Vitamin D can reduce colon cancer risk by helping the body get rid of its own highly toxic digestive acid, said a study in the journal Science (296: 1313-1316).

Researchers hope the findings will help in the development of drugs that can protect against cancer without some of vitamin D's dangerous side effects.

"We've known for a long time that high-animal fat, Western-style diets, and colon cancer go hand-in-hand," said study author David Mangelsdorf, PhD, a Howard Hughes Medical Institute investigator at the University of Texas Southwestern Medical Center in Dallas. "We also know that vitamin D is protective for colon cancer."

Vitamin D helps the body absorb calcium and maintain strong bones. That's why it is added to milk and other foods. In countries where vitamin D supplies are low, the population is more susceptible to diseases such as rickets — and colon cancer.

In Western countries, colon cancer has been linked to diets high in red meat and animal fat. To digest these foods, the body makes lithocholic acid. Mangelsdorf says it is probably one of the most toxic compounds that the body naturally makes.

"If you inject lithocholic acid into an animal's colon, they'll get colon cancer," he said.

Humans have evolved to deal with this acid by detoxifying it. What surprised Mangelsdorf and his colleagues is that the body uses the vitamin D receptor — which is normally thought of as a way to help the body maintain normal levels of calcium — to do it. Both vitamin D and lithocholic acid can attach to the receptor and switch on a detoxifying gene.

Unfortunately, many Americans are pushing their body's detoxification system past its limit, Mangelsdorf said.

"A lot of people could have a hamburger and fries for breakfast, lunch and dinner, and they do," he says. "This is a chronic increase in lithocholic acid. Our bodies just can't handle it."

Colon cancer patients have been found to have high concentrations of lithocholic acid, he said.

Vitamin D deficiency is also associated with colon cancer. "If you didn't have enough vitamin D in your diet, the vitamin D receptor wouldn't be functioning at its optimum," Mangelsdorf said.

Taking extra vitamin D to protect against colon cancer is not the answer, according to an American Cancer Society (ACS) expert.

"This study has more to do with understanding the mechanism of how colon cancer occurs," said Marji McCullough, ScD, an ACS senior epidemiologist and a registered dietician. "It doesn't mean that you should be taking more vitamin D. You should make sure that you are getting enough vitamin D, but not too much because it can be toxic."

According to the National Institutes of Health's Office of Dietary Supplements, adults should get 400 IU (International Units) of vitamin D each day. Those over 70 years old need 600 IU daily, but no one should consume more than 2,000 IU a day.

Too much vitamin D causes too much calcium in the blood, leading to kidney and bone problems, buildup of calcium in muscles, and heart trouble.

Researchers hope to eventually find another substance that will trigger the protective effects of the vitamin D receptor without endangering the body.

For people interested in protecting against colon cancer today, Mangelsdorf suggested, "The best thing is to modify your diet. Take in less fat and get some exercise. That's the number one thing that an American can do to prolong their life."