Study under review: Can dietary viscous fiber affect body weight independently of an energy-restrictive diet? A systematic review and meta-analysis of randomized controlled trials
Dietary fiber, which refers to food matter, primarily from plants, that cannot be digested by the body, has a health halo around it. Dietary fiber as a group consists of different types that differ by their specific characteristics and physiological effects. The most common way of grouping dietary fiber is according to its solubility (soluble and insoluble fiber). As shown in Figure 1, insoluble fiber tends to increase stool size, while viscous, soluble fiber appears to have beneficial metabolic effects, in part because it can be metabolized by colonic bacteria and hence serve as a prebiotic.
|Soluble fiber||Insoluble fiber|
|What is it?||Fiber that is soluble in hot water and that humans can’t digest, though colon bacteria can ferment it, at least in part.|
Examples include fructans, β-glucan, mucilage from psyllium husk, some hemicellulose, pectin, and gums.
|Fiber that can’t be dissolved in hot water and passes through the digestive tract intact.|
Examples include cellulose, some hemicellulose, and lignin.
|What good is it?||Bacteria in the colon can make short-chain fatty acids out of soluble fiber, which feed the lining of the colon, and might lower inflammation, improve cholesterol, and reduce the risk of cardiovascular disease. It also promotes the growth of healthy gut bacteria.||Insoluble fiber increases fecal bulk and can help relieve constipation. It could also bind to both intestinal toxins and cholesterol, improving the body’s ability to eliminate them.|
|What are some good food sources?||Some veggies, including carrots, broccoli, and artichokes.|
Some fruits like berries, apples, pears, and bananas. Legumes, oats, and barley.
|Whole grains, including whole wheat, bran, nuts, and seeds.|
Some fruits and veggies.
Reference: Soliman GA. Nutrients. 2019 May 23;11(5). doi: 10.3390/nu11051155.
One of the purported beneficial effects of fiber is that it could help with weight loss, as epidemiological trials tend to show an association between fiber intake and reduced weight gain over time. Several mechanisms have been proposed to explain this effect, particularly for viscous fiber, which are soluble fiber types with high viscosity that thicken when mixed with fluids. These include increased viscosity of the chyme and subsequently reduced absorption speed and delayed gastric emptying, and promotion of gut hormones that increase satiety and fullness.
Even though previous studies appear to find an effect of soluble fiber on weight loss, there was heterogeneity between trials, suggesting differences among sources of soluble fiber or variability in the study design. In addition, the body of research has been analyzed in the context of energy restricted diets, without taking viscosity into account. Thus, the authors of the current study wanted to determine the effects of viscous soluble fiber supplemented on top of a non-energy restricted, habitual diet on bodyweight and adiposity-related measures.
A higher dietary fiber intake has been associated with health benefits and reduced weight gain in epidemiological studies. In particular, soluble fiber with high viscosity (viscous fiber), which can serve as a prebiotic and has been proposed to increase fullness and reduce appetite, could aid in weight loss. However, previous meta-analyses have not evaluated the effects of particularly viscous soluble fiber and its effects in the absence of calorie restriction.
Other Articles in Issue #65 (March 2020)
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In this first installment of NERD News, we cover a new drug that lowers the chance of severe allergic reaction to peanuts in children who already have peanut allergy.
Investigating vitamin D for reducing arterial stiffness
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Probiotics for stool solidity during cancer treatment
Probiotics seem to help with diarrhea caused by radiation treatment, but better quality evidence is needed to confirm how well it works, and what doses and strains work best.
Can supplementation help silence tinnitus symptoms?
This study found that a blend of lots vitamins and antioxidants had a significant impact on tinnitus, but it’s far from clear which ingredients played a role and which sat on the sidelines.
Deep Dive: Can synbiotics help prevent respiratory tract infections?
This meta-analysis found a modest, but reliable, effect in adults, but no apparent effect in children. However, what doses and strains work best is far from clear. There’s also no strong reason to suspect that these results carry over to more serious RTIs, like COVID-19.
Safety Spotlight: Dairy, dietary supplement use, and breast cancer
In this first-ever installment of Safety Spotlight, we examine two observational studies that explored the association of breast cancer with dairy intake and breast cancer outcomes with supplement use.
Investigating inulin-type fructans for glycemic control
These prebiotics seem to make a decent dent in glycemic control problems, especially for people with prediabetes and type 2 diabetes.