. Others think it increases the risk of those cancers. Then there are those who claim that flaxseeds reduce menopausal symptoms and lower cholesterol. These are only a few of the benefits (and drawbacks) claimed for flaxseeds.

Should you therefore ignore the high price and short shelf life of flaxseed oil and use it in place of olive, canola, and other vegetable oils? Should you make sure to eat some flaxseeds daily? There’s no denying that flaxseeds contain interesting components, and there has been a lot of promising research. But the benefits of flaxseeds and their oil remain hard to pin down.

Since Biblical times . . .

Flax is an ancient and useful crop, domesticated in the Fertile Crescent thousands of years ago, yielding linen fibers for cloth, as well as seeds and oil. The oil, also called linseed oil, has many commercial uses, as an ingredient in paints, varnishes, and linoleum (named for linseed). Edible flaxseed oil is rich in unsaturated fats and is in the same healthful category as canola and olive oil.

Flaxseeds are very rich in lignans, a kind of plant estrogen, or phytoestrogen, similar to the phytoestrogens in soy and to human estrogen. When you eat lignans, bacteria in your digestive tract convert them into estrogen-like substances called enterodiol and enterolactone (two mammalian lignans). Like the phytoestrogens in soy, these may have anti-cancer effects. They may also have antioxidant properties, which would mean they deactivate cell-damaging free radicals. But all this remains theoretical. Also, there is nothing magical about the lignans in flax. Lignans are widespread in plants—found, for example, in sesame and pumpkin seeds, cranberries, and even tea and coffee. Flaxseed oil, by the way, has no lignans, though some manufacturers add them to the oil.

But there’s more to flaxseeds than that. The seeds and their oil are the best food sources of an essential fatty acid called alpha-linolenic acid (ALA). "Essential" means the body needs it but does not manufacture it, so we must consume it. ALA is an omega-3 fat, similar to the fats in fish. It may reduce blood clotting and lessen the chance of a heart attack. It may also lower blood cholesterol. However, the body cannot utilize omega-3s from plants as efficiently as those from fish.

Lots of maybes

The operative word for the health benefits of flaxseeds and flaxseed oil is "may." It is still too early to say that flax can prevent or treat any kind of cancer. Plant estrogens are not always benign. At high doses, lignans might turn into cancer promoters. Indeed, some doctors warn men with prostate cancer not to eat flaxseeds—though, again, the evidence is not clear. As for the benefits of flaxseeds in lowering blood cholesterol and preventing heart attacks, these too are unclear. Most studies have not been of top quality. Until a great deal more is known, flaxseeds and their oil should be regarded—like oats, soy, fish, and nuts—as good foods, best consumed in moderation.

Bottom line: Flaxseeds and their oil contain beneficial chemicals, but they are not the only sources of ALA or lignans. Walnuts, canola oil, and soybean oil contain ALA. Indeed, the studies showing that ALA may help prevent heart attacks were done not with flaxseeds but with canola oil and/or walnuts. Lignans come from many other plant foods. You can have a very healthy diet without ever eating flaxseeds or their oil. Still, flaxseeds are nutritious. They have a pleasant, nutty flavor, and taste good when added to other foods or baked in quick breads, such as muffins.

Some flax tips

• Whole flaxseeds simply pass through the body, so grind them just before eating (to preserve flavor), then refrigerate them. There is no nutritional difference between brown and yellow seeds.

• Flaxseed oil is tasty, though expensive and quick to turn rancid. You are just as well off with olive or canola oil. If you do buy it, keep it refrigerated. Like all oils, it is high in calories. It is not suitable for high-temperature frying.

• Children and pregnant or lactating women should not eat lots of flax because of its potential hormonal effects.

• Flaxseeds and their oil can have a laxative effect.

• If you take any medication or are undergoing cancer treatment, talk to your doctor before adding lots of flaxseeds to your diet. Flax can interact with some drugs, including warfarin and other blood thinners such as aspirin (increasing the risk of bleeding) as well as tamoxifen, a breast cancer drug.

UC Berkeley Wellness Letter, August 2007