“Whole grain intake can be modified relatively easily by replacing refined grains and could have a large effect on the burden of chronic disease if adopted in the general population,” conclude researchers led by Dagfinn Aune at Imperial College London after they systematically reviewed and analyzed 45 published studies on how whole grain consumption relates to several health outcomes and all cause mortality.
Their analysis, published in The BMJ June 14, included more than 7,000 cases of heart disease, 2,000 cases of stroke, 26,000 cases of cardiovascular disease, 34,000 deaths from cancer and 100,000 deaths from 700,000 patients, and it revealed reductions in the relative risk of each.
Specifically, they found when people ate 90 grams of whole grains per day, or about three servings, the relative risk of coronary heart disease dropped 19%, cardiovascular disease fell 22%, mortality from stroke decreased 14%, cancer 15%, and all cause mortality 17%. They also found incidents of respiratory disease fell 22%, infection disease 26% and diabetes 51%.
Considering that heart disease and cancer are the two most common causes of death, claiming 25.5 million people worldwide in 2013, and that grains provide 56% of the energy on average around the world, this connection could have a significant public health impact, they say.
The study also found the relationship between whole grain consumption and disease risk reduction was linear, meaning the risk fell more the more whole grains consumed. It also means that if eating three servings of whole grains a day is too challenging, people likely still will see a benefit if they shifted from eating no whole grains to just two per day, the study notes.
An accompanying editorial by Cecilie Kyrø and Anne Tjønneland from the Danish Cancer Society Research Center also published in The BMJ, agrees with the study’s recommendation that “small individual improvements could have a relatively large effect across whole populations.”
For that reason, they go one step farther and argue that even replacing one serving of grain with whole grains could have a substantial benefit. That said, they also argued that eating up to seven servings of whole grains a day “might be ambitious, but it is feasible.”
How whole grains could help
“Several mechanisms could explain the beneficial effect observed between whole grain intake and coronary heart disease, cardiovascular disease, cancer and all cause and cause specific mortality,” the researchers explain.
Fiber in particular could play a key role in many of the associations. For example, it can reduce the postprandial glucose and insulin responses leading to better glycemic control. It also might reduce cholesterol concentrations by inhibiting reabsorption of bile acid and by bacterial formation of fiber in the colon, producing short chain fatty acids that inhibit cholesterol synthesis in the liver, the researchers explain.
In addition, they note, “dietary fiber can reduce the risk of cancer by mechanic removal of damaged cells from the digestive track, increasing stool bulk, diluting carcinogens, decreasing transit time, altering the gut microbiota.”
Finally, a high intake of whole grains could reduce the risk of chronic disease simply by displacing unhealthy foods or drinks that are associated with the development of such diseases, they add.
Limitations and looking forward
The meta-analysis was far from perfect, the researchers acknowledge. Among its main weaknesses were including poor information on the assessment of whole grain intake in many of the studies reviewed. In addition, there were limited studies on some endpoints, such as diabetes and infectious disease related deaths.
Kyrø and Tjønneland added: “We still need more and better research on the biological mechanisms of health effects and the contribution of health of different grain types,” such as whole grain oats and rye on heart disease compared to whole grain wheat.
With this in mind they suggest, “future studies should improve the assessment of whole grain intake by reporting intakes in a similar way, using biomarkers to track compliance in randomized trials and using validated assessment methods in observational studies.”