ORLANDO, Fla. (AP) _ Getting a little sunshine may be one way for men to cut their risk of prostate cancer.
A large study presented at a cancer conference Thursday found that men with higher levels vitamin D in their blood were half as likely to develop aggressive forms of the disease than those with lower amounts.
Doctors are not ready to recommend the ``sunshine vitamin'' without more study, but many see little harm in getting the 15 minutes a day that the body needs to make enough of this nutrient.
``When you were little and your mother said, `Go outside and play,' it wasn't just to get you out of her hair,'' but may have been instinctive advice about something good for health, said Dr. Eric Klein, a prostate cancer specialist from the Cleveland Clinic.
He had no role in the research, which involved nearly 15,000 men in the Physicians' Health Study at Brigham and Women's Hospital and Harvard Medical School in Boston. Five years ago, this study found that men who consumed a lot of calcium had modestly higher rates of prostate cancer.
The new findings fit with that notion, because too much calcium lowers vitamin D, and are especially believable because researchers got them by measuring blood samples rather than relying on what men said they ate _ an imprecision that has hurt past studies of food and cancer risk.
Blood samples were taken in 1982, when the study began. Eighteen years later, 1,082 of the men had developed prostate cancer. Their levels of two common forms of vitamin D in the stored blood samples were compared with those of 1,701 men in the study who did not get cancer.
Levels of one or the other vitamin D derivative did not make much difference in prostate cancer risk. However, men with higher levels of both had roughly half the risk of developing aggressive tumors _ the kind most likely to kill _ than men with lower levels, said Dr. Haojie Li, who led the study.
That is in keeping with what previous studies have shown about prostate cancer, Klein noted.
Men in northern latitudes have higher cancer death rates, and vitamin D levels are lower in older men, who are most prone to prostate cancer.
Melanin, which gives skin its color, blocks ultraviolet light that spurs vitamin D production. Blacks, who have a lot of melanin, also have the highest rates of prostate cancer.
Experiments also suggest vitamin D inhibits cell growth. ``So there is some lab evidence that vitamin D may be anti-cancer,'' Klein said.
It could be that the risk comes from too little vitamin D, and that consuming lots of vitamin D is not helpful, doctors say.
How much should people get? The recommended daily amount is 400 international units, but most scientists think that is probably low, Li said.
Most milk is fortified with vitamin D, but drinking a lot of it might raise the risk of prostate cancer because of its calcium content. Getting enough vitamin D from food is difficult, but doctors do not recommend supplements because they can cause unsafe levels of calcium to build up.
``If you start overloading on vitamin D you're going to cause other problems,'' said Dr. Durado Brooks, chief of prostate cancer research at the American Cancer Society.
Hence the advice to get a little sunlight _ but not too much, because that can raise the risk of skin cancer.
Researchers presented two other studies from the same group of 15,000 doctors. One found that men who were overweight were 30 percent more likely to die of prostate cancer than normal-weight men. Those who were obese were nearly twice as likely to die.
The second study examined a protein in the blood, acid-labile subunit or ALS, that blocks the effects of a hormone that spurs cells to grow and has been linked to many types of cancer.
Compared with men with low levels of ALS, men with higher amounts of it were 40 percent to 60 percent more likely to develop prostate cancer, and their chances of having advanced cancer more than doubled, said Lorelei Mucci, a Harvard epidemiologist who led the study.
ALS needs more study, but may be a new marker for predicting cancer risk and may be a target for developing new treatments, Klein said.