According to observational studies, risk was reduced most for a range of critical outcomes from all-cause mortality to stroke when daily fiber consumption was between 25 grams and 29 grams, reported Jim Mann, PhD, of University of Otago in Dunedin, New Zealand, and colleagues in The Lancet.
By upping daily intake to 30 grams or more, people had even greater prevention of certain conditions: colorectal and breast cancer, type 2 diabetes, and cardiovascular diseases, according to dose-response curves the authors created.
In the systematic review, observational data showed a 15% to 30% decline in cardiovascular-related death, all-cause mortality, and incidence of stroke, coronary heart disease, type 2 diabetes, and colorectal cancer among people who consumed the most dietary fiber compared to those consuming the lowest amounts.
Whole grain intake yielded similar findings.
The analysis “provides compelling evidence that dietary fiber and whole grain are major determinants of numerous health outcomes and should form part of public health policy,” wrote Gary Frost, PhD, of Imperial College London in England, and colleagues in an accompanying editorial.
This confirmation that vegetables, fruit, and whole grains (the main sources of fiber) are healthy isn’t surprising, commented Marion Nestle, MPH, PhD, of New York University in New York City, who was not involved in the study. “Providers should continue to encourage largely plant-based diets, consistent with decades of dietary recommendations.”
But because most people consume under 20 grams of dietary fiber each day, “reinforcement of relevant nutrition policy will be required to achieve the potential reduction in non-communicable diseases,” the investigators concluded.
Quantitative guidelines relating to dietary fiber have not been available, the researchers said. With the GRADE method, they determined that there was moderate and low-to-moderate certainty of evidence for the benefits of dietary fiber consumption and whole grain consumption, respectively.
Included in the systematic review were 58 clinical trials and 185 prospective studies for a total of 4,635 adult participants with 135 million person-years of information (one trial in children was included, but analyzed separately from adults). Trials and prospective studies assessing weight loss, supplement use, and participants with a chronic disease were excluded.
The large size of pooled variance based on random-effects models were taken into account with meta-regression, subgroup analyses, dose-response testing, and sensitivity analyses.
In contrast, glycemic load had no observable impact on health. Overall, there was low-to-very low quality of evidence for the relationship between dietary glycemic index and critical outcomes, Mann’s group reported.
Frost’s group pointed out that total carbohydrate consumption was not taken into account in the meta-analysis, and the lack of objective and quantifiable biomarkers for evaluating carbohydrate consumption indicated dietary studies relied on self-reported information, which often leads to misreporting and errors.
“Although the absence of association between glycemic index and load with non-communicable disease and risk factors is consistent with another recent systematic review, caution is needed when interpreting these data, as the number of studies is small and findings are heterogeneous,” the editorialists cautioned.
The study was funded by the Health Research Council of New Zealand, WHO, the Riddet Centre of Research Excellence, the Healthier Lives National Science Challenge, the University of Otago, and the Otago Southland Diabetes Research Trust.