Although it is still early, 2016 is already shaping up to be a year to remember in science: Long-postulated gravitational waves were finally discovered, the CDC scrambles to battle Zika virus on multiple continents, and long-awaited clinical testing begins for a new HIV vaccine.

But these accomplishments all pale in comparison to the monumental study published in the American Journal of Gastroenterology titled “Anal Intercourse and Fecal Incontinence: Evidence from the 2009–2010 National Health and Nutrition Examination Survey.” This study sheds light on a rarely talked about and somewhat taboo area of human sexual behavior: anal sex.

The authors analyzed participants in the NHANES database, which is basically a large, diverse group of people who agreed to answer questions about all aspects of their health and behaviors. The data is then extracted, and studies can be generated. The study group comprised 4,170 adult men and women who answered questions regarding anal sex and fecal incontinence. Here are a few key points from this study (for the socially awkward, all of these are good “ice-breakers” at your next cocktail party … you can thank me later):

• About 37 percent of women and 5 percent of men have had receptive anal intercourse in their adult lives.
• Contrary to popular belief, women have the most anal sex in their twenties, and the rate drops off as they get older. There are two peak decades in anal activity in men: the 30s and the 50s.
• Whites and “non-Mexican Hispanics” had the most anal sex when compared to blacks.
• Graduating high school increases the rate of anal sex dramatically. Graduating from college then decreases the rate to a small degree. People who did not graduate high school had anal sex the least.
• A history of major depression was strongly associated with having anal sex in both men and women.

What is the association between anal sex and fecal incontinence? The authors found that receiving anal sex results in a 34 percent increased risk of fecal incontinence in women and a 119 percent increased risk of fecal incontinence in men! (For the purposes of the study, fecal incontinence was defined as leakage of liquid or solid stool or mucus, at least monthly.) Keep in mind these figures are relative risks, meaning that the absolute increases are only 2.5 percent in women and 6.3 percent in men. That being said, I would submit that any increased risk of fecal incontinence is a risk that is just not worth taking.

How might anal sex cause fecal incontinence? The authors point out that the internal anal sphincter muscle is responsible for maintaining the resting pressure of the anus (i.e., keeping the anus closed and keeping stool inside where it belongs), and previous studies show that men who receive anal intercourse have lower anal resting pressure. They postulate that anal sex may simply dilate and stretch the anal sphincter muscle and eventually cause damage to the muscles themselves and cause sensory nerve damage leading to loss of sphincter sensation and control.

Given the mechanism cited above, this study has several important weaknesses. Since it was a retrospective database study, more specific information about anal sex practices was not available to the researchers. For example, it would be helpful to know how frequently a person is engaging in anal sex since one could imagine that having it “once in a lifetime” vs. “once every week” would have different effects on fecal incontinence. The authors also point out that data was not available on non-penile anal insertions; e.g., objects, sex toys, and the authors specifically cite the practice of “fisting.” (You can use your imagination on that last one; this is the first time I have read a scientific paper from a major medical journal and thought, “What! Did they really write that?”)

Perhaps most importantly, since repeated stretching of the anal sphincter is the proposed mechanism of injury causing fecal incontinence from anal sex, this study provides no specific data regarding the sizes of the dilators used to do the stretching. Putting it more bluntly, it would be useful to know the penis sizes of the male participants since one could reasonably assume that a larger penis would cause more trauma to the sphincter muscle resulting in higher rates of incontinence. More specifically, the penile circumference (girth) is an important size since the length seems irrelevant for dilation. A quick search of PubMed reveals a study from the British Journal of Urology showing that the average (measured) girth of an erect penis is 11.66 cm (4.59 in). Some quick math (divide circumference by π) would show that this corresponds to an average penile diameter of 1.46 inches.

For reference, the diameter of an adult colonoscope, which is designed to be inserted into the anus, is 0.5 inches (Olympus CF-HQ190L, 12.8 mm OD). This is roughly one-third the diameter of the average erect penis.

So, after this careful analysis, here are my concluding thoughts on anal sex as a health care professional specializing in colon and rectal diseases:

• Anal sex seems to significantly raise your risk of having fecal incontinence.
• You probably shouldn’t do it.
• If you are going to do it anyway, don’t do it too frequently.
• It seems like common sense, but use lots of lube.
• When finding a partner for anal sex, smaller is probably better.

For as much as we know about anal sex, there is so much that we don’t know. Obviously, more research is needed.

Frederick Gandolfo is a gastroenterologist who blogs at Retroflexions.

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