Eating two handfuls of walnuts every day can improve sperm quality in healthy young men, researchers have found.
In the study, published in the journal Biology of Reproduction, scientists at the University of California, Los Angeles, in the USA randomly assigned 117 men aged between 21 and 35 to two groups. Members of one group ate a 75 gram pack of walnuts every day, while those in the other group avoided all tree nuts. Both groups otherwise followed a typical western diet.
After 12 weeks sperm samples from participants in each of the groups were analysed. To reduce bias the staff analysing the sperm did not know which samples belonged to which groups.
Improvements in sperm concentration, vitality, movement, shape and a reduction in chromosomal abnormalities were observed in the walnut-eating group. Although statistically significant, most of these changes were modest; for example, sperm concentration increased by just 3.1 per cent from 71.4 to 73.6 million per millilitre. The researchers saw no significant changes in any of these parameters in the control group.
With around 40 percent of couples' problems in conceiving related to male infertility, scientists have been keen to investigate the effects of diet on sperm health. Essential fatty acids such as omega-3 and omega-6 (found in oily fish, nuts and seeds) have been a particular focus in recent years. Men with fertility problems were not included in this study and so it is unknown whether a similar effect would be observed.
Dr Catherine Carpenter, one of the authors of the study, told the BBC that walnuts are 'a particularly rich source' of a type of omega-3 called alpha-linolenic acid, adding: 'We suspect [that] may have been responsible for the improvements we observed'.
Recent evidence may support this. In May, a study published in Human Reproduction found that a high omega-3 intake was associated with more 'normal' sperm size and shape compared to a diet high in saturated fats.
The influence of sperm health on fertility is still poorly understood, but the number, size, shape and swimming ability of sperm are thought to be important factors. According to the authors, the next step is to work with fertility clinics to investigate whether daily walnut supplements can actually improve a couple's chances of conception.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, told the BBC that 'it would be relatively easy to poke fun at studies like this, but there is increasing evidence to show that aspects of a man's diet can affect the number and quality of sperm produced by his testicles'.
Dr Pacey added that the trial had been 'well-executed' but expressed concern over the lack of control against a placebo effect. 'A better trial would be to produce tablets of walnut extract that looked identical to a placebo so that the study was completely blind', he said.