Eating three more portions of dietary fiber a day--say, two pieces of whole grain bread and a bowl of whole grain breakfast cereal--is associated with a lower risk for all cardiovascular diseases and for dying of cancer, diabetes, and respiratory and infectious diseases, a study just published in the BMJ has shown. The study is strong proof that consuming lots of whole grains is good for our health, says first author Dagfinn Aune, a PhD candidate at the Norwegian University of Science and Technology who is currently working at Imperial College, London.
The meta-analysis isn't the first study that links whole grains to positive health effects, but it is the first one to look at how much whole grain one should eat to minimize health risks and that has examined the connection with various causes of death. In general, the study showed that the higher the consumption, the better protected you are.
"We saw the lowest risk among people who ate between seven and seven and a half servings of whole grain products a day, which was the highest intake across all the studies. This corresponds to 210-225 grams of whole grain products in fresh weight and about 70-75 grams of whole grains in dry weight, and is about the same as the health authorities in Norway and other Nordic countries recommend as the minimum daily allowance," says Aune.
Applies only to whole grain products
The researchers' analyses showed fewer risk factors for people who consumed more bread and cereal with whole grains, as well as foods with added bran. On the other hand, people who ate a lot of white bread, rice or cereals with refined grains did not show reduced risk.
"A lot of folks eat plenty of grains, but they choose refined breads instead of varieties with more dietary fiber. Our study suggests that you can reduce the risk of premature death by replacing a big part of the white flour in your diet with whole grain products," said Aune.
Some of the beneficial health effects of eating whole grains may lie in their high fiber content. A high intake of dietary fiber can stabilize blood sugar levels, lower blood pressure, cholesterol, and levels of inflammatory markers in the blood, and can positively affect the gut environment. All of this can contribute to a reduced risk of diabetes, cardiovascular disease and cancer.
Whole grain products also contain several other biologically active substances, such as antioxidants, B vitamins, iron, magnesium, and zinc. It is probably the whole package of favorable components that contribute to the positive effect on the risk of chronic disease and mortality.
Something is much better than nothing
Aune and an international group of researchers collected data from 45 different population studies that investigated whole grain intake in relation to risk of future illness or death due to specific causes.
To be included, the scientists required risk estimates in the studies to be adjusted for other factors that could affect the relationship between dietary fiber intake and health. The findings are now being published in The BMJ, one of the highest ranked medical journals, and reviewed in an editorial in the same issue.
In the editorial, Cecilie Kyrø and Anne Tjønneland from the research centre of the Danish Cancer Society, wrote, "Today, the Nordic countries have the most specific recommendations in the world for how much dietary fiber you should eat: 70-90 grams a day in dry weight, which corresponds to about 210-270 grams of product.
In Denmark, the intake of whole grains has doubled over the last ten years, thanks to the food and health authorities ensuring that food may not be labeled as whole grain if it does not meet strict content requirements. But even in Denmark, no more than 6% of the population follow the recommendations to eat seven or more servings of whole grains a day.
Nevertheless, the researchers believe it is possible to get more people to eat that much whole grain, both in Scandinavia and other countries.
"It may seem ambitious, but effective campaigns and product development can lead to much greater consumption of whole grains. When the health authorities implement measures to increase the population's dietary fibre intake, it is important not to promote whole grain products with a high sugar and salt content," write the Danish researchers, and they point out that greatest health impact at the population level can be achieved by getting more people to eat some whole grains instead of none.
This idea is also supported by the meta-analysis findings.
"We found the greatest risk reduction in people who consumed 0 grams up to those who ate 50 grams of whole grain products a day. The risk continued to decrease - but to a lesser extent - up to an intake of 225 grams a day," said Aune.
Reduced mortality risk for many diseases
Nine studies with a total of more than 700,000 participants examined the risk for all types of cardiovascular disease and correlated cardiovascular deaths. The half of the study subjects who ate the most whole grains had a 16% lower risk than those who consumed less, and every third portion more of whole grain a day reduced the risk by 22%.
The seven studies that looked at coronary heart disease (heart attack and angina) showed similar risk estimates, whereas the reduced risk of stroke was slightly lower in the combined analyses of the six studies that examined this. Analyses of whole grains and mortality from stroke showed a statistically significant 14% reduction in risk.
The risk of dying prematurely from all causes was 18% lower for individuals who consumed a lot of whole grains compared to those who consumed lesser amounts, while three additional servings each day were associated with a 17% reduction in mortality. The risk for deaths associated with cancer (15%), respiratory diseases (22%), diabetes (51%) and infectious diseases (26%) was also lower the more whole grains individuals consumed.
Still need more research
The researchers acknowledge the limitations in measuring how much grain was eaten via self-reported data and that the various studies may have had different definitions for what was regarded as whole grain products. The researchers also cannot rule out that the results may be influenced by people who eat lots of whole grains also living healthier in other ways, but point out that they got the same results when they only analysed studies that adjusted for smoking, alcohol consumption, BMI, physical activity and consumption of other foods such as red meat, sugary drinks and fruits and vegetables.
"Future studies need to improve the way grain intake is measured, and more research that looks at whether the different types of whole grains can have different effects on health is also needed. For example, some studies indicate that oats and rye are better whole grain sources than wheat in terms of reducing the risk of cardiovascular disease," the Danish researchers Kyrø and Tjønneland write.
And a lot of research is still missing on the relationship between dietary fibre intake and relatively rare causes of death.
Aune says that much of the research on whole grains so far has been focused on the most widespread illnesses such as cardiovascular diseases, cancers (mainly colorectal cancer) and type 2 diabetes. "However, our results suggest that consuming whole grain products may have beneficial effects on several other diseases, like mortality from respiratory diseases and infections, and the literature suggests that whole grains may even protect against rare cancers.
Now several very large population-based studies have been established, allowing us to examine less common diseases in more detail than previously. So we'll continue to examine the relationship between whole grains and diseases that have only been studied minimally or not at all," he says.
Reference: Whole grain consumption and risk of cardiovascular disease, cancer, and all cause and cause specific mortality: systematic review and dose-response meta-analysis of prospective studies. Dagfinn Aune, NaNa Keum, Edward Giovannucci, Lars T. Fadnes, Paolo Boffetta, Darren C. Greenwood, Serena Tonstad, Lars J. Vatten, Elio Riboli, Teresa Norat. BMJ 2016;353:i2716