"We observed no evidence of association with fresh fruit intake or with total vegetable intake, even though a protective effect of total vegetables and onion and garlic was suggested for intestinal [cancer]," wrote lead author Carlos Gonzalez in the International Journal of Cancer (Vol. 118, pp. 2559-2566).
Stomach cancer is the fourth most frequent cancer in the world, according to the European School of Oncology, and there are 800,000 new cases every year. It is Japan's most common form of cancer.
The new study, part of the European Prospective Investigation into Cancer and Nutrition (EPIC), followed 521,457 subjects in 10 European countries with an average age of 52. It is said to be the largest cohort study on fruit and vegetable intake and the incidence of stomach (gastric) cancer in Western countries and the first to look at adenocarcinoma of the oesophagus.
Usual dietary intake over the previous year was measured using a country-specific validated questionnaire. Most countries used a self-administered technique with questionnaires typically containing between 88 and 266 food items.
After an average of 6.5 years of follow-up only 400 cases of gastric cancer and 188 cases of oesophagus cancer had been reported. While total vegetable intake was not associated with a decreased risk of gastric cancer, the researchers report that a protective effect by total vegetable intake was "suggested".
An increase in the intake of onions and garlic of 10 grams per day was associated with a 30 per cent reduction in the risk of intestinal gastric cancer, a statistic that was said to be only a "borderline significant negative association."
Citrus fruit intake was also associated with negative risk of gastric cancer, although this 38 per cent decrease in risk was considered non-significant.
This leads on to the main limitation of the study, being that the number of actual cases of the diseases was small, which significantly limited the power of the study. Other limitations include the fact that fruit and vegetable consumption was relatively high, even in the lowest quartile.
"It may be possible that most subjects are above the biological level needed to have a beneficial effect of chemical compound contained in fruit and vegetables," said Gonzalez.
This is not the first time that onions have been linked to reduction in the risk of certain cancers. US researchers recently reported that onion extracts could inhibit the growth of liver and colon cancers in vitro.
Dr Julie Sharp, science information manager at UK charity Cancer Research UK, told NutraIngredients.com that the results agree with others linking fruit and vegetable intake and incidence of cancer, but noted that the limitations of the study meant that further studies were needed.
"We know that a healthy diet, including plenty of fresh fruit and vegetables, can reduce the risk of certain cancers, especially bowel cancer. The authors of this study report a weak link between fruit and vegetables and a reduced risk of stomach cancer, as well as cancer of the oesophagus. But they point out that the numbers of cases studied was very small and so these results need further confirmation," she said.
The EPIC researchers called for future cohort studies with more cases to further investigate the relationship between fruit and vegetable intake and the incidence of gastrointestinal cancer.