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The study of around 125,000 Americans found that people with the highest intake of vitamin A lowered their risk of squamous cell skin cancer by around 15%. Most of the vitamin A they consumed came from foods.
"These findings just add another reason to have a healthy diet with fruits and vegetables. Vitamin A from plant sources is safe," said Eunyoung Cho, the study's senior author. She is an associate professor of dermatology and epidemiology at Brown University.
Healthy food sources of vitamin A include sweet potato, cantaloupe, carrots, black-eyed peas, sweet red peppers, broccoli, spinach, dairy foods, fish and meat, especially liver, according to the U.S. National Institutes of Health (NIH).
Vitamin A is a fat soluble vitamin. That means it can collect in the fat cells. But when taken in large amounts -- like those in supplements -- you can potentially reach an unsafe level, according to the NIH. Adults shouldn't consume more than 10,000 international units of preformed vitamin A daily, the NIH said.
Squamous cell carcinoma is a common type of skin cancer. Over a lifetime, as many as 11% of Americans will have squamous cell skin cancer, the researchers said. It tends to occur in areas exposed to a lot of sunlight, such as the face and head.
The study included data from more than 75,000 women who took part in the Nurses' Health Study and almost 50,000 men in the Health Professionals Follow-up Study. Participants' average age was in the early 50s.
Study volunteers provided information on their average diet and supplement use.
People with higher levels of vitamin A tended to be older. They also exercised more and were less likely to consume alcohol or caffeine. Women with higher levels of vitamin A were more likely to use postmenopausal hormones.
Nearly 4,000 people ended up with squamous cell skin cancer during more than 25 years of follow-up, the findings showed.
Average daily vitamin A intake was around 7,000 I.U. daily for the lowest group in both studies. The highest group in both studies had more than 21,000 I.U. daily. Most of this came from dietary sources, the study authors said.
The study wasn't designed to prove a cause-and-effect link, but Cho said that vitamin A works to keep skin cells healthy, and that may be why it's linked to a lower risk of squamous cell cancers.
"While this study seems promising, it should not change any current sun protection behaviors or recommendations. The group involved in this study did show a slight decrease in squamous cell carcinoma incidence and it was only after each study participant ingested vitamin A in excess," said Ratner, a dermatologist in New York City.
In addition, vitamin A didn't prevent squamous cell skin cancers entirely, she noted. The study also didn't look at vitamin A's effect on other forms of skin cancer, such as basal cell carcinoma and melanoma.
"The best way to protect yourself against skin cancer is to develop a complete sun protection strategy that includes seeking shade, wearing protective clothing and daily sunscreen application," Ratner advised.
The study was published online July 31 in JAMA Dermatology.
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