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After 12 weeks, those who ate walnuts had better sperm vitality, motility, and morphology compared with those who avoided tree nuts altogether (P≤0.03 for all), according to Wendie Robbins, PhD, of the University of California Los Angeles, and colleagues.
Walnuts also were associated with improvements in serum omega-6 and omega-3 fatty acids, the researchers reported online in Biology of Reproduction.
"Whether adding walnuts to the diet will go beyond the shifts in sperm parameters as seen in this study to improving birth outcomes for men within fertility clinic populations or in the general population is not yet known and will require further research," they wrote.
Polyunsaturated fatty acids -- which are found in high concentrations in fish, fish oil supplements, flax seed, and tree nuts -- have been shown in animal and human laboratory studies to be involved in sperm maturation and membrane function. However, not all clinical studies of male fertility have shown differences in polyunsaturated fatty acid concentrations between fertile and infertile participants.
Nuts, and walnuts in particular, have a high concentration of alpha-linolenic acid -- a natural source of omega-3 -- in addition to omega-6 fatty acids, antioxidants, and micronutrients like folic acid.
Robbins and colleagues set out to explore whether adding walnuts to a Western-style diet would improve semen quality. They randomized 117 healthy men ages 21 to 35 (mean age 25) with no known history of infertility to add 75 grams of whole-shelled walnuts a day to their usual diet or to continue their usual eating habits while avoiding tree nuts.
The walnuts were supplied by the California Walnut Commission, which provided funding for the study.
At baseline, the two groups were similar in age, education level, race, body mass index (BMI), body weight, and physical activity levels. During the 12-week study, there were no significant changes in BMI, body weight, or physical activity in either group.
Sperm parameters improved in the group that consumed walnuts (but not in the control group), resulting in significantly better sperm vitality, motility, and morphology.
Serum fatty acid profiles were similar in the two groups at baseline, and only the participants eating walnuts had significant increases in omega-6 (P=0.0004) and omega-3 (P=0.0007) fatty acids through 12 weeks.
The only omega-3 that increased was alpha-linolenic acid (P=0.0001).
Despite the change in serum fatty acids, the fatty acid profile in the sperm did not show changes within each group. However, a nonsignificant increase in eicosadienoic acid in the walnut group, coupled with a nonsignificant decrease in the control group, resulted in a significant between-group difference (P=0.02).
There were no differences between the groups in sperm aneuploidy for chromosomes X, Y, and 18 at baseline or at the end of the study, but in the walnut group there were significant decrease in sex chromosome disomy and sperm missing a sex chromosome (P≤0.01 for both).
Increasing amounts of alpha-linolenic acid in the sperm were associated with a lower proportion of sperm missing a sex chromosome and percentage of sperm with any numerical chromosomal abnormality at the 12-week visit (P≤0.01 for both).
The authors acknowledged that the study was limited by the collection of blood specimens for hormone analysis at all times during the day.