US researchers say men who regularly eat processed meats like bacon, sausages and hamburgers have significantly lower amounts of normal sperm.

The Harvard University researchers examined 156 men who were having fertility problems. They looked at their diet, and the size and shape of their sperm.

Men who regularly ate processed meat had significantly lower amounts of normal sperm, compared with those who did not.

The results are being presented this week at a meeting of the American Society for Reproductive Medicine in Boston.

Professor Chris O'Neill, who specialises in reproductive and developmental medicine at the University of Sydney, says he is not surprised by the findings.

"These are diets that tend to result in low-level chronic inflammation of tissues within the body, and when you get that sort of low-level inflammation it's associated with a whole range of adverse health outcomes, such as heart disease and stroke and diabetes," he said.

"And it is also associated with reduced fertility... particularly in men, so any aspect of a man's environment which reduces his health will almost invariable have some impact on his sperm quality and function."

The Harvard researcher say men who want to boost their odds of having kids should instead eat white fish.

They say species such as cod seem to have a particularly positive effect on fertility.

However, Professor O'Neill believes the findings need further scrutiny.

"It is a very preliminary finding which will then need to be verified by a much more powerful prospective study, and that means doing a comparison of men on different diets and men looking at their fertility and their sperm function," he said.

Professor O'Neill's view is backed by the president of the Fertility Society of Australia, Mark Bowman.

"I think it is one of those things where you can’t draw judgements and make hasty conclusions for example of an entire branch of the food industry," Dr Bowman said.

"I think that would be very unwise and unfair to do that. The difficulty with sperm morphology, which is sperm appearance, is that is has a very wide reference range anyway.

"In other words, not all different looking sperm are actually abnormal sperm. It is a complicated area.

"And when you are dealing with low numbers in the sample size that makes the statistical relevance more difficult to draw judgements upon."

Dr Bowman says what is clear is that lifestyle choices impact on a couple's chances of having children.

"There is increasing evidence that diet and lifestyle has an effect on human fertility in all sorts of ways, and we known that quite clearly in terms of detrimental things like cigarette or excessive alcohol intake," he said.

"I guess what is progressively emerging is that there may be other influences of our diet upon fertility, and this is another study that is leading in that direction.

"So, I don't think we should be raising massive alarm bells that people are absolutely ruining their fertility just simply through one particular type of food.

"I think it just gives us this general trend that the healthier we are, the more likely our fertility is going to be good."

Dr Bowman has this advice for those struggling to have kids: "The number one thing is, don't be too over interpretive about these kinds of things."

"But yes, make absolutely sure that you are having a well-rounded, well-balanced diet - all of the food groups in their appropriate proportions - and that you are undertaking a reasonable level of exercise," he added.