The study, which analysed dietary data from over 36,000 women and 45,000 men, suggests that the high carotenoid content of these types of vegetables, particularly beta-carotene.
"Green leafy vegetables and root vegetables are rich in beta-carotene, which is an antioxidant and can also be converted into vitamin A (involved in the regulation of cell growth)," lead researcher Susanna Larsson from the Karolinska Institutet, Stockholm told NutraIngredients.com.
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 research by Dr. Larsson and Alicja Wolk from the Karolinska Institutet, and Leif Bergkvist from the Central Hospital, Vasteras, prospectively investigated fruit and vegetable consumption and the incidence of stomach cancer amongst the 45,338 men of the Cohort of Swedish Men and the 36,664 women in the Swedish Mammography Cohort.
Dietary assessment was performed using self-administered 96-item food frequency questionnaires in 1997. The researchers combined fruit and fruit juice consumption, and categorised vegetables into five distinct groups: green leafy vegetables, root vegetables, fruiting vegetables, cabbages, and onions, leek and garlic.
During the average seven years of follow-up, 139 incident cases of stomach (gastric) cancer were determined. General vegetable consumption was inversely associated with stomach cancer risk, but not relation between fruit consumption and the cancer was found.
"Risk was 44 per cent lower in the highest [more than 2.5 servings per day] compared with the lowest category [less than one serving per day] of vegetable consumption," wrote Larsson in the journal Cancer Epidemiology Biomarkers & Prevention (Vol. 15, pp. 1998 -2001).
For similar servings of fruit, the difference was 14 per cent, but this was not statistically significant.
When the researchers focussed on green leafy vegetables, like spinach, lettuce, and green salad, people who ate more than three servings per week were associated with a 46 per cent lower risk of stomach cancer than those who ate less than half a serving every week.
For root vegetables, the same difference in servings was associated with a 57 per cent lower risk of developing gastric cancer, said the researchers.
The study does have several limitations that should be noted, including an inability to distinguish between different types and stages of gastric cancer, as well as relying on food frequency questionnaires which can "inevitably lead to some misclassification of fruit and vegetable consumption."
Nevertheless, the researchers concluded: "The findings from this prospective study suggest that frequent consumption of vegetables may reduce the risk of gastric cancer. These findings add further support to current dietary recommendations to increase vegetable consumption."
Dr. Larsson told this website that the high carotenoid content, particularly beta-carotene, could be behind the observations, and apparent benefits of these vegetables. "We are evaluating the association between dietary intake of carotenoids and risk of gastric cancer," she said.
The results are at odds with another European-based study, published earlier this year in the International Journal of Cancer (Vol. 118, pp. 2559-2566). The 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, and was 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.
The EPIC study reported that garlic and onions could help protect against stomach cancer but total fruit and vegetable intake has no benefit for this disease.