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1 From the Departments of Epidemiology and Nutrition, Harvard School of Public Health, and the Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston.
See corresponding article on page 93.
Since the first publication of federal guidelines for healthy eating >50 y ago, the consumption of adequate amounts of fruit and vegetables has been paramount. Because data on the long-term health consequences of such a diet were not available at that time, this advice was based primarily on the documentation of essential micronutrients in fruit and vegetables and on the fact that an increase in the consumption of fruit and vegetables would displace other foods deemed less healthy (eg, meats and sweets). Over the past decade there has been an increasing number of studies documenting the long-term health effects of diets high in fruit and vegetables (1). Results from many of the first prospective studies are mixed and do not suggest a consistent association. This lack of clarity may have been caused by the use of inadequate dietary assessment methods or relatively short follow-up periods. More recent long-term prospective studies, including the study by Bazzano et al (2) in this issue of the Journal, have supported an important role of fruit and vegetables in lowering heart disease, stroke, and total mortality (3–5). Although a long-term trial of fruit and vegetable intakes is likely not feasible, short-term trials have shown that diets high in fruit and vegetables substantially lower the risk of hypertension (6) and secondarily lower the occurrence of cardiovascular disease (7). The Food Guide Pyramid (8) and the most recent US Department of Agriculture dietary guidelines (9) continue to emphasize the importance of fruit and vegetables in the diet.
It is likely that the true benefits of fruit and vegetables are underestimated by Bazzano et al in their analysis of data from the National Health and Nutrition Examination Survey Epidemiologic Follow-up Study because only a single question on intake was used to classify individuals as moderate (3 servings/d) or low (<1 serving/d) consumers. Furthermore, this single assessment was used to predict chronic disease after up to 19 y of follow-up. Capturing changes in intake over time would probably refine and strengthen the measure of association (10), particularly because the assessment of fruit and vegetables at baseline was limited to a single question, which did not assess portion size, the amount of processing (ie, fresh, canned, or cooked), or the quality of the food. In their analysis, Bazzano et al tried to account for the importance of other characteristics of the diet (differences in vitamin use or consumption of meat and fish) to determine whether the benefits could truly be attributable to fruit and vegetables. With the addition of more foods to the model, it can become increasingly difficult to interpret the results because the intakes of some foods are highly correlated. Thus, more recent advances in the development of eating pattern scores are important and translate more readily into complete dietary guidelines (11,12). For example, in a recent analysis of dietary patterns and coronary heart disease in the Health Professionals Follow-up Study, we found that men with a "prudent" dietary pattern characterized by a high intake of fruit, vegetables, legumes, whole grains, fish, and poultry had almost one-half the risk of coronary heart disease than did men with a low intake of these foods (12).
The generalizability of results across cultures, ethnic populations, or even sex is an important concept to consider when making general dietary guidelines. A quick scan of the relative risk estimates in Table 4 of Bazzano et al's article suggests that the risk reductions associated with a higher consumption of fruit and vegetables differ dramatically by race. The authors found a strong reduction in the risk of cardiovascular disease among whites but not among nonwhite populations. This illustrates the problems with trying to conduct health surveys in representable samples. With so little power in any one minority subgroup, the CIs include a wide range of risk estimates. Bazzano et al cite the generalizability of their results as an important contribution to the literature, yet the data in Table 4 suggest that the general population studied offered no real advantage over more focused populations, such as nurses, male health professionals, and Framingham residents (3–5). What should not be lost in this discussion is the important fact that true ethnic differences exist in the incidence of chronic disease and potentially in the quantity, quality, and preparation of fruit and vegetables. Thus, a policy of all-inclusiveness regarding sex and ethnicity in every study may be misplaced and will probably provide studies that are underpowered to examine true differences across populations. A more important goal should be to have a portfolio of individual studies that are large enough to address hypotheses in any one focused population and overall are balanced for ethnicity and sex across a given funding agency. This would allow for the careful scrutiny of an association within a subpopulation and eventually allow for the pooling of results across studies to determine whether true heterogeneity exists. It is hoped that the importance of a diet high in fruit and vegetables in reducing the risk of chronic disease among nonwhite populations will be addressed in several ongoing cohort studies that specifically target minority populations (13,14).
Although it is important to establish the benefits of fruit and vegetables across a wide range of intakes in populations around the world, true causality may only come after we delve further into the underlying mechanisms. We learned from our past enthusiasm for the benefits of fruit and vegetables on lung cancer that a predominant micronutrient such as ß-carotene when taken out of context and packaged into a pill is not a magic bullet that can be used to replace food. In fact, as pointed out by Bazzano et al, a combination of many different factors—including folate, fiber, magnesium, and potassium or potentially a group of other compounds such as flavonoids or plant sterols—probably contribute in a small way to the overall benefits of fruit and vegetables. Now that it is firmly established that the intake of fruit and vegetables lowers the risk of chronic disease, our focus should be on widening the scope of our research to include populations defined by ethnicity, preexisting disease, or other unique characteristics and on carefully exploring the biological mechanisms responsible for this benefit.
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