Study Lets the Sun in on Vitamin D as Cancer Fighter

Of the many nutrients needed to stay healthy, vitamin D seems to be having a moment in the spotlight.

More than 1,000 clinical studies are currently examining its role in the body, according to the National Institutes of Health, and with about 1 billion people around the world not getting enough vitamin D, including half the population of North America, experts say proof of its disease-fighting benefits could have a profound effect on public health.

A new study published today (Sept. 21) may point out one way to overcome some of the limitations of previous studies that have made tentative links between the vitamin and its role in health.

Vitamin D and cancer

Preliminary studies have linked vitamin D to decreased risk of colorectal, breast, prostate, ovarian, bladder, lung and skin cancers. An analysis of the vitamin published last month in the journal Genome Research concluded that vitamin D interacts with a number of genes associated with cancer risk.

Still, limitations in experiment design and methods - small study sizes, for example - have prevented scientists from establishing with more certainty whether vitamin D can protect humans the way it seems to protect animals in experiments from getting cancer.

A report published today (Sept. 21) in the journal Cancer Prevention Research demonstrates that, in the case of endometrial cancer, it could help to focus on specific subgroups in the population.

Because obesity is known to increase a woman's risk of endometrial cancer by 200 percent, a team of researchers led by oncologist Leena Hilakivi-Clarke of Georgetown Lombardi Comprehensive Cancer Center in Washington, D.C., decided to examine the development of endometrial cancer in obese mice to that in non-obese mice.

The researchers gave vitamin D supplements to obese and non-obese mice genetically engineered to be predisposed to endometrial cancer. They found that 75 percent of the obese mice remained cancer-free, while 33 percent of the non-obese did.

Hilakivi-Clarke said she was surprised by how well the vitamin worked.

"Other studies in humans showed that vitamin D has no effect on this type of cancer," Hilakivi-Clarke said, "but those studies looked at women of all sizes. Our study suggests this vitamin may prevent the increase in risk posed by obesity."

How it diminishes risk is uncertain, but it could be that vitamin D counteracts some of the harmful effects of obesity, such as insulin resistance, that can increase cancer risk, Hilakivi-Clarke said.

"This is speculation, but there may be different biological mechanisms driving endometrial cancer development in non-obese and obese women," Hilakivi-Clarke said.

Should women take vitamin D supplements to prevent cancer?

Hilakivi-Clarke plans to next study the connection between obesity, vitamin D and breast cancer in clinical trials, but it will be several years before any scientists are able to conclusively say whether vitamin D supplementation reduces disease risk, and what dose of the vitamin maximizes its benefits.

"Most obese women are vitamin D deficient, and for that reason, they should be supplemented with vitamin D, but it's not clear how much they need and current recommendations are likely to be too low," Hilakivi-Clarke told My Health News Daily.

A newly launched clinical study, the Vitamin D and Omega-3 Trial (VITAL), may offer the first definitive answers. Led by Harvard Medical School researchers, VITAL will examine the effects of a daily dose of 2,000 international units - much higher than current recommendations - in 20,000 men and women over a 5-year period.

In the meantime, Hilakivi-Clarke recommends a daily dose of 600 to 1,000 international units of a vitamin D supplement - "make sure its cholecalciferol, aka vitamin D3," she said. Or spend more time outside, she said, because humans need sunlight to produce vitamin D.

"Sunscreen actually inhibits production of the active form of vitamin D, so expose sunscreen-free arms and legs to indirect sun for 15 to 30 minutes every day," Hilakivi-Clarke said.

The findings are published today (Sept. 21) in the journal Cancer Prevention Research.