A man's testes might be safe from yellow dye no. 5 and cellphones (maybe), but apparently soya-based foods pose another problem.   A new study by the Harvard school of public health found certain ingredients in soya that could lower sperm production.

Soya is found in roughly two-thirds of manufactured food, including biscuits, pasta, sweets, and certain breads.  The protein substitute has risen steadily in western diets since the 1940s.  Today about 20% of young European men have low sperm counts, and in America, in 30-40% of couples experiencing fertility problems--it's because of the man.  

The study found that men who consumed more than two portions of soya-based foods a week had, on average, 41m fewer sperm per milliliter of semen than men who had never eaten soya products.  80m and 120 m per ml is considered average, but anything lower than 20m per ml is regarded as clinically subfertile.  

The study conducted by Jorge Chavarro was written about in the Guardian:

In the biggest human study into the effects of soya on fertility, Chavarro and colleagues at Massachusetts General hospital recruited 99 men who had visited a fertility clinic between 2000 and 2006. The men were asked to fill out a questionnaire which asked them about the amounts of 15 different soya foods they had eaten over the previous three months. The researchers then put the men into four groups according to the levels of chemicals called isoflavones in their diets. Isoflavones are ingredients in soya products that mimic the female sex hormone, oestrogen. Each man then provided a sperm sample for testing.

Chavarro found that men who consumed at least half a portion of soya food a day had the lowest sperm counts.

The study is to be published in the journal Human Reproduction, and doctors are already warning men with low sperm counts to stay clear away from soya.