Carbohydrate Quality and Human Health: A Series of Systematic Reviews and Meta-Analyses
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This is a series of systematic reviews and meta-analyses that evaluated the interplay between dietary carbohydrate quality and health outcomes and the predictive potential of certain markers. In all, 185 prospective studies and 58 clinical trials are presented here, including 4635 participants and 135 million person-years of data. Increased intake of dietary fiber was associated with lower bodyweight, systolic blood pressure, and total cholesterol, with maximal benefit observed in association with 25 to 29 g of fiber intake daily. The authors report that whole grain intake provided similar benefits. Diets focusing on reducing glycemic load had a less pronounced impact on reducing the risk of adverse health outcomes.
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The results indicate that dietary fiber and whole grain intake is associated with reduced risk of adverse health outcomes.
abstract
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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. We 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 fibre.
Methods
We did a series of systematic reviews and meta-analyses of prospective studies published from database inception to April 30, 2017, and randomised 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. We 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 quality of evidence.
Findings
Just 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 fibre consumers with the lowest consumers. Clinical trials show significantly lower bodyweight, systolic blood pressure, and total cholesterol when comparing higher with lower intakes of dietary fibre. Risk reduction associated with a range of critical outcomes was greatest when daily intake of dietary fibre was between 25 g and 29 g. Dose-response curves suggested that higher intakes of dietary fibre 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 characterised 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 fibre, 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.
Interpretation
Findings from prospective studies and clinical trials associated with relatively high intakes of dietary fibre and whole grains were complementary, and striking dose-response evidence indicates that the relationships to several non-communicable diseases could be causal. Implementation of recommendations to increase dietary fibre intake and to replace refined grains with whole grains is expected to benefit human health. A major strength of the study was the ability to examine key indicators of carbohydrate quality in relation to a range of non-communicable disease outcomes from cohort studies and randomised trials in a single study. Our findings are limited to risk reduction in the population at large rather than those with chronic disease.
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Additional Info
Disclosure statements are available on the authors' profiles:
When we look at cost, quality of evidence, and health outcomes, fiber may be one of our most value-based therapies. Two meta-analyses and systematic reviews published this week offer support.
The first study by Jovanovski et al1 looks at fiber supplementation.
For a drug to be approved by the FDA to treat diabetes, it needs to reduce HbA1c by 0.3%. This meta-analysis showed that an average of 13.1 g daily of soluble fiber reduced HbA1c levels by 0.58% while also improving insulin sensitivity and reducing fasting blood sugar levels.
How Does Fiber Work?
Soluble fiber soaks up water, creating a mucilage in the GI track that slows the absorption of sugar, reduces cholesterol absorption, and makes one feel full (satiety). As it passes through the GI tract, it stimulates mucous production, creating a home for a diverse microbiome. It is a powerful prebiotic that feeds and ferments bacteria producing short-chain fatty acids, which support a healthy GI lining, reducing the risk of colon cancer.
How to Prescribe Soluble Fiber Through a Supplement
There are many forms of soluble fiber, including psyllium, guar gum, ground flax seed, konjac, beta-glucan, and pectin, to name a few. Let’s focus on psyllium because it is often accessible in multiple forms. The amount of water you mix with a soluble fiber can affect bowels. Too little water can constipate; too much can loosen stools. Drinking 500 mL of water without fiber before meals can result in a 5- to 6-lb weight loss over 12 weeks.2 Adding a soluble fiber to the water can enhance the weight loss effect, while also providing glucose-lowering effects. See the graphic below for a strategy that costs as little as 10 cents per day.
Note: If this amount causes stools to be too loose, reduce the dose.
The second study by Reynolds et al3 looked at the amount of fiber in the diet.
This review of 135 million person-years of data showed convincing results. When high versus low consumers of fiber were compared, there was a relative risk of 0.85 for all-cause mortality, a relative risk of 0.69 for mortality from CAD, and a relative risk of 0.84 for incidence of type II diabetes. On average, higher intake of fiber was associated with a 15% to 31% reduction in noncommunicable diseases.
The greatest benefit came with a total daily fiber intake between 25 and 29 g per day, with a dose–response relationship showing greater benefit with higher intake.
The authors conclude that the fiber research supports striking reductions in, and dose-response relationships with, all-cause mortality, total cancer deaths, total cardiovascular disease deaths and incidence, stroke incidence, and incidence of colorectal, breast, and oesophageal cancer.
How to Recommend More Fiber Through Diet
Eating a high-fiber breakfast is an opportunity to start the day with significant head start toward the 25-g goal of daily fiber. There are two strategies that can help with this. The first is to look at the food label of a cereal box and make sure it has at least 5 g per serving of fiber. Second, encourage soy milk (or a nut milk) instead of cow’s milk. These milks have less sugar and more fiber.
References
This research makes a timely contribution to the literature on dietary carbohydrates and risk for noncommunicable diseases and mortality. The investigators scoured the literature and conducted scientifically rigorous systematic reviews and meta-analyses, including 243 cohort studies and clinical trials spanning decades. This particular research is timely because the need for ample carbohydrate in the human diet is being called into question and hotly debated among contemporary nutrition scientists and preventive medicine practitioners. The findings provide confirmation of current dietary guidelines in developed countries, with clear answers about the role of dietary carbohydrate quality and food groups.
Plant-based, whole foods with minimal processing, including whole grains, whole fruits, vegetables, legumes, nuts, and seeds, form the foundation of a prudent dietary pattern based on the body of scientific evidence as reflected in this research study by Reynolds and colleagues. Dietary fiber is the key measurable and defining feature of a high-quality, plant-based diet. Indeed, as described by Reynolds and colleagues, the evidence on dietary fiber is stronger than any other dietary characteristic, with graded inverse dose–response associations with all-cause mortality, cardiovascular disease, diabetes, colon cancer, and body weight. Recommendations to eat a plant-based, prudent diet, to replace refined grains with whole grains, and to accumulate 25+ grams of dietary fiber per day are appropriate, important, and strongly supported by the totality of scientific literature on diet and chronic diseases.