The presence of fat in the stomach may override the anti-cancer effects of vitamin C, research suggests.
Oranges are a good source of Vitamin C
In laboratory experiments, a team at the University of Glasgow simulated what happens in the human stomach.
They found vitamin C (ascorbic acid) mopped up potential cancer-causing compounds that are made when saliva and food mixes with stomach acid.
But when they added fat to the mix, the ascorbic acid could no longer convert the hazardous compounds into safe ones.
The scientists say their findings show how diet might be linked to certain stomach cancers.
Nitrite, which is present in saliva and comes from nitrate in our diet, is thought to be able to trigger gastric cancer.
When it is swallowed and enters the acidic environment of the stomach, nitrite spontaneously forms nitrosating species, and these are able to convert a range of other stomach chemicals into potential cancer-causing agents called N-nitrosocompounds.
Antioxidants like ascorbic acid protect against the formation of these nitrosocompounds by converting the nitrosating species into nitric oxide.
However, when fat is present, it reacts with the nitric oxide to reform nitrosating species, the scientists found.
Lead author Emilie Combet said: "These results show that the presence of lipid can markedly alter the protective effects of antioxidants, and how a diet rich in fat can directly influence gastric biochemistry."
She presented her work at the annual meeting of the Society of Experimental Biology.
Bridget Aisbitt, nutrition scientist for the British Nutrition Foundation, said: "Cancer is a complex disease developed over many years due to a number of genetic and environmental factors, so it's important not to imply that any one factor can 'cause cancer'.
"However, this research is interesting."
She said fat-compatible antioxidants in the body, such as beta-carotene, could also neutralise the nitrosocompounds.
She advised: "This is another reason to underline the importance of a healthy balanced diet where meals high in fat should not be frequent and five portions of fruit and vegetables - our main source of vitamin C - are eaten each day."