Showell's group said that semen contains vitamins E, C and other antioxidants, but the natural defenses can weaken due to lifestyle factors, infection and disease. The oxidative stress may cause sperm cell damage, but that damage can be remedied by natural and supplemental antioxidants.
Subfertility, a condition in which a person is less fertile than average but still capable of contributing to a pregnency, affects about one in 20 men. Up to 80 percent of subfertility could be due to sperm damage caused by oxidative stress or harmful particles produced by a person's own body.
While infertility has historically been seen as a woman's problem, male infertility is present in up to half of infertile couples.
Varicocele, a condition in which a man has varicose veins in the scrotum, is the most common cause of male subfertility. Other causes include hormonal disorders, problems with quantity and motility of sperm, infections, obesity, drug use and exposure to radiation and chemotherapy. Depending on the condition, male infertility and subfertility can be treated with surgery, hormone therapy and assisted reproductive technologies.
While, as we've seen, some doctors dismissed the link between antioxidants and fertility, others said they were not surprised by the results because healthy nutrition and lifestyle are already recommended for couples planning to conceive.
Dr. Jeanne O'Brien, associate professor of urology at University of Rochester Medical Center, is one of those doctors. O'Brien said she routinely suggests that her patients take antioxidants if they want a baby. She also recommends decreasing alcohol consumption, smoking cessation, exercising and stress reduction.
"I tell them the antioxidants may help and certainly don't harm them at the recommended dosages," said O'Brien. "I believe it is the lifestyle changes and increased health awareness that accompanies antioxidant consumption that may make a difference in terms of increased fertility."
O'Brien said antioxidants are never her sole recommendations. "There are always additional factors, testing, medication or alternatives offered in conjunction with them," O'Brien said.
"I use antioxidants for select men with impaired semen quality and demonstration of elevated oxidative stress, which is manifest by increased levels of reactive oxygen species," said Kim. "However, I do not assess for oxidative stress if identifiable causes of male infertility are present. In clinical practice, improvements in pregnancy and live birth rates are certainly not as robust as suggested by the Cochrane review."
And Kim warns that, despite the hype around antioxidant supplements, they should not be consumed without a watchful eye. For example, excessive consumption of Vitamin E, or more than 400 units per day, has been shown to have possible negative cardiac and vascular effects.
"Although generally considered safe in moderation, numerous large-scale studies have indicated that many supplemental antioxidants can actually be detrimental to overall health, said Kim. "With an understanding of these caveats, an assessment of the severity of male factor infertility, and very modest expectations of possible benefit, a man could try antioxidants before attempting assisted reproduction."