FOOD NUTRITION: DIETARY FIBER
Diagram
Two kinds of fibre, two jobs in the gut
Soluble dietary fiber
Fibre that dissolves in water and forms a gel. It is fermented by gut bacteria and slows digestion.
↓
Insoluble dietary fiber
Fibre that does not dissolve in water. It adds bulk to the stool and speeds its passage through the gut.
Figure 1: Soluble fibre dissolves in water and feeds gut bacteria, while insoluble fibre adds bulk and speeds transit. Drawn in our own style from the dietary fiber discussion in Stephen, Food Polysaccharides and Their Applications, and in deMan et al., Principles of Food Chemistry.
Introduction
Soluble dietary fiber and insoluble dietary fiber are both dietary fiber, which means both are plant parts the human gut cannot digest, but they behave differently in water and in the gut. Soluble fiber dissolves in water and forms a gel, and gut bacteria ferment it in the colon. Insoluble fiber does not dissolve in water, it adds bulk to the stool, and it speeds the passage of food through the gut. This pair is about the two kinds of fiber themselves, and it is different from the built Page Dietary Fiber vs Crude Fiber, which compares fiber with the laboratory measure called crude fiber.
Quick comparison
Table 1: Soluble Dietary Fiber compared with Insoluble Dietary Fiber
| Aspect | Soluble Dietary Fiber | Insoluble Dietary Fiber |
|---|---|---|
| Dissolves in water | Yes, it dissolves and forms a gel | No, it does not dissolve |
| Examples | Pectin in apples, beta-glucan in oats, psyllium, gums | Cellulose and lignin in wheat bran, vegetable skins, whole grains |
| Fermented by gut bacteria | Yes, readily fermented in the colon | Little, it is fermented slowly or not at all |
| Effect on digestion | Slows digestion and the emptying of the stomach | Speeds the passage of food through the gut |
| Effect on stool | Softens the stool by holding water in the gel | Adds bulk and makes the stool heavier |
| Effect on blood sugar | Slows the rise of blood sugar after a meal | Little direct effect on blood sugar |
| Effect on cholesterol | Helps lower blood cholesterol, by binding bile acids in the gut | Little direct effect on cholesterol |
| Food sources | Oats, barley, beans, lentils, apples, citrus, psyllium | Wheat bran, whole grains, vegetable skins, nuts |
| Texture in food | Gives a smooth, thick, gel-like texture | Gives a coarse, gritty texture |
| Water holding | Holds water in a gel | Holds water in its structure and adds bulk |
Scientific explanation
Definition
Soluble dietary fiber is the part of dietary fiber that dissolves in water to form a gel and is readily fermented by gut bacteria in the colon. Insoluble dietary fiber is the part of dietary fiber that does not dissolve in water, is little fermented, and adds bulk to the stool and speeds its passage.
Working principle
Soluble fiber works by dissolving in the water of the gut and forming a viscous gel. The gel slows the mixing of food with digestive juices and slows the movement of sugars into the blood, and it binds bile acids, which the body then replaces from cholesterol. In the colon, bacteria ferment soluble fiber into short chain fatty acids. Insoluble fiber works by staying intact: it holds water in its structure, adds bulk to the stool, and stretches the gut wall, which speeds the movement of food through the gut and shortens transit time.
Apparatus
The books used here name no laboratory apparatus for measuring or comparing either term, so no apparatus list is given.
Measurement procedure
The books used here give no standard laboratory procedure with volumes and steps for either term, so no procedure is invented here.
Factors affecting readings
For both kinds of fiber, the food source decides the mix: oats and beans are rich in soluble fiber, wheat bran is rich in insoluble fiber. Water matters: fiber needs water to work, and without enough water insoluble fiber can make the stool hard instead of bulky. Processing matters too: milling removes the bran, so refined flour loses most of its insoluble fiber, while cooking softens fiber without removing it.
Formulas and calculations
The books used here give no formula for either term, so no formula is given here.
Industrial applications
- Soluble dietary fiber: oats and barley foods, where beta-glucan gives a thick texture and slows digestion.
- Soluble dietary fiber: psyllium and pectin added to foods and supplements as a soluble fiber source.
- Soluble dietary fiber: beans and lentils, where soluble fiber slows the rise of blood sugar.
- Insoluble dietary fiber: whole wheat bread and wheat bran, where the bran adds bulk.
- Insoluble dietary fiber: breakfast cereals and bakery foods made with whole grains and bran.
- Both kinds: whole fruits, vegetables and pulses, which carry soluble and insoluble fiber together.
Advantages and limitations
- Soluble dietary fiber, advantage: it slows digestion and the rise of blood sugar, and it helps lower blood cholesterol.
- Soluble dietary fiber, limitation: it gives no bulk on its own, and a large dose can cause gas as bacteria ferment it.
- Insoluble dietary fiber, advantage: it adds bulk and speeds transit, which helps regular bowel movement.
- Insoluble dietary fiber, limitation: it does little for blood sugar or cholesterol, and without enough water it can make the stool hard.
FAQ
What is the difference between soluble and insoluble fiber?
Soluble fiber dissolves in water, forms a gel and is fermented by gut bacteria, while insoluble fiber does not dissolve, adds bulk and speeds transit.
Which foods are rich in soluble fiber?
Oats, barley, beans, lentils, apples, citrus fruit and psyllium.
Which foods are rich in insoluble fiber?
Wheat bran, whole grains, vegetable skins and nuts.
Is this the same as dietary fiber vs crude fiber?
No. That Page compares fiber with the laboratory measure called crude fiber. This article compares the two kinds of dietary fiber with each other.
Do I need both kinds?
Yes. Both do different jobs, and whole plant foods carry both together.
References and further reading
- Stephen, Alistair M. Food Polysaccharides and Their Applications. CRC Press, 1995, chapter Dietary Fiber, p. 547.
- deMan, John M. et al. Principles of Food Chemistry. Springer, 2018, chapter Carbohydrates.
- Google Books pages read for this article:
- Gropper, Sareen S., Jack L. Smith and Timothy P. Carr. Advanced Nutrition and Human Metabolism. 7th ed., Cengage, 2016 (Google Books page read)
