Protection against skin cancer can be added to the list of health benefits that come with drinking coffee, a new study says.

Women who drank more than three cups of coffee daily were 21 percent less likely to develop basal cell carcinoma (BCC), compared with women who drank less than one cup of caffeinated coffee per month, the study showed. For men, this risk reduction was 10 percent.

“Most likely, the protective effect is due to caffeine,” said lead author Jiali Han, an associate professor at Harvard Medical School and Harvard School of Public Health in Boston. People in the study who drank decaffeinated coffee did not appear to have a lower risk of developing the skin cancer.

Additionally, the researchers found that the more caffeinated coffee that people in the study drank, the lower their risk of developing BCC, the most common type of skin cancer.

But the findings don't mean that your cup of joe can substitute for daily sunscreen.

“I would hope that people would not decide to spend a lot more time in the sun because they are drinking coffee,” said Lorelei Mucci, an associate professor of epidemiology at the Harvard School of Public Health, who was not involved in the study. “There is a lot more about the prevention of BCC that we need to understand,” Mucci said.

Caffeine and skin cancer

BCC accounts for about 80 percent of all skin cancer cases, according to the American Cancer Society. An estimated 2.8 million cases are diagnosed each year in the U.S., according to the Skin Cancer Foundation. BCC does not readily spread to other parts of the body, and so it is rarely deadly. Chronic exposure to the sun or ultraviolet radiation in tanning booths is the major environmental factor that causes BCC.

The researchers analyzed data gathered from 113,000 nurses and health professionals during two long-term studies. Study participants completed questionnaires about their diets, and provided information regarding their cancer risk factors, including family history of melanoma, sunburn reactions, complexion and exposure to direct sunlight. They were also monitored for signs of skin cancer.

Over the 20-year study, 22,786 participants developed basal cell carcinoma, while 1,953 developed squamous cell carcinoma and 741 participants developed melanoma.

The researchers found that the reduction in the risk of developing BCC seen in those who drank coffee was similar to the reduction in risk in people who consumed similar amounts of caffeine from other sources, including tea, chocolate and soda. Still, coffee was the major source of caffeine among the study population, accounting for 78.5 percent of all caffeine intake.

No link was found between caffeinated coffee intake and melanoma, the most deadly form of skin cancer, or squamous cell carcinoma (SCC). However, because the number of study participants diagnosed with melanoma or SCC were small, it is unclear whether caffeine truly has no effect on these skin cancers, or whether more time would be needed to see an effect, Han said.

“In another 10 years or more, it may be clearer whether caffeine also helps stave off these other types of skin cancer,” Han said.

The study is not conclusive — it showed an association, not a direct cause-and-effect relationship between caffeinated coffee and skin cancer risk. Although mouse studies have shown that caffeine may prevent the development of SCC due to UV exposure, there is still no direct, convincing data showing coffee prevents skin cancer in people.

Han also emphasized that while it seems likely the benefit of the coffee comes from caffeine, researchers cannot yet know for sure. “There are lots of compounds in the coffee, including antioxidants. The process of decaffeination can wash out other compounds in the coffee, so we cannot 100 percent tease out that caffeine is the only factor responsible for the effect,” Han said.

Who reaps the most cancer-protective benefits from caffeine?

“Not everyone equally benefits from caffeine consumption,” Han said. The researchers would like to investigate which genes may explain why some people gain cancer protection from drinking caffeine, he said.

Coffee has recently been found to lower people's risk of dying over a given period, and to decrease the risk of prostate, breast and endometrial cancer, said Mucci.

But the mechanisms at play in these conditions may be different, Mucci said. “For prostate cancer and endometrial cancers, the data show the same benefit of lower risk from caffeinated and decaffeinated coffee,” she said.

Coffee influences several body processes — it has antioxidant effects, helps insulin regulation and may lower inflammation, Mucci said. “It may be that different components of coffee are important for different cancers.”

The study is published today (July 2) in the journal Cancer Research; some of the results were presented at a 2011 cancer research meeting.

Pass it on: At least three cups of caffeinated coffee a day appear to protect against basal cell carcinoma, the most common form of skin cancer, but more studies are necessary to confirm the association.