Unpacking the tofu-sperm connection

London - There was an awkward pause on the phone the day before my last dinner party. A friend called to check details of my promised Chinese feast.

"Because the thing is," she mumbled, "we're trying for a baby and I read that tofu can lower men's sperm counts."

This was news to me. I'd come to think of all soy products as health-boosting "superfoods". Weren't they meant to lower cholesterol, help with menopausal symptoms and help us resist all sorts of cancer?

Our confusion is understandable. Over the past decade a range of scientific studies conducted around the world have concluded that these tasty little East Asian beans (and the many products derived from them) might help prevent or cause all manner of life-threatening or undesirable conditions.

So last year's "lowered sperm count" story (based on a study of the diets of 99 men by the Harvard School of Public Health) have just been counterbalanced by the results of research conducted by scientists from the Vanderbilt University school of medicine in Nashville, Tennessee, who studied the diets and health of 68 412 women aged 40-70 in Shanghai and found that those in the top third of soy food intake reduced their risk of bowel cancer by 30 percent.

Separate research from Taiwan last month also suggested that soy supplements could be used to ease menopausal symptoms instead of hormone replacement therapy, which has been linked to cancer.

Soy is believed to have an impact on many diseases because it contains isoflavones - phytoestrogens or plant hormones - which appear to act as a milder form of oestrogen.

And this is where things get tricky, as many human cancers have been linked to high levels of oestrogen, but many scientists also believe that by binding to oestrogen receptors and blocking access from the stronger oestrogens, isoflavones can reduce the risk of hormonal cancers.

Most of the human studies to date have involved Asian populations with different lifestyles, diets, genetic make-up and ability to metabolise soy and phytoestrogens which they traditionally eat in far greater quantities than the average Westerner.

Based on a 2000 adult survey, the mean population consumption of soya products in the UK is about 3g a day. And the Food Standards Agency believes that consumption of soy at this level is unlikely to have any health benefits or risks.

Doctor Allan Pacey, senior lecturer in andrology at The University of Sheffield, says that the plant hormones "may affect the initial development of the testicles if a pregnant woman eats lots of soya products".

"Adult sperm counts are largely determined by how big the testicles get at puberty, which is controlled by their cellular structure, laid down in the foetus. There is evidence to suggest that if a male foetus is exposed to compounds mimicking oestrogen, they may have a less well developed reproductive system that could lead to a variety of conditions, including a risk of infertility in adulthood."

But the Food Standards Agency advises that pregnant women don't need to avoid soya products if they're eaten as part of a healthy balanced diet. And in their 2003 report on Phytoestrogens and Health, the Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (Cot) considered that animal and human studies on soy-based infant formulae did not provide definitive evidence that soy-based infant formulae can adversely affect the health of infants.

However, the findings provided evidence of potential risks. The Department of Health advises that soya-based formula should only be given on the advice of a GP.

Doctor Justine Butler, senior health campaigner for the Vegetarian and Vegan Foundation, is one of the few scientists to have made a complete study of all the health risks and benefits of soy. She believes the Department of Health's "highly cautious approach is based largely on unfounded anecdotal evidence and animal-based experiments". Butler stresses that "millions of infants have been raised on soya formulas in the UK and US, many of whom are now well into their late 30s and early 40s."

Butler emphasises that soy is a good source of protein, containing essential amino acids which the human body needs.

"If my research flagged up any areas of potential risk," says Butler, "it would be for post-menopausal women with oestrogen-dependent breast cancer and possibly people with hypothyroidism, as a limited number of studies have suggested that isoflavones may affect thyroid function by lowering free concentrations of thyroxine."

These are the other two groups considered to be at potential risk by the COT report, too.

Meanwhile, the Food Standards Agency is commissioning a research project to consider potential adverse effects of phytoestrogens in men with low testosterone levels. But for the rest of us, it seems that so long as it's eaten in moderation and as part of a healthy balanced diet, soy can remain on the menu. - The Independent

• This article was originally published on page 7 of The Cape Times on May 08, 2009