Dose-Response Benefits for Fiber Intake Seen Across Wide Swath of Acquired Diseases
Low-carb, glycemic index, glycemic load: all have been dietary buzz words in recent years. But fiber intake appears simpler and more predictive.
A massive review and meta-analysis representing nearly 135 million person-years, 195 prospective studies, and 58 clinical trials is cementing the dose-response relationship between intake of dietary fiber and a host of noncommunicable diseases, including coronary and cardiovascular diseases.
The study, commissioned by the World Health Organization (WHO), provides “convincing evidence” linking high consumption of fiber and whole grains to improved primary prevention of chronic diseases—a connection that appears more clear-cut than other popular metrics of dietary carbohydrate intake, including glycemic load and glycemic index.
“The key finding is that carbohydrate quality is important,” lead author Andrew Reynolds, PhD (University of Otago, Dunedin, New Zealand), told TCTMD. “A lot of the discussion now is about carbohydrate amount, but it's the quality as well.”
Low-carb diets, in various forms, have been a long-standing nutritional craze, but focusing solely on carbohydrates will not produce risk factor reductions or hard outcomes on the same scale. “You can have a low-carb diet that’s full of sugar that will increase the risk of certain diseases, just like you can have a very high-carb, but high-fiber and high-whole-grain diet that doesn't increase your risk for disease. That carbohydrate quality is actually confounding any considerations of carbohydrate quantity.”
Rather than cutting carbs to improve health, Reynolds stressed, “we should focus more on the carbohydrates we should eat, which is the whole grains, pulses, vegetables, and fruits.”
The paper was published online January 10, 2019, ahead of print in the Lancet.
Old Advice, New Insights
While the benefits of fiber in disease prevention are well known, the current study was designed in part to help identify which markers of carbohydrate quality are more effective at influencing disease development. An additional aim of the study was to look at fiber’s impact, and at what intake levels, across a “massive array of really important diseases,” said Reynolds. The WHO will use this information to update its recommendations on fiber intake, he added.
In all, information from 4,635 individuals on dietary fiber and whole-grains consumption, as well as a broad range of noncommunicable diseases and risk factors, were included in the analysis. In observational studies, mortality from cardiovascular disease was reduced 15% to 30% among people eating the highest amounts of dietary fiber as compared with the lowest fiber consumers. Similar relationships were seen for coronary heart disease, stroke incidence and mortality, diabetes, and colorectal cancer. In clinical trials, higher intake of dietary fiber was associated with reductions in a range of risk factors, including body weight, systolic blood pressure, and total cholesterol.
Analyses using “whole-grain intake” as the metric of interest followed a similar pattern as high fiber intake; however, smaller reductions in disease incidence—and in some cases, no impact whatsoever—were seen when the marker studied was glycemic load or glycemic index.
The analysis was not intended to address mechanisms, but the authors point to the more than 100 years of research into the chemical, physical, and metabolic properties of fiber. To TCTMD, Reynolds speculated that fiber’s cardiovascular benefits are likely mediated through its effect on blood pressure, weight, lipids, and glucose. The microbiome likely also plays a role. Reductions in stroke seen at the highest levels of fiber consumption, he pointed out, were seen for both hemorrhagic and ischemic stroke.
The sweet spot for primary prevention associated with fiber intake appeared to be in the range of 25 to 29 grams per day, with dose-response data for certain diseases, including coronary artery disease, suggesting that intake above 30 grams per day confers additional health benefits. Recommended daily intake is even higher for people with certain cardiometabolic diseases, Reynolds noted.
Worldwide, however, average fiber consumption is less than 20 grams per day; in the United States, fiber intake is in the range of 15 grams per day.
That means increasing fiber intake is one relatively easy and inexpensive step that people can take to help ward off some of the world’s deadliest diseases—a much easier step than cutting smoking or losing weight, for example. “There are a range of foods that have high fiber in them, so I don't think there are access issues as much as there are for other food groups, which are sometimes more expensive,” Reynolds said. “And most of us base our diet on carbohydrates, so it's really an effective part of our diet to [tackle].”
What’s more, he added, the switch from refined to whole grains carries a lot of bang for the buck. “Those little changes add up to a lot,” Reynolds stressed. “Inherently people know . . . fiber is good, but we didn't really know how good it was until we started looking at all of these conditions together and how broadly it affects our risk of these pretty prevalent diseases.”
Reynolds A, Mann J, Cummings J, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;Epub ahead of print.
- Reynolds reports no relevant conflicts of interest.