Higher Dietary Fiber Intake Associated with Reduction in Incidence of Non-Communicable Diseases

A series of systematic reviews and meta-analyses have depicted how findings from prospective studies and clinical trials associated with relatively high intakes of dietary fiber and whole grains were complementary, and striking dose-response evidence indicated that the relationships to several non-communicable diseases could be causal. According to the publication in The Lancet, the implementation of recommendations to increase dietary fiber intake and to replace refined grains with whole grains is expected to benefit human health.

Foods containing carbohydrate consist principally of sugars, starches, and dietary fiber (non-starch polysaccharide) and provide the main source of dietary energy for people worldwide. The role of free sugars as a determinant of adverse health outcomes has been clarified, and clear guidelines relating to their restriction issued. Dietary fiber and some starches are associated with health benefits. Dietary guidelines typically encourage regular consumption of vegetables, cereals, pulses, and whole fruit, which are rich sources of dietary fiber and some starches, as well as other health-promoting nutrients. However, previous systematic reviews and meta-analyses explaining the relationship between carbohydrate quality and health had usually examined a single marker and a limited number of clinical outcomes. In this study, the Reynolds et al. aimed to more precisely quantify the predictive potential of several markers, to determine which markers are most useful, and to establish an evidence base for quantitative recommendations for intakes of dietary fiber.

The investigators performed a series of systematic reviews and meta-analyses of prospective studies published from database inception to April 30, 2017, and randomized controlled trials published from database inception to Feb 28, 2018, which reported on indicators of carbohydrate quality and non-communicable disease incidence, mortality, and risk factors. Studies were identified by searches in PubMed, Ovid MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials, and by hand searching of previous publications. They excluded prospective studies and trials reporting on participants with a chronic disease, and weight loss trials or trials involving supplements. Searches, data extraction, and bias assessment were duplicated independently. Robustness of pooled estimates from random-effects models was considered with sensitivity analyses, meta-regression, dose-response testing, and subgroup analyses. The GRADE approach was used to assess the quality of evidence.

A little under 135 million person-years of data from 185 prospective studies and 58 clinical trials with 4635 adult participants were included in the analyses. Observational data suggest a 15–30% decrease in all-cause and cardiovascular-related mortality, and incidence of coronary heart disease, stroke incidence and mortality, type 2 diabetes, and colorectal cancer when comparing the highest dietary fiber consumers with the lowest consumers. Clinical trials show significantly lower body weight, systolic blood pressure, and total cholesterol when comparing higher with lower intakes of dietary fiber. Risk reduction associated with a range of critical outcomes was greatest when the daily intake of dietary fiber was between 25 g and 29 g. Dose-response curves suggested that higher intakes of dietary fiber could confer even greater benefit to protect against cardiovascular diseases, type 2 diabetes, and colorectal and breast cancer. Similar findings for whole grain intake were observed. Smaller or no risk reductions were found with the observational data when comparing the effects of diets characterized by low rather than higher glycaemic index or load. The certainty of evidence for relationships between carbohydrate quality and critical outcomes was graded as moderate for dietary fiber, low to moderate for whole grains, and low to very low for dietary glycaemic index and glycaemic load. Data relating to other dietary exposures are scarce. Stressing on the importance of dietary fibre content over the glycaemic index, corresponding author Dr. Lisa Te Morenga wrote, “Our findings, based on a series of systematic reviews and meta-analyses, provide convincing evidence for the importance of including advice regarding the nature and source of other carbohydrates in dietary guidelines aimed at reducing the risk of NCDs. High intakes of dietary fiber and whole grains are more clearly associated with good health outcomes than measures of the glycaemic index or glycaemic load. Although the glycaemic index provides a measure of the glycaemic potential of the carbohydrate content of foods, some low glycaemic index foods might have other attributes that are not health promoting. Foods containing added fructose or sucrose and composite foods containing both saturated fat and carbohydrate (e.g., confectionary products) can have a low glycaemic index.”

The complementary findings from prospective studies and clinical trials, which show that higher intakes of dietary fiber or whole grains are associated with a reduction in the risk of mortality and incidence of a wide range of non-communicable diseases and their risk factors, provide convincing evidence for nutrition recommendations to replace refined grains with whole grains and increase dietary fibre to at least 25–29 g per day, with additional benefits likely to accrue with greater intakes. Considering current evidence, dietary glycaemic index or glycaemic load might be less useful as overall measures of carbohydrate quality than dietary fiber and whole grain content. Highlighting the importance of the study, author Dr. Andrew Reynolds commented, “Our study contributes to the growing body of evidence that carbohydrate quality rather than quantity determines major health outcomes. Translating these findings regarding dietary fiber and whole grains into dietary advice for individuals and populations should be accompanied by a caveat. Dietary fiber as defined by Codex Alimentarius is naturally occurring in foods but can be extracted from foods or synthesized and added into manufactured foods. Whole grain foods are required to have a nutrient composition similar to that of the original grain, without regard to the degree of processing. Scarce, but quite striking evidence exists that consumption of whole grains that have undergone increased processing can result in a deterioration of several biomarkers of cardiometabolic disease. As these are relatively recent developments, no epidemiological evidence exists of the consequences of such changes in the food supply on clinical outcomes and mortality. Until evidence is available, it seems appropriate that dietary advice should emphasize the benefits of naturally occurring dietary fiber in whole grains, vegetables, and fruits that have been minimally processed.” Given that most people worldwide currently consume less than 20 g of dietary fiber per day, reinforcement of relevant nutrition policy will be required to achieve the potential reduction in NCDs.