Deep Dive: Thick fiber for slimming down

Introduction

Dietary fiber, which refers to food matter, primarily from plants, that cannot be digested by the body, has a health halo around^[1] 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^[2] to have beneficial metabolic effects, in part because it can be metabolized by colonic bacteria and hence serve as a prebiotic^[3].

Figure 1: Soluble versus insoluble fibers
	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.^[4]

One of the purported beneficial effects of fiber is that it could help with weight loss, as epidemiological trials tend to show^[5] 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^[6] 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^[7] 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.

What was studied?

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