Whole grain intake is related to a clear dose-dependent reduction in the risk for coronary heart disease, stroke, cardiovascular disease, total cancer deaths, and all-cause mortality, the authors of a new meta-analysis report.
They observed a similar relationship between whole grains and the risk for respiratory disease, diabetes, infectious disease, and deaths not related to cardiovascular disease or cancer.
The findings "strongly support existing dietary recommendations to increase whole grain consumption in the general population," lead author Dagfinn Aune, from the School of Public Health, Imperial College, London, United Kingdom, and colleagues write in an article published June 14 in the BMJ.
Despite a growing body of evidence demonstrating the benefits of whole grain consumption, "dietary recommendations have often been unclear or inconsistent with regard to the amount of whole grains that should be eaten to reduce the risk of chronic disease," most likely because of study limitations such as challenges in measuring intake, variations in consumption patterns among populations, or insufficient dose–response information, the authors write. They designed this meta-analysis to clarify the relationship between the level of whole grain intake and the risk for several common chronic ailments, which could help public health experts develop more specific dietary guidelines.
To conduct the comparison, the authors defined one serving of all grains or whole or refined grains as 30 g, equivalent to one slice of bread or one serving of breakfast cereal. The researchers defined a serving of pasta as 150 g, and a serving of white or brown rice as 167.25 g. They studied changes in the risk for illness or mortality per 90-g increase in whole grain intake and between the lowest and highest intakes, up to 210 to 225 g (7 - 7.5 servings) per day.
The analysis included prospective cohort studies published between May 31, 2014, and April 3, 2016, that examined the relationship between whole grain consumption and the incidence of coronary heart disease, stroke, cardiovascular disease, and total cancer, as well as all-cause and cause-specific mortality. There were 20 studies conducted in Europe, 16 in the United States, and nine in Asia, with the number of participants in each study ranging from 245,012 to 705,253.
The summary relative risk for coronary heart disease for high vs low whole grain consumption was 0.79 (P heterogeneity = 0.63; n = 7 studies), equivalent to a risk reduction of 21%. For stroke, the pooled relative risk for high vs low intake was 0.87 (P heterogeneity = .21; n = 6 studies), a risk reduction of 13%. High vs low whole grain intake also was associated with a 16% reduction in the risk for cardiovascular disease (summary relative risk, 0.84; P heterogeneity = .48; n = 10 studies). In a similar comparison for total cancer, the summary relative risk was 0.89 (P heterogeneity = .003; n = 6 studies), and for all-cause mortality, the pooled relative risk was 0.82 (P heterogeneity < 0.001; n = 11 studies), translating into risk reductions of 11% and 18%, respectively.
In the dose–response analyses, the summary relative risk for coronary heart disease per 90 g/day was 0.81 (n = 7 studies), or a 19% reduction in risk. The summary relative risk for stroke per 90 g/day was 0.88 (n = 6), equivalent to a 12% risk reduction. For cardiovascular disease, the summary relative risk was 0.78 (n = 10) or a 22% risk reduction per 90 g/day. Total cancer was associated with a summary relative risk per 90 g/day of 0.85 (P heterogeneity = .16), a 15% reduction in risk. The summary relative risk for all-cause mortality per 90 g/day was 0.83 (P heterogeneity < .001), a reduction of 17%.
The authors also observed reductions of 19%, 36%, 20%, and 21%, respectively, in the relative risk for mortality from respiratory disease, diabetes, infectious disease, and all deaths not related to cancer or cardiovascular disease between high and low intakes of whole grains.
Most of the studies showed "a clear dose-response relation with further reductions with intakes up to seven to seven and a half servings a day (210-225 g/day)," the authors write. These findings suggest that "even moderate increases in whole grain intake could reduce the risk of premature mortality."
Study limitations include high heterogeneity in the analysis between whole grain consumption and all-cause mortality, although excluding two outlying studies "reduced the heterogeneity...but did not substantially alter the summary estimates," the authors write. Studies also may have differed according to the type of grain consumed, the way in which whole grains were defined, and the accuracy of measuring whole grain intake, especially given the diversity of grain products available. Variations in lifestyle might also account for some of the differences observed, although those differences persisted in studies that adjusted for criteria such as smoking, alcohol intake, exercise, and body mass index. The number of studies that teased out any associations with subtypes of whole grains and total or refined grains also was limited.
Current evidence points to a causal relationship in the associations reported in this study, Cecilie Kyrø, PhD, and Anne Tjønneland, PhD, from the Danish Cancer Society Research Center, Copenhagen, Denmark, write in a linked editorial. They agree with the recommendation to increase whole grain intake, but warn that "the authorities should take great care not to promote whole grain foods with high sugar and salt content," and cite recent findings in the United Kingdom that "children and adolescents with the highest intake of whole grains also had the highest intake of sugar."
This analysis provides further evidence for the beneficial effects of diets high in whole grains, Aune and colleagues write. The findings "support dietary recommendations to increase intake of whole grains and as much as possible to choose whole grains rather than refined grains."