Fruit and vegetables and heart disease

We know that eating fruit and vegetables is associated with a reduced risk of heart disease. This meta-analysis tries to quantify that association and asks by how much ?

Message

It is unclear by how much fruit and vegetable consumption reduces the risk of heart disease. If low consumers double vegetable and quadruple fruit consumption it is likely they will reduce their risk of heart disease by approximately 15%. It is not possible to equate this increase to a number of servings, as baseline consumption is not available.

Reference

MR Law and JK Morris. By how much does fruit and vegetable consumption reduce the risk of ischaemic heart disease? European Journal of Clinical Nutrition 1998 52: 549-556.

Search

Cohort studies were identified using MEDLINE (from 1965 onwards) and the Science Citation Index (from 1981 onwards). Review articles and bibliographies were also examined. Small studies (reporting fewer than 50 ischaemic heart disease cases) and case control studies were excluded. Eleven were identified. Overall, there were approximately equal proportions of adult men and women. Participant numbers were 300 (one study), 1,000-10,000 (four studies) and 10,000-80,000 (six studies). They were followed for a number of years, ranging between 4 and 24.

Diet was assessed using food frequency questionnaires in eight studies; three used diet histories (over seven days, four weeks and one year). Direct estimates of fruit and vegetable consumption were only given in three and two studies respectively. The remaining papers estimated fruit and vegetable consumption by measuring nutrients found in fruit and vegetables in sufficiently large quantities (carotenoids, vitamin C, fruit fibre and vegetable fibre).

The studies reported the association between nutrient intake and heart disease in different ways, e.g. as risk in subgroups relative to another subgroup. Moreover, when this design was used, the average dietary consumption of nutrients in the subgroups was often not reported. To quantify the associations in a consistent way, individuals at the 90th percentile of consumption were compared with individuals at the 10th percentile (highest versus lowest consumers).

Results

Risk of heart disease was estimated to be between 12% and 19% lower among the highest consumers of fruit and vegetables and their nutrients, compared with the lowest consumers. Highest consumers ate the equivalent of about four times as much fruit and twice as many vegetables as the lowest consumers. All results were adjusted for age, but it seems not all studies adjusted for other heart disease risk factors. Some had adjusted for smoking, hypertension and body mass index, but it is not clear how many. Table 1 shows the relative risks and 95% confidence intervals for the six measures of fruit and vegetable consumption.

Table 1. Relative risk of heart disease at the 90th percentile of consumption compared to the 10th percentile.

Fruit/Vegetables/Nutrient

Relative Risk 10-90 (95% CIs)

All fruit

0.86 (0.71-1.05)

All vegetables

0.82 (0.66-1.02)

Carotenoids

0.85 (0.77-0.93)

Vitamin C

0.88 (0.79-0.97)

Fruit fibre

0.86 (0.79-0.95)

Vegetable fibre

0.81 (0.71-0.93)

 

Comment

There were too many differences between the studies and insufficient information for this meta-analysis to be informative. There was also significant variability between the studies contributing to the pooled estimates.