What does excessive tanning have to do with endometriosis?

According to a new study, tanning bed use and sunbathing not only increases the risk of developing malignant melanoma, the deadliest form of skin cancer, but may also be associated with a higher likelihood of endometriosis. The findings of American researchers published in the journal Human Reproduction point to such a relationship, although they do not explain the mechanisms behind it.

Researchers studied more than 116,000 women living in different parts of the United States. Endometriosis – a disease in which the endometrium also grows in other places, such as the ovaries and abdominal cavity – was more common in young women who used tanning beds, got sunburns or used sunscreen during their teenage and young adult years. At the same time, they found that women living in parts of the country with high levels of ultraviolet radiation year-round, such as southern parts of the U.S., were less likely to be diagnosed with this painful condition.

Earlier analyses indicated a higher risk of melanoma in patients with endometriosis. The reasons were that the disease was more likely to occur in women who are sensitive to sunlight, do not tan easily, have red hair, light eyes, freckles or a large number of moles. The researchers concluded that these links may reflect a common genetic basis for endometriosis and melanoma or a hidden link between sun exposure and endometriosis risk.

The study was conducted between 1989 and 2015. It included 116,429 women who joined the US Nurses’ Health Study when they were between the ages of 25 and 42. Every two years participants completed questionnaires that asked about their medical history and exposure to risk factors for several chronic diseases. When entering the study in 1989, participants were asked about their susceptibility to sunburn, the number of moles on their legs, and the number of serious sunburns when they were between the ages of 15 and 20. In 1993, they provided information about their use of sunscreen, and in 2005, they provided information about their use of tanning beds in their teens and early adulthood and between the ages of 25 and 35.

Since 1993, participants were also asked if they had endometriosis, diagnosed by laparoscopy, which is the gold standard for correctly diagnosing this condition. There were a total of 4,791 such cases. The subjects’ home addresses were updated every two years and were linked to annual ultraviolet A and ultraviolet B intensity data for the area. The researchers also restricted the analyses to white women, taking into account known racial and ethnic disparities in the diagnosis of endometriosis and differences in the effects of UVA and UVB radiation for different races.

After adjusting the analyses to account for factors that might influence the results, the researchers found that compared with women who never used a tanning bed, those who used one six or more times a year when they were teenagers and young adults had a 19% increased risk of endometriosis. If they used tanning beds six or more times a year between the ages of 25 and 35, the risk increased by 24%, and if they used tanning beds three or more times a year during both lifetimes, the risk of endometriosis increased by 30%.

A history of five or more sunburns between the ages of 15 and 20 was associated with a 12% increased risk of endometriosis compared to women who never tanned intensively. At the same time, women who lived in the sunniest parts of the country at birth, at age 15 and at age 30, had a 19%, 21% and 10% reduced risk of endometriosis, respectively, compared to women living in parts of the U.S. with the least annual sunshine.

According to the researchers, it may be that avoiding excessive recreational sun exposure and using tanning beds may reduce the risk of endometriosis. However, the mechanisms behind this are not clear. The study was observational, so it could not show direct causes of the problem. The authors note that intense UV exposure is associated with DNA damage, cell damage, inflammation, and melanoma risk, and that tanning beds emit mainly UVA radiation, which is associated with an increased risk of cell damage and a weakened immune response.

All of these factors have been linked to the risk of endometriosis. In contrast, if women live in sunnier parts of the world, they are exposed to more UVB light, which stimulates vitamin D production in the skin; vitamin D has been shown to inhibit inflammation and boost immunity.