New research out of Hong Kong suggests that sunlight across the lifespan may decrease the risk of breast cancer.
One strong risk factor for breast cancer are particularly dense breasts. Density and density patterns can be assessed using the well-known screening tool, mammograms. When looking at breast density via a mammogram, there are a few different scales developed to describe the density and density pattern of the breast, with scales generally ranging from non-dense to dense.
One such classification scale is the Tabár classification, which grades patterns I-V. Patterns I, II and III are considered low risk for breast cancer, while patterns IV and V are considered high risk for breast cancer.
In this recent study, Dr Sheng-Hui Wu and colleagues wanted to know, does past sunlight exposure correlate with breast density? And subsequently, does sunlight correlate with breast cancer risk?
As part of an ongoing study, the researchers surveyed 650 Hong Kong Chinese women via a phone call, asking the women questions related to sun exposure. The questionnaire they used has been used in past studies estimating sun exposure. The questionnaire looked at how much sun exposure the women got at different periods of their lives [6-12, 13-19, and 20-34 years, and from 35 years to present (mean age of the women was 43)].
The researchers then assessed breast density via a mammogram and classified women on the Tabár classification system. The health care providers who assessed breast density were blind to the questionnaire results.
The researchers then compared results. How did the amount of sun exposure throughout the lifespan compare to breast patterns? Which women have IV-V patterns and which women have I-III patterns and what is their sun exposure history? Here’s what they found:
- Women aged 40 to 44 years who were in the highest tertile of lifetime total hours spent in the sun had a 59% decreased risk of having high risk breast pattern compared to those is lowest tertile (OR 0.41, 95% CI, 0.18-0.92).
- Women aged 40 to 44 years belonging to the highest tertile of hours spent in the sun during the ages of 6 to 12 years had a 63% decreased risk of having high risk breast pattern compared to those is lowest tertile (OR, 0.37; 95% CI, 0.15-0.91).
- Women with Tabár scores of I-III (low density) got more sun exposure at every age across the lifespan compared to Tabár scores of IV-V (high density), though this wasn’t always statistically significant.
- Women with Tabár scores of IV-V were more likely to use sun protection than women with scores of I-III, particularly during the years they were 35 and older (p=0.04).
The authors conclude,
“The present findings indicate that increased sunlight exposure throughout one’s lifetime and across ages 6 to 12 years seem to be associated with a protective effect of breast cancer risk measured by breast density among Hong Kong premenopausal women.”
While this is not the first study to look at sun exposure and breast cancer, it is the first study to particularly examine sun exposure and breast density (to the researcher’s knowledge).
The greatest limitation of this study was the way they measured sun exposure. While the questionnaire used has proven in past studies to be fairly accurate, there is still great room for error in asking people to recall their sun exposure habits throughout their life. Also keep in mind that the study was cross-sectional, so it was only designed to look at associations and not prove causation.
The researchers call for further research in this realm, particularly research looking into possible mechanisms why sun exposure and vitamin D in early life would affect breast density later in life.