"Phytoestrogens" are substances found in plants that can imitate (to a certain extent) the activity of estrogens in humans. Plants cannot make human estrogens! In fact, even though plants do make some hormone-like substances that help to regulate their reproduction, none of these substances are the same as the human reproductive hormones we call estrogens. But phytoestrogens from plants can definitely impact hormonal metabolism in humans—in both males and females. Because much of the research on phytoestrogens has been related to breast cancer there has been a natural emphasis on the role of phytoestrogens in women's health. But men are affected by phytoestrogens as well, even though the research in this area is a little more scattered and a little less conclusive.
You asked about flaxseeds, which is a food that contains phytoestrogens. The phytoestrogens found in flaxseeds fall primarily into one category of phytoestrogenic substances called lignans. (Matairesinol and secoisolariciresinol are examples of well-studied phytoestrogens that fall into this lignan category.) But there are many categories of plant substances that are known to function as phytoestrogens in humans. The isoflavones found in soybeans are one good example. (This phytoestrogen category contains the important phytoestrogens genistein and daidzein.) Trans-resveratrol (one of the fascinating health-supportive substances found in red grapes) also functions as a phytoestrogen.
The best studies I've seen on dietary phytoestrogens and male hormonal balance has involved the cardiovascular system. These studies show that men with high intake of phytoestrogen-containing foods have increased dilation (expansion) in some of their small arteries, improving their blood flow, the distribution of their blood, and their blood pressure regulation. Some of these effects are likely to involve changes in hormonal metabolism brought about by these phytoestrogens. In other words, the hormonal impact of phytoestrogens in this situation seems to be a good thing!
I've seen studies on male rats showing decreased fertility following consumption of diets high in phytoestrogens, but the connections here seem indirect and appear to involve more complicated relationships involving not only hormonal but other mechanisms (including oxidative stress). I've also seen studies on ganders (male geese), but like the rat studies, I am unable to come away with any hard and fast conclusions about the impact of phytoestrogen-containing foods for men from these studies.
It's important to remember that phytoestrogens like lignans or isoflavones have a wide variety of roles, not only roles related to hormone metabolism. For example, perhaps unrelated to their estrogen-like effects, these substances appear to be helpful in improving blood-sugar balance, insulin metabolism, and weight control in both men and women. Moreover, foods like flaxseed and soybeans have hundreds of other constituents and are known to bring health benefits when included in the diet.
I believe the jury is still out with respect to the precise impact of phytoestrogens on male hormone balance. But I believe that there is likely to be far more good news here than bad.
For more information on this topic, please see:
Comhaire FH, Mahmoud A. The Role of Food Supplements in the Treatment of the Infertile Man. Reprod Biomed Online. 2003;7(4):385-91.
Lin X, Switzer BR, Demark-Wahnefried W. Effect of Mammalian Lignans on the Growth of Prostate Cancer Cell Lines. Anticancer Res. 2001;21(6A):3995-9.
McCann MJ, Gill CI, McGlynn H, et al. Role of Mammalian Lignans in the Prevention and Treatment of Prostate Cancer. Nutr Cancer. 2005;52(1):1-14.
van der Schouw YT, Sampson L, Willett WC, et al. The Usual Intake of Lignans but Not That of Isoflavones May Be Related to Cardiovascular Risk Factors in U.S. Men. J Nutr. 2005;135(2):260-6.
War WE, Yuan YV, Cheung AM, Thompson LU. Exposure to Flaxseed and Its Purified Lignan Reduces Bone Strength in Young but Not Older Male Rats. J Toxicol Environ Health A. 2001;63(1):53-65.