Previous systematic reviews and meta-analyses explaining the relationship between carbohydrate quality and health have usually examined a single marker and a limited number of clinical outcomes. For this analysis, researchers 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 results suggest a 15-30% decrease in all-cause and cardiovascular related mortality when comparing people who ate the highest amount of fiber to those who ate the least. Eating fiber-rich foods also reduced incidence of coronary heart disease, stroke, type 2 diabetes, and colorectal cancer by 16-24%. Per 1,000 participants, the impact translates into 13 fewer deaths and six fewer cases of coronary heart disease.
In addition, a meta-analysis of clinical trials suggested that increasing fiber intakes was associated with lower bodyweight and cholesterol, compared with lower intakes.
The study was commissioned by the World Health Organization to inform the development of new recommendations for optimal daily fiber intake and to determine which types of carbohydrates provide the best protection against non-communicable diseases (NCDs) and weight gain.
Most people worldwide consume less than 20 grams of dietary fiber per day. In 2015, the U.K. Scientific Advisory Committee on Nutrition recommended an increase in dietary fiber intake to 30 grams per day, but only 9% of U.K. adults manage to reach this target. In the U.S., fiber intake among adults averages 15 grams a day.
"Previous reviews and meta-analyses have usually examined a single indicator of carbohydrate quality and a limited number of diseases so it has not been possible to establish which foods to recommend for protecting against a range of conditions," said corresponding author Professor Jim Mann, the University of Otago, New Zealand. "Our findings provide convincing evidence for nutrition guidelines to focus on increasing dietary fiber and on replacing refined grains with whole grains. This reduces incidence risk and mortality from a broad range of important diseases."
The researchers included 185 observational studies containing data that relate to 135 million person years and 58 clinical trials involving 4,635 adult participants. They focused on premature deaths from and incidence of coronary heart disease, cardiovascular disease and stroke, as well as incidence of type 2 diabetes, colorectal cancer and cancers associated with obesity: breast, endometrial, esophageal, and prostate cancer. The authors only included studies with healthy participants, so the findings cannot be applied to people with existing chronic diseases.
For every 8-gram increase of dietary fiber eaten per day, total deaths and incidence of coronary heart disease, type 2 diabetes and colorectal cancer decreased by 5-27%. Protection against stroke, and breast cancer also increased. Consuming 25-29 grams each day was adequate but the data suggest higher intakes of dietary fiber could provide even greater protection.
For every 15-gram increase of whole grains eaten per day, total deaths and incidence of coronary heart disease, type 2 diabetes, and colorectal cancer decreased by 2-19%. Higher intakes of whole grains were associated with a 13-33% reduction in NCD risk—translating into 26 fewer deaths per 1,000 people from all-cause mortality and seven fewer cases of coronary heart disease per 1,000 people. The meta-analysis of clinical trials involving whole grains showed a reduction in bodyweight. Whole grains are high in dietary fiber, which could explain their beneficial effects.
The study also found that diets with a low glycemic index and low glycemic load provided limited support for protection against type 2 diabetes and stroke only. Foods with a low glycemic index or low glycemic load may also contain added sugars, saturated fats, and sodium. This may account for the links to health being less clear.
"The health benefits of fiber are supported by over 100 years of research into its chemistry, physical properties, physiology, and effects on metabolism,” said Prof Mann. “Fiber-rich whole foods that require chewing and retain much of their structure in the gut increase satiety and help weight control and can favorably influence lipid and glucose levels. The breakdown of fiber in the large bowel by the resident bacteria has additional wide-ranging effects including protection from colorectal cancer."
While their study did not show any risks associated with dietary fiber, the authors noted that high intakes might have ill-effects for people with low iron or mineral levels, for whom high levels of whole grains can further reduce iron levels. They also noted the study mainly relates to naturally-occurring fiber rich foods rather than synthetic and extracted fiber, such as powders, that can be added to foods.
Commenting on the implications and limitations of the study, Professor Gary Frost, Imperial College London, U.K., said: "[The authors] report findings from both prospective cohort studies and randomized controlled trials in tandem. This method enables us to understand how altering the quality of carbohydrate intake in randomized controlled trials affects non-communicable disease risk factors and how these changes in diet quality align with disease incidence in prospective cohort studies. This alignment is seen beautifully for dietary fiber intake, in which observational studies reveal a reduction in all-cause and cardiovascular mortality, which is associated with a reduction in bodyweight, total cholesterol, LDL cholesterol, and systolic blood pressure reported in randomized controlled trials.
“There are some important considerations that arise from this Article,” he continued. “First, total carbohydrate intake was not considered in the systematic review and meta-analysis. Second, 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. Third, the absence of quantifiable and objective biomarkers for assessing carbohydrate intake means dietary research relies on self-reported intake, which is prone to error and misreporting. Improving the accuracy of dietary assessment is a priority area for nutrition research. The analyses presented by Reynolds and colleagues provides compelling evidence that dietary fiber and whole grain are major determinants of numerous health outcomes and should form part of public health policy."