HEALTH WATCH: Flax seed and prostate cancer

By Michael Greger M.D
Why is there such a huge disparity in prostate cancer rates around the world? The incidence of malignant prostate cancer is highest in African Americans, some 30 times greater than in Japanese men, and 120 times greater than in Chinese men.

The conventional thinking is that this may be due to the higher intake of animal fat and protein in the Western diet, but it could also be the protective phytoestrogens found in plant foods. There are two major types of phytoestrogens: soy isoflavones and lignans.

Researchers have found higher levels of lignans in the prostate fluids of men in countries with relatively low rates of prostate cancer and in vitro studies show lignans can slow the growth of prostate cancer cells in a petri dish, so a pilot studywas performed on flaxseed supplementation in men with prostate cancer. Why flaxseeds? Because while lignans are found throughout the plant kingdom, flax has up to 800 times more than any other food.
The research team took a bunch of men with prostate cancer, about a month before they were scheduled for surgery to get their prostates removed, and put them on a relatively low fat diet with three tablespoons a day of ground flax. Though the scientists were skeptical that they would observe any differences in tumor biology in the diet-treated patients in such a short time span, they found significantly lower cancer proliferation rates and significantly higher rates of cancer cell death. That was compared to so-called “historical controls,” meaning compared to the kind of growth one typically sees in their situation, not to an actual randomized control group.
A few years later, though, a controlled study was published.
Researchers enrolled men who recently had their prostates biopsied and were scheduled to have repeat biopsies in six months. Then they did the same thing as the previous study: they reduced the fat in their diet and put them on ground flaxseeds to see if it made their repeat biopsy look any different. These were men with what’s called PIN (prostatic intraepithelial neoplasia), which is like the prostate equivalent of ductal carcinoma in situ in the breast. That’s why they were getting repeat biopsies–to make sure it wasn’t spreading.
There hadn’t been much research on this kind of prostatic hyperplasia, with only four epidemiologic studies reported at the time. They yielded varying findings, with increased risk associated with higher energy, protein, and animal product intake, and decreased risk related to the consumption of alcohol, fruit, and green and yellow vegetables—in sum, a low-fat, plant-based diet, high in phytoestrogens. The researchers wanted to know if that kind of diet could be used to treat it too.
Study subjects experienced a significant drop in PSA levels (a biomarker of prostate cell growth), a drop in cholesterol (what one would expect with a lower fat diet with extra fiber), and most importantly, a significant decrease in the cellular proliferation rate. In fact in two of the men, their PSA levels dropped so much they didn’t even have to go through with the second biopsy!
Since the dietary intervention involved both reducing fat intake and flaxseed consumption, how do we know the flax had anything to do with it? Given the composite nature of the intervention—both a lower fat diet and flaxseeds, it was unknown whether the effects could be attributed to flaxseed supplementation, a fat-restricted diet, or both factors working together. To figure that out you’d have to do a study where you split men into four groups, a control group, a flaxseed only group, a lower-fat only group, and then a flaxseed and lower fat group. And that’s exactly what they did. Find out the results in my follow-up video Was It the Flaxseed, Fat Restriction, or Both?.
That reminds me of the experiment described in Is It the Diet, the Exercise, or Both? in which researchers try to tease out the individual effects of a similar composite treatment—a plant-based diet and walking—on the growth of prostate cancer cells in vitro. They both appeared to help, but diet appeared to be more powerfully protective.
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