A study on ejaculation and prostate cancer risk, which made a big splash at last year's annual meeting of the American Urological Association (AUA), was published online March 29 in European Urology.
The publication provides greater detail on the main finding: that men might be able to lower their risk for prostate cancer by ejaculating frequently.
"This large prospective study provides the strongest evidence to date of a beneficial role of ejaculation in prevention of prostate cancer," write the researchers, led by Jennifer Rider, ScD, MPH, a cancer epidemiologist at the Boston University School of Public Health.
However, another expert threw the requisite cold water on any firm conclusion.
"Association does not mean causation, so one has to be cautious about interpretation," Janet Stanford, PhD, MPH, a prostate cancer researcher at the Fred Hutchison Cancer Research Center in Seattle, who was not involved in the study, said about the observational data.
The data come from 31,925 men in the prospective Health Professionals Follow-up Study, who were followed from 1992 to 2010. The average age of the men in 1992 was about 59 years.
During the 18-year follow-up, 3839 men were diagnosed with prostate cancer, and 384 of those cases were lethal.
In a 1992 questionnaire, men were asked to report their average monthly ejaculation frequency during three periods: age 20 to 29 years, age 40 to 49 years, and the previous year.
After potential confounders were controlled for in multivariate analyses, the relative risk for prostate cancer was about 20% lower in men who ejaculated at least 21 times a month than in men who ejaculated four to seven times a month. For high-frequency ejaculators, this risk reduction was seen in all three time periods (P trend < .0001 for all).
After the results were first reported at the AUA meeting, the study received a lot of mainstream news coverage, Dr Rider noted. But in some of the reporting, the emphasis was on the men who ejaculated at least 21 times per month, she explained.
It is true that risk reduction was more pronounced in high-frequency ejaculators than in lower-frequency ejaculators, she acknowledged. (Too few men reported zero to three ejaculations per month, so those reporting four to seven ejaculations per month served as the reference group.)
However, there was a significant relative risk reduction of 10% in men who reported eight to 12 ejaculations per month at 40 to 49 years, and of 20% in men who reported 13 to 20 ejaculations at 40 to 49 years (P trend < .0001).
In an interview with Medscape Medical News last year, Dr Rider warned against overemphasizing the high-end numbers
"We shouldn't dwell on the exact numbers of ejaculation, but instead should focus on the dose–response relation," Dr Rider advised at that time.
She summarized: "Safe sexual activity could be good for prostate health."
Notably, there was no association between ejaculation frequency and high-grade, advanced, or lethal disease. The reason for this exception is not known.
The risk reduction effect seen in the study is "modest," according to Dr Rider's team and Dr Stanford. And they acknowledge that other studies have pointed to sexual activity as a possible modifiable risk factor for prostate cancer development.
Portrait of High-Frequency Ejaculators
The men with the highest frequency of monthly ejaculations (at least 21) "are an interesting group," Dr Rider told Medscape Medical News this week.
They ate more calories than men who ejaculated less frequently, drank more alcohol, contracted more sexually transmitted diseases (gonorrhea and syphilis), and were more likely to be smokers or exsmokers, she reported.
They were was also less likely to have undergone prostate-specific antigen screening.
All of these factors concerned the researchers. They point out that these men "had some exposure patterns that might put them at higher risk of mortality and morbidity due to other causes."
"We were concerned that the reduction in prostate cancer risk we observed in this group might be attributable to premature death from other causes among men who may have had undiagnosed prostate cancer," they explain.
But using a "model of semi-competing risks," the researchers found that the reduction in prostate cancer risk in high-volume ejaculators "seemingly cannot be explained" by any premature death from other causes alone.
Dr Stanford was impressed with the thoroughness of the research.
"The analyses were comprehensive and considered alternative explanations for the observed inverse association," she summarized in an email to Medscape Medical News.
Dr Stanford called the study data "high quality," and said she is excited about possible mechanisms of action to be investigated in the future.
"These interesting results should stimulate more research on how ejaculation may alter the prostate microenvironment," she noted.
The researchers speculate what could be at work, mechanically, and offer one explanation: the prostate might accumulate potentially carcinogenic secretions that can lead to prostate cancer. This idea, known as the prostate stagnation hypothesis, has been around for decades, Dr Rider reported.
That theory might have parallels in folk wisdom. When these results were reported last year, a Medscapereader commented that the results make common sense, and urged his fellow male readers to "keep the pipes clean boys!"
Dr Rider and some of her coauthors were supported by Prostate Cancer Foundation Young Investigator Awards. Dr Stanford has disclosed no relevant financial relationships.
Eur Urol. Published online March 29, 2016.