BOSTON -- We know that too much sun without protection can cause skin cancer. That's why a controversial theory by a Boston doctor about the benefits of direct rays on the production of vitamin D has become a hot button issue.

Dr. Michael Holick says people don't get enough direct sunlight, which can cause vitamin D deficiency.

"We think that vitamin D deficiency is the most unrecognized health problem in the U.S.," said Holick.

The head of the Vitamin D lab at Boston Medical Center, and author of "The Vitamin D Solution," said vitamin D is essential not only for healthy bones, but in decreasing the risk for a number of diseases.

"It reduces risk of common cancers, like breast and colon cancer, by about 50 percent. It reduces type 2 diabetes by about 33 percent."

But it's Holick's theory on sun exposure that's the most controversial. He said direct rays without protection should be part of our vitamin D intake regimen.

"When you make vitamin D in the skin, it lasts two to three times as long in your body," said Holick. "As a result, exposing your arms and legs a couple of times a week for about 5 to 10 minutes is not going to significantly increase your risk of cancer."

Deb Girard of the Melanoma Foundation agreed that vitamin D is needed, but said unprotected sun exposure is more than risky business.

"We would say get your vitamin D in the cleanest way possible. Ultraviolet radiation, we know, is a carcinogen," said Girard.

Holick said he doesn't condone tanning, only moderate direct exposure, and never on the face. He also recommends dietary sources.

"Wild salmon is a good source and you can also get it from fortified foods like milk, and even now some orange juices are fortified with vitamin D. Then also to take a vitamin D supplement," Holick said.

Girard and Holick agree about the dietary sources and supplements. But the controversy doesn't end with natural sunlight. Holick takes the controversial stance that tanning beds can also be a good way to get vitamin D.

"As soon as you are exposed to a tanning bed, you instantly make vitamin D. Put sun protection on your face and go in 50 percent of the time that's recommended for tanning," said Holick.

"We know if you use a tanning bed under the age of 35, that your risk of melanoma is increased by 75 percent, said Girard. "Again, this is a situation where people are getting their vitamin D through a known carcinogen."

Holick said Americans are sun-phobic. His theories are at odds with many, from the American Cancer Society to the Institute of Medicine.