By Harvard Health Publications

If you push a shopping cart down the aisles of your supermarket, you may get a bit confused. Are you at a market or a drugstore? Why do many products proclaim their role in fighting heart disease and cancer instead of their flavor or price? And why are so many dietary supplements filling shelves that once held real food?

It’s all part of a growing interest in "functional foods," products containing nutrients that may help fight disease. It’s no surprise that many of these foods can actually be beneficial; a healthful, well-balanced diet is, after all, one of the best ways to preserve health (see Harvard Men’s Health Watch, December 2002).

As you fill your cart with food that’s healthful as well as enjoyable, should you also make room for food extracts selected strictly for their purported health benefits? Ironically, perhaps, food extracts that are sold as dietary supplements claim many more benefits than the foods from which they were derived. That’s because the FDA must review food labels for scientific accuracy, but it lacks a similar jurisdiction over the supplement industry.

In many cases, there is just not enough reliable information to say whether a food extract is helpful or harmful. That’s the case with flaxseed oil, but since it’s one of the hottest supplements in the health food department, you should review the current data to decide for yourself.

What are flaxseeds?
They sound exotic, but if you visit cool farmlands in Canada, northern Europe, or even North Dakota, you may see the pretty blue flowers of the flax plant. When the flower fades, the plant forms a pod containing small, flat, brownish seeds. The seeds have a nutty flavor; they can be eaten whole, sprinkled on cereal or yogurt, or ground and added to baked goods. Finally, flaxseed oil can be extracted from the seeds and sold as a dietary supplement.

The flaxseed bonus
Flaxseeds have not become popular for their flavor or texture but for their potential health benefits. The ancient Greeks and Romans used them as a laxative, and a few modern consumers have the same goal. But most people buy flaxseeds and flaxseed oil because they’ve heard that they will help prevent cancer and heart disease.

The main components of flaxseeds are fat (41%), dietary fiber (28%), and protein (21%). Despite their tiny size, they are packed with hundreds of phytochemicals. But the health claims are based on just two of them. One is a lignan called secoisolariciresinol (SRL); the other is an omega-3 fatty acid known as alpha-linolenic acid (ALA). Both ingredients appear in whole flaxseeds, but most of the SRL is removed from the oil as it is extracted. SRL is responsible for most of the cancer-prevention claims for flaxseeds; ALA is the ingredient advocated for heart disease.

SRL, the flaxseed lignan
After you eat flaxseeds, the bacteria in your intestinal tract go to work on SRL, rapidly converting it into two other lignans, enterolactone and enterodiol. The names of these substances are not important, but their biological attributes may be: both are phytoestrogens, plant chemicals whose activities mimic those of female hormones.

Flaxseeds are not the only food with phytoestrogens. Far from it; in fact, soy is the most popular source. For years, women have been using soy in the hope that it will alleviate menopausal symptoms and reduce the risk of breast cancer. Similarly, men are attracted to soy in the hope that it will help prevent prostate cancer. The scientific evidence behind these hopes is preliminary and mixed, but there is sound evidence that eating a good amount of soy can lower blood cholesterol levels and reduce the risk of heart disease (see HMHW, October 2002). How do flaxseeds compare?

Flaxseeds won’t lower cholesterol the way soy can, and they lag far behind in vitamin E content (0.08 mg/gm vs. 0.94 mg/gm). They do generate much more enterolactone and enterodiol, but soy has other phytoestrogens. And in terms of actual cancer prevention, there is even less hard evidence supporting flaxseeds than soy. Flaxseeds do show some activity against breast cancer in rats, but there is no assurance that women will respond in the same way. Estrogens reduce the growth of prostate cells, but the effects of flaxseeds are mixed. A 5% flaxseed diet reduces prostate growth in rats, but a 10% diet increases testosterone levels and has the opposite effect. In mice that are genetically altered to increase the risk of prostate cancer, a 5% flaxseed diet appears to reduce that risk.

All in all, there is just not enough evidence to support a role for flaxseed lignans in fighting human cancers. But the fatty acids in flaxseed oil are another story. While ALA appears to have a protective role in cardiovascular disease, it may increase a man’s risk of prostate cancer.

