A large study published Monday that looked at the dietary patterns of more than 100,000 Americans discovered an unexpected link between high consumption of citrus – specifically whole grapefruit and orange juice – and risk of melanoma, the most dangerous form of skin cancer.

Researchers found that 1,840 of the study participants developed melanoma and that those who had a serving of citrus fruit or juice 1.6 times daily had a 36 percent higher risk of the cancer as compared with those who consume it less than two times a week. A serving was defined as half a grapefruit, one orange or a 6 ounce glass of juice.

Writing in the Journal of Clinical Oncology, the authors theorize that the link may be due to high levels of something called furocoumarins found in citrus fruit. These substances are produced by plants as a defense mechanism and are photoactive, meaning that their toxicity is enhanced by ultraviolet radiation. They have been known to cause skin to be more sensitive to sunlight.

The findings were independent of age, and other factors including cigarette smoking, alcohol intake and other lifestyle factors linked to cancer.

Shaowei Wu, the lead study author and a postdoctoral research fellow at the Warren Alpert Medical School at Brown University, said that in a statement that the findings suggest that “those who consume a lot of grapefruit and/or orange juice should be particularly careful to avoid prolonged sun exposure.”