TORONTO - Men who are concerned about infertility and contemplating fatherhood are often advised to avoid smoking, excessive alcohol intake and tight underwear. But they don't need to worry about consuming soy, according to research on semen quality, described by one leading urologist as "quite reassuring."
Evidence is mounting that soy consumption may be beneficial in reducing the risk of heart disease and prostate cancer, as well as in alleviating hot flashes and night sweats in menopausal women.
However, over the years, there have been questions about whether ingesting soy might have negative consequences for men's fertility.
"Soy beans are a plant and they have phytochemicals in them, and a common phytochemical is isoflavones," explained Alison Duncan of the department of human health and nutritional sciences at the University of Guelph in Ontario. "And isoflavones are a type of phytochemical classified as a phytoestrogen, which means that they have a very structural similarity to estrogen in our bodies, and they can bind to the estrogen receptor.
"A lot of interest from the health aspect of isoflavones has been from the perspective of hormone-related conditions like menopause, menopausal symptoms, osteoporosis, heart disease, breast cancer, prostate cancer. And a lot, a majority, of these studies have shown beneficial effects," she said.
"In the face of all this, there's been concern that maybe these estrogenic isoflavones could have adverse effects on men."
Dr. Keith Jarvi, a professor of urology at the University of Toronto, said fertility specialists have never actually stopped men from consuming soy-based products because of this.
"It was more or less a theoretical concern, and there were a few animal studies that indicated that maybe in some rodent models that they might have an adverse effect on fertility -- but typically those doses were pretty high that were given," said Jarvi, director of the Murray Koffler Urologic Wellness Centre at Mount Sinai Hospital in Toronto.
"We didn't really think that that was a serious concern, so it hasn't really been, for us, something that we've been discussing with patients. On the other hand, it's always been sitting at the back of our minds that, yet, this is a possibility that could occur."
Duncan's team was already doing work funded by the American Institute for Cancer Research on the effects of soy on prostate cancer risk in men, and she decided it made sense to collect other samples that might be of interest at the same time.
"That's why we decided to look at the semen samples and look at the sperm motility and morphology in response to consumption of soy that had high isoflavones and low isoflavones, to see if there is any possible dose response."
Thirty-two healthy young men agreed to take part and drink a daily shake made from reconstituted powder. For three 57-day periods, they consumed a milk protein isolate, a low-isoflavone soy protein isolate or a high-isoflavone soy protein isolate, until they had gone through all three kinds of drinks. In between, there were 28-day washout periods when they took a break.
Isoflavones were measured in urine samples collected near the end of each treatment period, and semen was collected on the first and last day of each treatment period.
"We found absolutely no effects of consuming the soy, regardless of how much isoflavones were in it, on the semen parameters, either semen volume, sperm concentration, sperm counts, sperm morphology, sperm motility. There was no changes at all," Duncan said of the findings, which were published late last year in the journal Fertility and Sterility.
Duncan said there's always room for further research on the subject. She said she approached the soy products company Solae to provide protein powder for the study, and noted the company is unbiased and doesn't put any pressure on researchers. She wanted to use a consistent product rather than have the men eat tofu, for instance, which could have meant more variable intake.
Jarvi said the finding of no impact on men's sperm is reassuring.
"As in all science, someone's going to come along and say it's got to be repeated and done somewhere else, right? But as a first pass, this is a mammoth study," he said.
"It's reassuring because of the widespread use of soya that it doesn't have this impact -- at least we had some animal studies that gave us a little bit of concern before that it might have an effect on the size of the testicles in rodents.
"Again, you know, rats and men are actually, despite what you might think, quite different," he added with a chuckle.
