Lead author Wendie Robbins, a professor at the University of California, Los Angeles, School of Nursing and Fielding School of Public Health, said the study involved 117 men ages 21-35, divided into two groups.
During the 12-week study, one group added 75 grams, or 2.6 ounces of whole-shelled walnuts to their diet daily. The other group continued their usual diet but avoided eating tree nuts. Both groups ate a typical Western-style diet.
"We found a significant improvement in sperm parameters in the group that consumed the walnuts," Robbins said in a statement. "The men who ate no tree nuts saw no change."
Food has been linked to human reproductive success throughout history, but dietary habits and essential nutrients to promote reproductive outcomes focused on women, Robbins said.
"Women are not the only ones who should be paying attention to what they eat when they are trying to get pregnant," she added. "This study shows that what men eat is important too."
Catherine Carpenter of the UCLA Center for Human Nutrition and UCLA schools of nursing and medicine said walnuts provide a particularly rich source of a-linolenic acid, a natural plant source of omega-3, which we suspect may have been responsible for the improvements we observed."
The findings were published in the journal Biology of Reproduction.
The California Walnut Commission partially funded the study.