Similar protection was also observed for an increased intake of potatoes, cabbage, turnip tops, and lettuce report Galician researchers in the journal Nutrition .
The protective effects of the vegetables was possibly due to the antioxidant content of the food, known to be rich source of vitamins A and C, as well as flavonoids.
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In light of these findings, the protective effect detected for total greens, cabbage, turnip tops, lettuce, tomatoes, green beans, and potatoes, rather than being due to a specific nutrient, could be due to their combined effect and to interactions between the various component nutrients ," wrote lead author Olga Dosil-Diaz from the Galician Public Foundation for Health Emergencies.
The study adds to a rapidly expanding body of science linking an increased intake of vegetables to a reduced risk of certain cancers.
No benefit from fruit The study analysed dietary intakes of 617 people, comprising 295 people with lung cancer and 322 healthy controls, using a food-frequency questionnaire (FFQ).
Dosil-Diaz and co-workers report no benefits from fruit, regardless of the level of intake, after adjusting for sex, age, tobacco use, and occupation.
On the other hand, consumption of at least one portion of green leafy vegetables a day was associated with a 50 per cent reduction in the risk of developing lung cancer than consumption of less than five times per week.
"Similarly, a significant protective effect was detected for potatoes, cabbage, turnip tops, and lettuce in the highest intake-frequency category.
Consumption of tomatoes and green beans showed a protective but non-significant effect," added the researchers.
With regard to the nutrient content of the vegetables, versus fruit, Dosil-Diaz and co-workers report that: "Beta-carotene doses in fruit and vegetables, the vitamin A content of fruit is in the order of 10 to 100 times lower than that of green leafy and other vegetables.
"Vitamin A possesses the greatest protective effect against lung cancer by reducing the risk posed by different mechanisms, particularly with regard to its antioxidant potential and role in cellular differentiation."
They also note that vegetables are a rich source of vitamin C and flavonoids, which may play a role individually or collectively in protecting against oxidative stress and potential cancer risk.
Supporting science Researchers from Finland reported that a diet rich in vitamin E may protect middle-aged male smokers from dying from diseases such as certain cancers and coronary heart disease ( American Journal of Clinical Nutrition , Nov. 2006, Vol. 84, pp.1200-1207).
The 19-year study reported that men with the highest serum alpha-tocopherol levels (more than 13.5 mg/L) had significantly reduced risk of cause-specific mortality than those with the lowest levels (less than 10 mg/L).
Indeed, mortality due to lung cancer, prostate cancer, coronary heart disease, stroke, and respiratory diseases were found to be reduced by 21, 32, 16, 36, 42 per cent, respectively, for men with the highest serum levels, compared to men with the lowest levels.
The results contrast with the results of supplementation trials.
"Because supplemental vitamin E has not been shown to reduce mortality in randomised trials, efforts to improve vitamin E status through dietary means may be warranted, particularly if future prospective studies show similar serum alpha-tocopherol -mortality associations in diverse populations, including non-smokers," concluded the Finnish researchers.
Beyond lung cancer A diet rich in antioxidants like vitamins C and E, and beta-carotene has been reported to protect certain sub-populations, particularly smokers, against prostate cancer, a disease that is becoming more common, with incidence rates haven risen by almost two per cent over 15 years.
Indeed, the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, (Journal of the National Cancer Institute, Vol. 98, pp. 245-254), and the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) study (1998), have both reported protective effects of vitamin E against prostate cancer amongst smokers.
However, the Heart Outcomes Prevention Evaluation (HOPE) Trial (2005) reported that a daily supplement of 400 IU of vitamin E had no effect on either prostate or any other type of cancer.
Source: Nutrition (Elsevier) Published online ahead of print 7 March 2008, doi:10.1016/j.nut.2008.01.005 "Consumption of fruit and vegetables and risk of lung cancer: A case-control study in Galicia, Spain" Authors: O. Dosil-Diaz, A. Ruano-Ravina, J.J. Gestal-Otero, J.M. Barros-Dios