There is considerable evidence that vitamin C helps fight cancer. A new study, however, suggests that in people who eat a lot of fat, vitamin C may do the opposite — promote, rather than prevent, the production of cancer-causing chemicals.

Published ahead of print in the September, 2007 online journal, Gut, the study analyzed the impact of both fat and vitamin C on nitrite chemistry in the upper stomach, an area especially vulnerable to pre-cancerous changes and tumor growth.

Nitrites, which are present in human saliva and in preserved meats and other foods, can be converted to cancer-causing compounds called nitrosamines.

Nitrosamines are formed in acidic environments, such as the human stomach, but vitamin C inhibits their formation by converting nitrite to nitric oxide.

The study team, made up mostly of researchers from the University of Glasgow in Scotland, recreated the chemical conditions of the upper stomach and measured the formation of four nitrosamines, oxygen and nitric oxide.

Without fat, vitamin C curbed levels of two nitrosamines and completely eliminated the production of the other two.

When an amount of fat was added that approximates what the upper stomach might contain after a large meal, however, the presence of vitamin C actually boosted the production of nitrosamines between 8 and 140-fold.