The frothy advice is not new but is now backed up by the “strongest evidence to date” on the subject, according to lead author Jennifer Rider, ScD, MPH, an epidemiologist at the Harvard T.H. Chan School of Public Health in Boston.

“There is no modifiable risk factor for developing prostate cancer,” Dr Rider told Medscape Medical News. “It would be exciting to tell men that there was a way to modify their risk.”

However, she noted that these are observational data and urged caution when “interpreting them.”

The results are “fascinating,” said Jesse Sammon, MD, a urologist at the Henry Ford Hospital in Detroit, who attended Dr Rider’s presentation. “It was the highlight of the session on cancer epidemiology; the moderator called it the ‘study most likely to be tweeted’.”

These are “incredibly high-quality data,” said Dr Sammon, who was not involved with the study.

The data come from nearly 32,000 men in the prospective Health Professionals Follow-up Study, who now have been followed for 18 years.

During the study period, 3839 men have been diagnosed with incident prostate cancer, 384 cases of which were lethal.

At recruitment in 1992, all participating men were asked to report their average monthly frequency of ejaculation from the ages of 20 to 29 years and 40 to 49 years, and during the previous year. A lifetime average was then computed from these reports.

After potential confounders were controlled for, the risk for prostate cancer was 20% lower in men who ejaculated at least 21 times a month than in men who ejaculated 4 to 7 times a month. The 20% risk reduction was seen at ages 20 to 29 and 40 to 49, and for the lifetime average (P trend < .0001 for all).

At ages 40 to 49, men most (38.0%) reported 8 to 12 ejaculations per month; only 8.8% reported at least 21 ejaculations per month.

“We shouldn’t dwell on the exact numbers of ejaculation, but instead should focus on the dose-response relation,” Dr Rider advised.

She summarized: “Safe sexual activity could be good for prostate health.”

Notably, there was no association between ejaculation frequency and the risk for high-grade, advanced, or lethal disease, she reported. The reason for this exception is not known.

These results are an update of the last major report from the Health Professionals Follow-up Study, which was published about 10 years ago (JAMA. 2004;291:1578-1586). At that time, investigators concluded that “high ejaculation frequency may possibly be associated with a lower risk of total and organ-confined prostate cancer,” as reported by Medscape Medical News.

Many other studies have likewise reported that ejaculation frequency might be tied to prostate cancer risk, with more orgasms being protective.

But these new data have three outstanding strengths, Dr Rider said.

First, the study is prospective, whereas most other studies have been retrospective, and the data are long term. Second, the study involves the largest cohort to date. And third, the study has specific information on ejaculation, she explained.

Whereas previous studies have tended to rely on “proxies” for ejaculation, such as age at marriage, number of children, and number of sex partners, the Health Professionals Follow-up Study investigators were bold; they explicitly asked about ejaculations from sexual intercourse, masturbation, and nocturnal emissions.

The average age of the men in the study was about 59 years, and they had undergone an average of five PSA tests by 2008. Most of the men were married, but the men who reported at least 21 ejaculations per month at ages 40 to 49 were more likely to be divorced than less robust ejaculators (11.8% vs 4% – 7%).

American Urological Association (AUA) 2015 Annual Meeting: Abstract PD6-07. Presented May 15, 2015.