Does flax or soy increase the risk of breast and prostate cancer?

I said, a long time ago, that I would tell you when I changed my mind. Here is one of those times. Back in 2005, I wrote that flax probably increased prostate cancer risk because of its conversion to estrone, a less favorable estrogen metabolite that accumulates after menopause and andropause. I extended the worry to breast cancer. I was, in retrospect, heavy-handed about it.

Most of the studies I have seen since then suggest that the lignans in flax, the phytoestrogen precursors that show up in flaxseed and certain brans, are net positive for most people. The worry about prostate cancer rests largely on one older paper that has not been well replicated. The mechanism that originally concerned me, alpha-linolenic acid as a possible prostate cancer driver, also has not held up well in subsequent work.

The exception I would still respect

If you already have a hormone-sensitive cancer history, breast or prostate, I would still take the conservative approach and limit flax until you and your oncologist have agreed on a plan. The data is not strong in either direction, and the cost of being cautious is essentially zero.

The bigger problem with flax

This is the part of the conversation that gets lost. Even if flax is benign, it is a poor source of the omega-3 fatty acids most adults are actually short on. Plant-based ALA converts to EPA and DHA at a rate of roughly 5 percent in most adults, often lower. I have tested over a hundred long-term vegetarians and vegans who were diligently supplementing with flax oil or algae-derived omega-3. Not one of them had an omega-3 index in the protective range.

If you want the omega-3 benefit from flax alone, you are looking at roughly four ounces of straight oil per day for most people. That is a lot of oil and a lot of calories.

The cancer data on marine omega-3

Marine-source omega-3 supplementation has a much larger and more consistent literature than flax does for both prostate and breast cancer. The total study count for fish oil in human health applications runs in the thousands. The total study count for plant-based omega-3 sources runs in the dozens. The krill literature is smaller still and largely funded by krill companies, which colors how I read it.

On soy

The soy isoflavone story has shifted similarly. The early concern was that phytoestrogens would drive breast cancer recurrence. The longer-term data, including a large cohort follow-up from the Shanghai Breast Cancer Survival Study, suggests that moderate soy intake from whole soy foods, not isolated isoflavone supplements, is at worst neutral and at best mildly protective in postmenopausal women. The American Cancer Society updated its guidance accordingly.

I would still avoid concentrated soy protein isolates and soy-based supplement products of unclear provenance. The Monsanto connection to industrial soy production is a separate concern I have written about elsewhere.

On ovarian cancer

The data on flax, soy, and ovarian cancer is sparse and mostly uninformative. I would not draw conclusions in either direction yet.

What I tell patients now

• Use flax in food, ground fresh, as one part of a varied diet. Do not rely on it for omega-3.
• Eat whole soy foods, tempeh, edamame, miso, in moderation if you tolerate them. Avoid concentrated soy supplements.
• Get your omega-3 from marine sources, clinically dosed, and test your omega-3 index every year or two.
• If you have a hormone-sensitive cancer history, talk to your oncologist before adding either.

I should have hedged in 2005. I am hedging now. The good clinicians I respect change their minds when the evidence does.

— Doc