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

I have said, I would admit when I was wrong and when I changed my thinking.  Back in 2005, I mentioned an article that suggested that flax increased the risk of prostate cancer. Specifically, it referred to alpha linolenic acid, the main short chain source of Omega 3’s in flax. I suggested it might overlap to breast cancer as well, since (yes, a little ahead of my time here) the issue was of increased Estrone, a pathogenic form of estrogen that seems to accumulate in men and women, after menopause and andropause.

Similarly, there was a lot of concern about soy based phyto estrogens in people who already had breast cancer. Lately my stance is I would not worry about it too much. Most of the studies I have seen, since 2005, suggest positive health benefits from lignans (phytoestrogen precursors in brans and flax) of which flax does really seem to have a huge amount. There is at least one older study, suggesting an increase in prostate cancer, with flax, but there are a lot of unanswered questions, such as: how was the study done?  Most are either the notorious “food questionnaire”, or lab cell cultures meaning – well, who knows what they really mean in free living people.  If you have had either of these cancers though, you might want to avoid flax just in case!  You see, the issue is not totally settled.  But it is clear to me I was heavy handed back then and I want to make amends!

A much bigger issue is the poor conversion of medium (also called short in the industry) chain 18 carbon fatty acids, from plants and seeds, which is about 5%. I did a finger stick Omega 6/3 test on over 100 vegetarians and vegans, many of whom were supplementing with flax or algae-derived Omega 3’s.  Not one of them had anywhere near a healthy ratio. Bottom line, if you are going to use flax, you pretty much need to use the straight oil and consume a minimum of 4 ounces a day, to get your levels anywhere near a healthy ratio for primary prevention.

I would remind you that marine-based Omega 3 supplementation has positive effects on the incidence of both prostate and breast cancer, far outweighing flax.  The amount of studies is about 12,000 for fish oil in human health, with no specific manufacturer funding and 60-ish for flax and other plant-based sources. It’s about 10 and all company funded, for krill.

While I was looking at flax and soy, I also checked the stats for ovarian cancer.  No solid data either way, it seems. Nothing has reached statistical significance, but again, there seems to be no cause for concern.

What about colon cancer?  Preliminary data in rodents suggests a possible protective effect, but again no clear cut data.

When I reviewed the data on fermented versus non-fermented soy products, I could not find any differences with either type.

So the bottom line is that unless you have already had a breast or a prostate cancer, you probably should not be too concerned about soy consumption. It does not appear to protect you or put you at risk.  So why do we see diminished rates in Oriental populations, with a seemingly direct link to soy consumption? I think I can correctly invoke our old (or new!) friend epigenetics here.  Most of the populations involved in this type of study are more homogeneous genetically, environmentally, socially and epigenetically than in this country, or many Western nations.  And in most of the soy consuming nations, the soy consumption is ongoing from a very young age, including prior to puberty.  Not so in the West.

As far as flax, the conversion of lignans to phytoestrogens yields a different mix than soy and the data is a bit more promising. Remember again, about 5% of the Omega 3’s in flax are effectively converted to the long chain form and this may account for the small but positive data in breast, prostate and colon cancer.  If you are hard core vegan/vegetarian, you will need to consume a lot of flax seeds for lignans and oil for Omega 3, but I suppose it can be done. Also, remember the benefits of sulforaphane and DIM, both from broccoli and broccoli sprouts, and other cruciferous vegetables of the Brassica family.

You really do need to be careful about interpreting studies and of course, much more careful interpreting internet headlines!  A few tricks researchers play include claiming benefits to certain food groups when compared to other food groups.  A favorite is corn or other Omega 6 rich foods, which basically guarantee you will have a higher rate of tumor formation.  This was the case in more than one study I researched and it pretty much deflates those claims in terms of true prevention, although most people do eat too much Omega 6!

It took a long time and lots of studies for me to admit I was wrong, but you’ll pardon me.  Better late than never.

Humbly yours,

Dr Dave

Here are some references:

Soyfoods, isoflavones and risk of colonic cancer: a review of the in vitro and in vivo data


Consultant, Nutrition Matters, Inc.; Adjunct Associate Professor

Loma Linda University, California, USA


Professor, Department of Food Science and Human Nutrition

Michigan State University, East Lansing, Michigan, USA


Phytoestrogen consumption from foods and supplements and epithelial ovarian cancer risk: a population-based case control study

Elisa V Bandera1,2*, Melony King2, Urmila Chandran1,2, Lisa E Paddock2,3, Lorna Rodriguez-Rodriguez4 and Sara H Olson5

Anna Hsu, Carmen P Wong, Zhen Yu, David E Williams, Roderick H Dashwood, Emily Ho. Promoter de-methylation of cyclin D2 by sulforaphane in prostate cancer cellsClinical Epigenetics, 2011; 3 (1): 3 DOI:10.1186/1868-7083-3-3

Gynecol Oncol. 2012 Mar;124(3):410-6. Epub 2011 Nov 23.

A randomized parallel-group dietary study for stages II-IV ovarian cancer survivors.

Paxton RJ, Garcia-Prieto C, Berglund M, Hernandez M, Hajek RA, Handy B, Brown J, Jones LA

Nutr Cancer. 2006;54(2):216-22.

Chemopreventive effects of dietary flaxseed on colon tumor development.

Bommareddy A, Arasada BL, Mathees DP, Dwivedi C.


Department of Pharmaceutial Sciences, South Dakota State University, Brookings, SD 57007, USA.

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