Some research has found that being obese may have a protective effect when it comes to risk of non-melanoma skin cancers. A large new study, however, finds that those previous conclusions may be misleading.
The new data, published online ahead of print in Cancer, shows that when looking at everyone, those who are obese do tend to get skin cancer less often than those who aren't (2018; https://doi.org/10.1002/cncr.31810). But when you consider rates of sun exposure, that protective effect disappears, noted Delphine Lee, MD, PhD, Chief of the Division of Dermatology at Harbor-UCLA Medical Center, in an interview with Oncology Times.
"People who are obese do have less sun exposure than those who are normal weight," Lee said, citing a 2013 study (Cancer Epidemiol Biomarkers Prev 2013:(12):2412-2415). "However, when correcting for sun exposure and age, the 'protective effect from obesity' decreases with increasing sun exposure and with age."
The new research analyzed data that had been collected for 71,645 postmenopausal women, of whom 13,351 developed a non-melanoma skin cancer. Though some previous studies have considered the role of sun exposure, this dataset included more detailed information about sun exposure and time outside, and analyzed its interaction with body size (rather than just as another covariate).
With increasing age and sun exposure, the risk for skin cancer increases-"and interestingly, increases the risk more so and more exaggeratedly in an obese person compared with a normal person," Lee noted. Here's what else she said about the research.
1 What led you and your colleagues to do this research now?
"The literature actually pointed to the fact that perhaps obesity (measured by BMI) seems to be protective against non-melanoma skin cancers-meaning that the higher the BMI, the less of a risk of skin cancer. And we thought that was a little odd.
"People often suggest it's because people who are obese may cover up or they may not be participating in as many outdoor activities.
"We decided to look at the Women's Health Initiative, which is a very rich database with three clinical trials and one observational study of women who kept answering questions about their health. These women had been answering these questions since the 1990s, so a lot of data had been collected about them.
"And because the investigators who started that study were so careful and comprehensive in collecting so much different information, it allowed us to tap large numbers to really address this question in a very thorough way. We have information on the geographic location of the women, the UV watts exposure, and a few other measures of sun exposure, as well as some surveys that had been sent to the women asking about how much time they spent outdoors at different times in their lives."
2 So why did your data yield different conclusions than other studies that had also looked at sun exposure, obesity, and skin cancer risk?
"Other studies have investigated the association of obesity on non-melanoma skin cancer risk; some have even included sun exposure in multivariate analyses, controlling for a potential confounding effect of sun exposure. And some authors have concluded that the finding of increased weight inversely proportional to non-melanoma skin cancer risk may be due to lower sun exposure in overweight and obese people in univariate analyses (this was by Tang who also studied this population) (Cancer Epidemiol Biomarkers Prev 2013:(12):2412-2415).
"However, we explored this issue at greater depth. We estimated and tested the effect of increased weight along with two additional significant factors: age and sun exposure, and found a more complex interaction that allows one to see the effects of each of these in relation to each other on the risk of developing skin cancer.
"For example, in older age groups and those who have had increased sun exposure, the inverse relationship of increased weight is markedly less dramatic. By examining the role of sun exposure, one of the key risk factors along with age, we were able to discern that the association of obesity and less skin cancer is not due to less sun.
"If you just look at the people who are obese, the risk of getting non-melanoma skin cancer-if they've been having a high amount of sun exposure, as well as having spent a lot of time in the sun-they have a much higher risk compared to an obese person who has not been in the sun. And in some instances, [they] also have a higher risk compared to a person with a more normal BMI and less time in the sun."
3 What's the most important takeaway message about this research for cancer care providers and for the public?
"It's a matter of educating the public about prevention. A healthy lifestyle, including managing obesity, is important for many other factors. This study sends a public health message to protect from the sun using sun avoidance (such as trying to stay in the shade or avoiding being outside between the strongest times of sun exposure), covering up, and wearing sunscreen. All of those things will decrease your amount of UV exposure.
"Another important message is that people who are obese can, in some instances, have the same amount of risk or an even higher risk of developing non-melanoma skin cancer if they spend time in the sun. The message to those people is that you're not protected."