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THURSDAY, Sept. 6, 2007 (HealthDay News) -- When fat is present in the stomach, vitamin C transforms from a cancer-fighter to a possible contributor to malignancy, new research suggests.

Researchers at Western Infirmary in Glasgow, Scotland, analyzed the interaction between vitamin C and lipid (fat) in the upper stomach, which is particularly vulnerable to pre-cancerous changes and tumor growth. They focused on changes in nitrite chemistry.

Nitrites are present in human saliva and preserved foods. During the digestive process, they can be converted to cancer-causing compounds called nitrosamines, which form in acidic conditions. However, vitamin C usually inhibits their formation by converting nitrites to nitric oxide.

But the researchers found that when vitamin C and nitrites meet in environments with 10 percent fat, vitamin C multiplied the production of cancer-causing nitrosamines by eight to 140 times.

Without high fat levels, vitamin C curbed the levels of two nitrosamines by a factor of between five and 1,000. And it completely eliminated the production of the other two, the researchers said.

Fat remains in the stomach for some time after eating and also makes up the content of many cells lining the stomach, the study authors noted.

Writing in the September issue of Gut, the team theorized that this interaction explains why vitamin C supplements have not had significant success in reducing cancer risk.

More information

To learn more about dietary fats, visit the American Dietetic Association.