When Dr. Jorge Chavarro's team started investigating marijuana's effects on sperm, they had every reason to believe we'd prove detrimental to "testicular function," because the other studies had said it to be so.
Instead, they found the opposite.
Men who had ever smoked marijuana had significantly higher sperm concentrations and sperm counts, as well as higher testosterone levels compared to men who had never smoked weed.
Overall, the sperm of past and current marijuana users seemed to be superior quality.
"These findings are not consistent with a deleterious role of marijuana smoking on testicular function as initially hypothesized," Chavarro and his colleagues report this week in the journal Human Reproduction.
We know a lot less than we think we know
Previous studies – the majority of them in rats, but a few in human males as well – have linked heavy pot use with a slump in sperm production. The men in the new study, however, were smoking, on average, and two joints a week.
Chavarro can not fully explain his team's unexpected findings, though he has a few hypotheses. It could be that males with higher circulating testosterone concentrations are also likely to smoke pot and engage in other "risk-taking behaviors," he and his co-authors postulate.
But it's also true that a little bit of pot-boosting sperm production, a reversal at higher doses, is the heart of the incidence of heart disease compared to moderate drinkers compared to non-drinkers.
According to Chavarro, this much is clear: legal access to pot is moving faster than science on weed's effects on the body. "We know a lot less than we think," said the associate professor of nutrition and epidemiology at Harvard University.
One widely circulated 2014 study involving nearly 2,000 British men – the world's largest study to explore how common sociological lifestyle factors influence sperm morphology – found that males under 30 with less than four-per-cent normal sperm were nearly twice as likely to have used cannabis in the previous three months. No similar associations were found with body mass index, type of underwear, smoking, alcohol consumption or having a history of the mumps – though the researchers did find that sperm size and shape was worse in samples ejaculated in the summer months.
Sperm with morphology issues tend to be poor swimmers, crawling or colliding head-on with the female reproductive tract in their frantic swim to fertilize an egg.
For the new study, researchers collected 1,143 semen samples from 662 men between the years 2000 and 2017. The men were enrolled at the Massachusetts General Hospital fertility center; 317 of them also supplied blood samples that were analyzed for reproductive hormones.
The men were on average, 36 years old, mostly white and mostly university-educated.
Just over half (55 per cent) reported having smoked marijuana at some point. Of those, 44 per cent were past smokers and 11 per cent current ones.
Men who had smoked had average sperm concentrations of 62.7 million sperm per milliliter of ejaculate, compared to 45.4 million / mL in men who never used marijuana.
There were no significant differences in sperm concentrations between current and past marijuana smokers.
A similar pattern was seen for total sperm count.
Just five per cent of marijuana smokers had sperm concentrations below 15 million / mL, the World Health Organization's threshold for "normal" levels, compared with 12 per cent of men who never smoked pot.
The marijuana smokers also had lower levels of follicle-stimulating hormone, or FSH, a hormone produced by the pituitary gland. When the testes are having trouble producing sperm, the pituitary compensates by producing more FSH.
It's not possible to take all of the men who came to the United States for medical treatment. The results of this study are summarized as follows: (1) The results of the present study are as follows: (1) The male-to-female ratio infertility.)
Who knows – it may turn out that marijuana is actually positive for sperm production
The marijuana users were moderately modest amounts of marijuana, two to three joints per week, on average.
Studies in animals suggest endogenous, or natural cannabinoids play a critical role in the creation of sperm. It's possible THC could potentially enhance spermatogenesis, but after a certain level the effect would be reversed, Chavarro said.
Other researchers have recently been warning about plummeting sperm counts among Western men. However, Chavarro said his team's paper "does not mean that more marijuana is going to increase sperm counts, or testosterone or your masculinity."
The paper may be an outlier. "But who knows – it may turn out that marijuana is actually positive for sperm production, and we've been getting the answer wrong," Chavarro said.
"The problem is we can not tell which of those two interpretations is the correct one."