29/12/2019
Good Fiber, Bad Fiber - How The Different Types Affect You
Written by Joe Leech, MS on June 4, 2017
Fiber can influence many aspects of health. From gut bacteria to weight loss, it is often considered a fundamental part of a healthy diet. Most people have a very basic understanding of fiber, and tend to lump it all into one category. However, the truth is that not all fiber is created equal. Some types are highly beneficial, while others can cause digestive problems in some people.
What is Fiber and How is it Classified?
"Fiber" refers to a diverse group of carbohydrates that humans can not digest. We lack the digestive enzymes required to break them down, so they pass through most of the digestive system unchanged.
Fiber is mostly found in plant foods, including vegetables, fruits, legumes, whole grains, nuts and seeds (for more details, here is a list of 22 high-fiber foods). There is actually a huge variety of different fibers found in foods. The problem is that they are often classified in different ways, which can be highly confusing.
Fiber is formally classified into two main types (3):
Dietary fiber: Fiber found naturally in foods.
Functional fiber: Fiber that is extracted and isolated from whole foods, then added to processed foods.
A popular alternative method is to classify fiber based on its solubility (soluble vs insoluble), viscosity (viscous vs non-viscous) and fermentability (fermentable vs non-fermentable).
Soluble vs Insoluble Fiber
The solubility of fiber refers to its ability to dissolve in water.
Based on this, fiber has often been categorized as either soluble or insoluble:
Soluble fiber blends with water in the gut, forming a gel-like substance. It can reduce blood sugar spikes, and has various metabolic health benefits (4Trusted Source).
Insoluble fiber does not blend with the water and passes through the digestive system mostly intact. It functions mostly as a "bulking" agent, and may help speed the passage of food and waste through your gut (5Trusted Source).
Viscous Fiber
Some types of soluble fibers form a thick gel when they blend with water. These are known as viscous fibers. Put simply, the viscosity of a fluid refers to its "thickness." For example, the sweetener honey is more viscous than water.
When you eat viscous fiber, it forms a gel-like substance that "sits" in the gut. This slows down the digestion and absorption of nutrients, resulting in a prolonged feeling of fullness and reduced appetite (16Trusted Source, 17, 18Trusted Source).
A review of 44 studies on fiber treatments found that only viscous fibers reduced food intake and caused weight loss (19Trusted Source). Viscous fibers include glucomannan, beta-glucans, pectins, guar gum and psyllium. Good whole-food sources include legumes, asparagus, Brussels sprouts, oats and flax seeds.
Fermentable Fiber
An estimated 100 trillion live bacteria reside in the human gut, mainly in the large intestine (6Trusted Source). These bacteria are actually crucial for optimal health in humans. They play various roles related to weight management, blood sugar control, immunity, brain function and mental health (7Trusted Source, 8, 9Trusted Source, 10Trusted Source, 11, 12).
They are so important that they are often referred to as the "forgotten organ" (13Trusted Source).
Because humans can't digest fiber, it ends up reaching the large intestine mostly unchanged. This is where fermentable fiber comes into play. These are fibers that the friendly gut bacteria are able to digest (ferment) and use as fuel (14Trusted Source). This increases the number and balance of friendly gut bacteria, which also produce short-chain fatty acids with powerful health benefits (15Trusted Source).
Most fermentable fibers are soluble, but there are also some insoluble fibers that can function in this way.
Fermentable fibers include pectins, beta-glucans, guar gum, inulin and oligofructose.
The best whole-food sources of fermentable fibers are beans and legumes. A 1-cup serving often provides up to half of the recommended daily intake of fiber.
All that being said, one of the by-products of fiber fermentation is gas. This is why foods high in fermentable fiber can cause flatulence and stomach discomfort, especially if people are not used to eating a lot of fiber.
