The study, conducted at the University of Rochester and published in the journal Human Reproduction, formed part of the federally-funded multi-centre Study for Future Families, involving a cohort of pregnant women and their partners who provide data for scientists to investigate environmental causes for variations in reproductive health. The findings have been widely reported in the mainstream press, and there is a possibility that this could have some impact on beef consumption. But the American Meat Institute (AMI) has strongly criticised the methodology and conclusions, most notably because the association of the observed effect with chemical compounds in meat is purely speculative. For this arm of the SFF, 387 fathers-to-be, born between 1949 and 1983, asked their mothers to complete a questionnaire about their prenatal diet. During this period the researchers, led by Shanna Swan, PhD, said that "numerous chemical additives" were used in meat in the US, and it would have been difficult for women to avoid hormone residues. The researchers studied the men's semen quality for movement, concentration and other properties, and used statistical methods to relate each man's semen quality to the data on his mother's diet. They found that of the 51 men whose mothers reported eating the most beef during pregnancy, 18 per cent exhibited sperm counts of 20 mill/ml or less - classified by the WHO as sub-fertile. High beef consumption was defined as an average of more than one beef meal per day, and the average was 4.3 per week. Conversely, sperm concentrations were seen to be 24 per cent higher for men whose mothers ate less beef, and only five per cent were classified as sub-fertile. None of the men, no matter how much beef their mothers had eaten, were actually infertile: all had conceived a child without medical assistance. No correlation was seen between the men's sperm quality and their mothers' pre-natal consumption of other meats (pork, veal or lamb), fish, chicken, soy or vegetables. Moreover no correlation was seen between sperm quality and the men's own beef consumption. But although Dr Swan said that there were several possible explanations for the findings, including pesticides and other contaminants in cattle feed and lifestyle factors during pregnancy, she was cautious in her interpretation of the data. "This needs to be followed carefully before we can draw any conclusion". The researchers were not able to pinpoint any specific hormones, pesticides or other chemicals in the animal fat as a direct cause. The AMI noted that the study did not include any laboratory analysis of compounds suggested to be contained in beef - nor of the actual beef reportedly consumed decades ago. "To conclude that some undetected compound is the cause for an association seen in these data is of questionable validity," said AMI VP of scientific affairs Dr Randy Huffman. But Swan said: "What we're really trying to do here is raise an issue." Anabolic steroids are still used in the US today, although the FDA has set a limit on acceptable daily intake of the six hormones commonly used in cattle and diethylstilberstrol, the first synthetic hormone, was banned in 1979. In the EU, growth promoters in cattle were banned in 1988. "One way to determine if prenatal exposure to anabolic steroids is responsible for a change in sperm count would be to repeat this study in men born in Europe after 1988," said Dr Swann. An obvious limitation of the US study is that mothers were asked to report on their diet decades afterwards, which is likely to yield less reliable results than when someone reports on current food intake. However Swan believes that women are more likely than most to remember accurately what they have eaten during pregnancy, since at this time they pay particular attention to nutrient intake - and may avoid certain foods that make them feel sick. But the AMI called this procedure "absolutely absurd" to expect any degree of accuracy in the recall. Moreover, it criticised using the study subjects themselves to interview their mothers, in place of professional interviewers.