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Update on fruit and vegetable intake and cancer risk

 

Bottom line

Increased consumption of fruit and vegetables is protective against cancer risk at most cancer sites, but the association is strongest with case-control studies than with cohort studies.

Background

Modifiable determinants of cancer risk include smoking, alcohol intake, diet, obesity, and physical activity. For fruit and vegetable intake the evidence can be mixed, because of different study designs, different population characteristics, and because of the way studies measure dietary intake and deal with other variables, like smoking. Different bodies have assessed the evidence in different ways.

Reference

E Riboli, T Norat. Epidemiologic evidence of the protective effect of fruit and vegetables on cancer risk. American Journal of Clinical Nutrition 2003 78 (suppl): 559S-569S.

Study selection

This was a systematic review of case-control and cohort studies of fruit and vegetable intake and cancer risk. It searched for studies between 1973 and 2001, and assessed the risk for each 100-gram daily intake increase.

Results

The review found a large number of studies, and assessed the results for different cancer sites individually. For most cancer sites, case control studies found evidence of a significant risk reduction with increased intake of both fruit and vegetables (Figure 1). Cohort studies, by contract, were more often associated with a non-significant risk reduction.

Figure 1: Summary results for meta-analyses on fruit and vegetables and cancer risk

Comment

Quite why the different study designs come up with different answers is not clear. To produce a valid assessment means having accurate recall or measurement of dietary variables, and study populations with reasonable differences in dietary intake within populations. The former factor may be inaccurate in case-control studies, which are by nature retrospective. Prospective cohort studies may not have populations with large enough variation in diets to show an effect.

Then there is the nature of other factors that are associated with cancer risk, like smoking or physical activity. Where there are many associated factors, isolating the effects of any one will not be easy.

The best interpretation is that there is some protection against cancer in diets that are rich in fruit and vegetables. To minimise cancer risk, though, all modifiable factors should be addressed. The most importance is smoking, but will also include other risk factors.