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Eating fruit and vegetables reduces risk of breast cancer

Although a pattern is emerging among epidemiological studies showing women with diets high in fruit and vegetables have a decreased risk of breast cancer, the results are not consistent. This paper aims to summarise these results using meta-analysis. It also investigates carotenoids and vitamin C as these micronutrients may protect against breast cancer due to their role in antioxidant defence.

Message

Eating fruit and vegetables reduces risk of breast cancer and the more consumed, the greater the protection. Eating just one serving of vegetables a day reduces the risk of breast cancer by 21% and just one serving of fruit of day reduces the risk by 17%.

Search

The literature was searched using MEDLINE between 1982 and 1997. Bibliographies of identified papers were also reviewed. Studies were included if the following criteria were met:

sufficient information was provided for analysis (i.e. relative risks and 95% confidence intervals);

they were independent (i.e. their results had been analysed in one study only);

categories for the consumption of food items and intake of micronutrients were comparable;

diet was assessed using a food frequency questionnaire;

populations studied were similar (e.g. two studies were excluded because they examined pre-menopausal women and benign breast diseases).

Twenty-six studies were identified (21 case-control and 5 cohort) with 23,038 cases. Case-control studies used either hospital and/or population comparison groups. In studies reporting results for both groups, those from the population were used (in seven case-control studies). When results for overall fruit and vegetable intake were not available (in six studies), the next best definition was chosen, e.g. cooked vegetables. One serving of fruit or vegetables = 250g.

The authors note that adjustments to results differed between studies (e.g. age). If there was a choice, results with the most adjustments were used. No further details are given (i.e. numbers of papers adjusting for which factors). Although the meta-analysis was performed for case-control and cohort studies separately, only the combined results are reported.

Results

As the consumption of fruit and vegetables increased, risk of breast cancer decreased (see Figures 1 and 2). Similarly, as vitamin C and beta-carotene intake increased, risk decreased.

Figure 1: Increasing weekly consumption of vegetables and relative risk of breast cancer compared with one serving per week.

 

Figure 2: Increasing weekly consumption of fruit and relative risk of breast cancer compared with one serving per week.

 

Six servings of vegetables a week was associated with a 21% reduced risk and six servings of fruit a week was associated with a 17% reduced risk, compared with one serving a week (relative risks and 95% confidence intervals 0.79, 0.77 to 0.80 and 0.83, 0.79 to 0.87 respectively).

Compared with 50 mg of vitamin C a day, 400 mg was associated with a 23% reduced risk. Compared with 1000 micrograms of beta-carotene a day, 5000 micrograms was associated with a 9% reduced risk. (Relative risks and 95% confidence intervals 0.77, 0.72 to 0.83 and 0.91, 0.90 to 0.93 respectively).

Comment

A detailed description of the inclusion criteria and data extraction gives the impression this meta-analysis has been thoughtfully carried out. It is a shame that details such as participant numbers for individual studies have been excluded. Overall these omissions do not detract from the finding that risk of breast cancer can be reduced by eating very modest amounts of fruit and vegetables.

Reference

S Gandini et al. Meta-analysis of studies on breast cancer risk and diet: the role of fruit and vegetable consumption and the intake of associated micronutrients. European Journal of Cancer 2000 36: 636-646.