These are the findings of a new study that shows healthy American men in their 20s and 30s who ate a 75g (2.5 ozs) packet of walnuts a day were able to increase the vitality, motility and structure of their sperm compared to counterparts who did not eat walnuts.
A report on the study appeared online on 15 August in the Biology of Reproduction journal's papers-in-press section.
Infertility and subfertility is a common problem that affects about 70 million couples worldwide. Between a third and a half of cases are due to poor semen quality in the male partner, with scientists giving a number of reasons for this in industrialized societies: pollution, unhealthy lifestyles and the Western-style diet cited amongst them.
First author Wendie Robbins, of the University of California, Los Angeles (UCLA) and colleagues focused on the last of these, as they explained in their background information to the study:
"We tested the hypothesis that 75 gm of whole-shelled walnuts/day added to a Western-style diet of healthy young men would beneficially affect semen quality."
75 g is about 2.5 ozs, equivalent to one of those small snack-style packs you can get in the supermarket.
A small packet of walnuts a day appear to help improve the vitality of sperm
In the Western-style diet, PUFAs are usually found in fish, fish oil supplements, flax seed and walnuts. Walnuts also offer an important source of linolenic acid (ALA), a natural plant source of omega-3.
For their study, which was partially funded by the California Walnut Commission, Robbins and colleagues enrolled 117 healthy men aged 21 to 35 who followed a Western-style diet.
The participants were randomly assigned to one of two groups: 58 were asked to avoid eating tree nuts, and 59 were asked to eat 75 g of walnuts a day.
The researchers picked 75 g because other studies have suggested this is enough to change lipid levels in the blood but not enough to make healthy young men put on weight.
All the participants gave blood and semen samples before and after the study period, which lasted 12 weeks.
The researchers assessed semen quality using the traditional measures of male fertility. These include sperm concentration, vitality (living versus dead sperm), motility (how well they travel towards an egg), morphology (shape and structure), and chromosome abnormalities.
The results found at the end of the 12 weeks, neither group showed significant changes in body weight, body mass, or physical activity levels (these factors can also affect sperm quality).
However, the men in the walnut group had higher levels of omega-6 and omega-3 (ALA) fatty acids in their blood at the end of the study period than they did at the start.
The men in the walnut group also experienced improvements in sperm quality over the 12 weeks of eating walnuts, there were significant increases in measures of vitality, motility, and morphology. Their sperm also showed fewer chromosome abnormalities at the end of the 12 weeks than it did at the start of the study.
The control group, however, showed no such changes.
The researchers conclude that their:
"Findings demonstrated that walnuts added to a Western-style diet improved sperm vitality, motility and morphology."
Note that the study only looks at the effect of walnut consumption on semen quality in healthy young men: it doesn't show whether it would have the same effect in men with fertility problems, or whether the observed improvements in semen quality actually result in increased fertility.