ALA, the flaxseed omega-3
Omega-3s are polyunsaturated fatty acids that appear to promote cardiovascular health (see HMHW, May 2000). The evidence is strongest for eicosapentaenoicacid (EPA) and docosahexaenoic acid (DHA), the two long-chain omega-3s in fish. People who eat fish just twice a week appear to gain protection from heart attacks and sudden cardiac death. Fish oil extracts may also help, but the evidence is weaker. Other information suggests that men who eat a considerable amount of fish have a lower risk of prostate cancer than those who don’t (see HMHW, March 2001 and May 2003).

Plants don’t have EPA or DHA, but a few have another omega-3, ALA. Canola oil has been the chief dietary source of ALA, at least until flaxseeds came along; walnuts, wheat germ, and soybean oil also provide ALA.

The best evidence that ALA can fight heart disease comes from a study of canola oil. The Lyon Heart Study was a randomized clinical trial that tested a canola oil supplement in 605 men with coronary artery disease. Compared with a standard diet, the high-ALA diet was associated with a 72% reduction in heart attacks and cardiovascular deaths and a 56% lower risk of death from any cause during the four-year study. And two Harvard observational studies reported that men and women whose diets were high in ALA had a lower risk of dying from cardiovascular disease than those with low-ALA diets.

How do flaxseed oil and canola oil compare? Flaxseed oil has much more ALA, but much less of the monounsaturated fatty acids that give olive oil its good name. The table above compares the fat contents of flaxseed oil and some other foods.
This comparison may make flaxseed oil seem attractive — but there are two caveats. First, the demonstrated benefits of canola oil may depend in part on its monounsaturated fats rather than its ALA. Second, ALA itself may have a dark side.
The concern is prostate cancer, and it’s based principally on two Harvard studies of American men. In the 1993 Health Professionals Follow-up Study, the men with the highest dietary consumption of ALA had a 3.43-fold higher risk of advanced prostate cancer than those with the lowest consumption. Similarly, the 1994 U.S. Physicians’ Health Study reported that men with the highest blood ALA levels were 2 to 3.4 times more likely to develop prostate cancer than those with the lowest levels. Since then, two studies from other parts of the world linked high dietary ALA with prostate cancer, and two others found a link between blood ALA levels and prostate cancer. On the other hand, the men who used canola oil in the Lyon Heart Study had a reduced risk of urinary tract cancers — and three other studies have provided evidence that tends to exonerate ALA itself (see HMHW, January 2002).
None of these studies is conclusive, and none studied flaxseed oil itself. One 2001 study did test a daily supplement of 1 ounce of ground flaxseeds in men with prostate cancer. There was no drop in prostate specific antigen (PSA) levels, but the cancer cells appeared to be dividing less rapidly when pathologists examined surgical specimens. But the finding was subjective, and the study involved only 25 men and lasted for only a month, so it doesn’t shed much light on the safety — or effectiveness — of flaxseed supplements.

Are flaxseeds right for you?
Flaxseeds are heavily promoted in health food shops, and flaxseed oil is an increasingly popular dietary supplement. Flaxseeds are also finding their way into some commercially baked goods; at present, the FDA regards a flaxseed content of up to 12% as generally safe. And since flaxseeds are fed to some hens to increase the amount of omega-3 in their eggs, you may even find flaxseed-derived ALA in unexpected foods in your diet.

It’s too early to say whether flaxseeds are right for you. They do contain lignans that function as phytoestrogens — but so does soy. They are very rich in ALA, which may reduce the risk of heart disease — but may also increase a man’s risk of prostate cancer.

It will take time to learn whether flaxseeds are healthful or harmful for men. At present, the limited evidence suggests that the seeds are a better bet than the oil, though it’s still a betting proposition. While awaiting new studies, men who are attracted to flaxseeds should consider the alternatives. The present evidence favors fish as the best source of healthful omega-3 fatty acids; among the plant sources, walnuts and soy have the best track records. And, although the evidence is incomplete, soy may also provide lignans and proteins that may protect against cancer and heart disease.

Human nutrition is very complex; simple solutions to complex problems are not always best. Even when it comes to alternative medicine, there are alternatives.

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