The amount and type of fat in men's diets may affect the quality and concentration of sperm in their semen, according to a new small US study whose results need to be corroborated by a larger trial before we can say for sure whether this finding stacks up. But the researchers say in the meantime men already have much to gain by reducing the amount of saturated fat in their diet: we know too much of it is linked to poor health, and now it may also signify poorer fertility.

The study, by Jill Attaman and colleagues, is published in the 14 March online issue of the journal Human Reproduction.

Attaman was a clinical and research fellow in reproductive endocrinology and infertility at Massachusetts General Hospital, as well as an instructor in obstetrics, gynecology and reproductive biology at Harvard Medical School when they did the study.

In their study, of 99 American men, Attaman and colleagues found that a high total fat intake was linked to a lower total sperm count and concentration.

They also found that the sperm of men whose diets contained more omega-3 polyunsaturated fats, the type of fat that is often present in fish and plant oils, was better formed than that of men who ate less of these healthier fats.

However, they are careful to point out this was just a small study, the first to look at links between specific dietary fats and male fertility, and the findings need to be considered in the light of several limitations. These include, for instance, that the data came from food frequency questionnaires and may have missed some foods the men consumed, and there was only one sperm sample per participant. Also, being of cross-sectional design, it can only suggest links, it can't say there is a cause and effect relationship.

Attaman and colleagues suggest more research should now be done to find out for sure what role different types of dietary fat play in male fertility.

But in the meantime, they can still take action, says Attaman, "if men make changes to their diets so as to reduce the amount of saturated fat they eat and increase their omega-3 intake, then this may not only improve their general health, but could improve their reproductive health too."

"At a global level, adopting these lifestyle modifications may improve general health, as high saturated fat diets are known to be a risk factor for a range of cardiovascular diseases; but, in addition, our research suggests that it could be beneficial for reproductive health worldwide," she added.

Previous studies have looked at links between Body Mass Index (BMI) and semen quality, but the findings are not clear. Even less is known about the potential effects of different types of dietary fat on semen quality, which is what prompted Attaman and colleagues to start investigating.

They invited men attending a fertility clinic to take part in the study. Between the end of 2006 and the fall of 2010, 99 men answered questions about their diet, and gave semen samples for sperm analysis. The researchers were also able to measure levels of fatty acids in sperm and seminal plasma in 23 of the participants.

When they analyzed the results, the researchers put the men in three groups, according to the amount of fat in their diet: at the top was the third who ate the most fat, and at the bottom was the third who ate the least.

They found that compared with those in the bottom third, the top third, who ate the most fat, had a 43% lower total sperm count and a 38% lower sperm concentration.

Total sperm count is the total number of sperm in one ejaculation of semen. Sperm concentration is the amount of sperm in a millilitre of semen.

The results also showed that the relationships between fat in the diet and semen quality appeared to be largely driven by intake of saturated fats.

Compared to the third whose diets contained the least amount of saturated fats, the men in the top third saturated fat intake had a 35% lower total sperm count and a 38% lower sperm concentration.

Attaman said:

"The magnitude of the association is quite dramatic and provides further support for the health efforts to limit consumption of saturated fat given their relation with other health outcomes such as cardiovascular disease."

Another measure that experts take into account when assessing male fertility is sperm morphology, or the shape of the sperm. Normal sperm have an oval head and a long tail. Abnormal shapes occur when the head is too big or misshapen or the tail is crooked or there are two of them. Such defects can affect the ability of the sperm to get into and fertilize the egg.

In this study, the researchers found that the third of men who had the most omega-3 fats in their diets had slightly more normal-shaped sperm than the third who ate the least.

In their discussion, Attaman and colleagues point out that 71% of the men in the study were overweight or obese, which can also affect semen quality. However they did adjust for the potential effect of BMI, and they also noted that 71% overweight or obese is not very different from the 74% in the general US male population.

They conclude:

"Given the limitations of the current study, in particular the fact that it is a cross-sectional analysis and that it is the first report of a relation between dietary fat and semen quality, it is essential that these findings be reproduced in future work."

"Further, studies with larger samples are now required to confirm these findings," they urge.

Written by Catharine Paddock PhD