Dietary fiber includes plant-based carbohydrates such as whole-grain cereal, seeds and some legumes. Fiber's health benefits have been recorded "by over 100 years of research," Andrew Reynolds, a researcher at the University of Otago in New Zealand, wrote in an email. He is co-author of the new meta-analysis of existing research, which was published Thursday in the journal The Lancet.
The research shows that higher intakes of fiber "led to a reduced incidence of a surprisingly broad range of relevant diseases (heart disease, type 2 diabetes and colorectal cancer)," reduced body weight and total cholesterol, and reduced mortality, Reynolds wrote. Similar findings were shown with increasing whole-grain intakes.
Reynolds' team was commissioned by the World Health Organization to inform future fiber intake recommendations.
The researchers analyzed over 180 observational studies and 50 clinical trials from the past four decades; that's the strength of the analysis, explained co-author Jim Mann, professor of human nutrition and medicine at the University of Otago.
"The health benefits of dietary fiber appear to be even greater than we thought previously," Mann said of the results.
The analysis found a 15% to 30% reduced risk of death and chronic diseases in people who included the most fiber in their diets, compared with those with the lowest intake.
A fiber-rich diet was linked, on average, to a 22% reduced risk of stroke, a 16% lower risk of Type 2 diabetes and colorectal cancer, and a 30% reduced risk of death from coronary heart disease.
Most people globally consume about 20 grams (0.70 ounces) of dietary fiber per day, Mann said of the findings. Based on the research, he recommends 25 grams (0.88 ounces) to 29 grams (1.02 ounces) of fiber each day. Higher amounts are even more beneficial, according to the analysis.
A 15-gram (0.52 ounce) increase in whole grains consumed per day was associated with a 2% to 19% reduction in total deaths and incidences of coronary heart disease, Type 2 diabetes and colorectal cancer.
The study notes that the relationships between high fiber/whole grain consumption and reduced noncommunicable diseases could be causal.
The analysis found no dangers with a high fiber intake. But it adds that for people with an iron deficiency, high levels of whole grains can further reduce iron levels.
The authors note that carbohydrates include sugars, starches and dietary fiber. "However sugars, starches, and fibres are all carbohydrates that perform different roles in the body," Reynolds wrote.
Fiber content was shown to be a better indicator of a carbohydrate food's ability to prevent disease than glycemic index, the measure of the degree to which blood glucose goes up after a particular food is eaten.
The study found small risk reduction in stroke and Type 2 diabetes for people adhering to a low-glycemic-index diet, which involves foods like green vegetables, most fruits, kidney beans and bran breakfast cereals.
Glycemic index is not as good as dietary fiber when considering whether something is a good carbohydrate-containing food, Mann said. Foods that don't increase blood glucose may still be high in sugars, saturated fats and sodium. Ice cream, for example, has a low glycemic index but is high in sugar.
One limitation of the analysis is that the studies involved only healthy individuals, so the findings do not apply to people with pre-existing chronic conditions. Also, most studies were conducted in Western societies; it is not "100% certain" that the results therefore apply to less-privileged societies, Mann explained.
Brian Power, a dietician and lecturer in nutrition at University College London, said the analysis is "very robust" and "powerful." Power, who was not involved in the research, said it is the "highest form of evidence in terms of summarizing what we know."
"Any increase in dietary fiber has health benefits," he added, and it takes only small changes in diet to achieve a health benefit. A person could add 8 grams of fiber to their diet with a breakfast of a bran flakes, four dried apricots and a handful of almonds.
Reynolds advised, "Practical ways to increase fibre intake is to base meals and snacks around whole grains, vegetables, pulses, and whole fruits."
Helen Stokes-Lampard, chair of the Royal College of General Practitioners, wrote in an email that "living a healthy lifestyle is an obvious route to improving our health outcomes, and eating a balanced diet, as well as taking regular exercise, getting enough sleep, drinking alcohol in moderation and not smoking, is a key part of this."
"We've known for a long time that eating foods high in fibre is good for us and helps to aid digestion," wrote Stokes-Lampard, who was not involved in the new analysis, "so it's reassuring to see this high-quality research showing how far-reaching these benefits may be for our long-term health and wellbeing, and confirming why it's so important to include these foods in our diet."
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