Before we settle as to whether soluble fiber is superior or the insoluble fiber is, let’s get a clear understanding about what they are.
As the name suggests the soluble fiber is water-soluble. It dissolves completely in water. Whereas insoluble fiber doesn’t dissolve in water. It is worth nothing that insoluble fibers in general have the characteristic of absorbing water. They don’t dissolve in water, but they absorb water most of the time.
Even if you don’t explicitly drink water along with your food consumption, there is plenty of water present in the food sources. When you eat foods containing soluble fiber, the fiber will dissolve in the water that is present in the stomach. It will either form liquid solution or it will form a gel or liquid paste.
Some of the soluble fibers form very viscous gel. Viscous gel in the stomach causes stomach to empty slowly. The slow emptying stomach will make you feel full sooner.
This viscous gel also binds or traps nutrients and causes delays and interference in nutrient absorption in the small intestine. One can imagine, this may not bode well as it may potentially interfere with the absorption of vital nutrients like vitamins and minerals.
Luckily it seems these types of fibrous gel doesn’t hamper vital nutrient absorption. But it hampers bad cholesterol absorption up to certain extent. This means it helps reduce the blood cholesterol levels.
The viscous gel also seems to bind and trap simple carbohydrates. This means it would interfere with digestion and absorption of simple sugars. This would help with controlling blood sugar and insulin levels.
Although insoluble fiber doesn’t dissolve in water, it also can form viscous substance. But most of the insoluble fiber doesn’t form viscous gel. They rather swell in the presence of the water. So far the cholesterol lowering effects have only been attributed to the soluble fiber.
Another common characteristic of soluble fiber is the fermentability. Remember that neither soluble nor insoluble fiber is broken down by any of the digestive enzymes in the stomach or small intestine. This means the fiber reaches the colon pretty much intact.
Bacteria in the colon seem to thrive on the soluble fiber gel entering the colon. When bacterial ferment the fiber, they produce simple gases like hydrogen, oxygen and carbon dioxide. Along with the gases, they produce what is called short chain fatty acids. These fatty acids are absorbed through the colon wall into the blood stream.
It is not that the soluble fiber passes through the digestive tract without any caloric extraction. But we don’t get full caloric content of these soluble fiber. First bacteria ferment them and consume the energy from these foods. Remaining energy is consumed by the human body when those fatty acids are absorbed. You can see soluble fibers have low caloric value in general.
There is more than what meets the eyes. Those short chain fatty acids seem to be playing very important role in boosting the host immunity. They also seem to have a role in preventing the colon cancer. But the research in that area is still in its primary stages.
Although some of the insoluble fiber also seems to be fermented by the bacteria in the colon and offer those same benefits through short chain fatty acids. In general it is the soluble fibers which are mostly fermented.
Majority of insoluble fiber just passes through the gastrointestinal tract pretty much intact. This means they have no caloric value. In theory you can eat a lot of food that are high insoluble fiber without seeing a direct caloric impact.
Unfortunately the foods high in insoluble fibers are not the most palatable of the foods. Example of food high in insoluble fiber is wheat bran. Many people say that wheat bran tastes like paper.
Insoluble fibers have a tendency to swell up in the presence of water. This causes to give bulk to the food passing through the intestine. Enough bulk is important in stimulating intestinal muscular movement. This property of the insoluble fiber helps reduce constipation as it makes food move with enough speed through the intestinal tract.
As you can see both types of fibers have their own benefits. Soluble fibers and some insoluble fibers which are more viscous have the benefit of reducing cholesterol levels and better regulating blood sugar levels. But they will contain some calories as the fermentation byproducts will eventually be absorbed in the colon.
The soluble fibers which are not viscous may not help with cholesterol reduction or blood sugar but will provide the immunity benefits and potential colon cancer benefits as long as they ferment in the colon.
The insoluble fibers will not help with cholesterol, blood sugar or immunity but they will help with constipation. Besides they have no caloric value as they would pass through the digestive tract pretty much intact.
Moreover in most of the natural food both types of fiber are present in varying degrees. One can choose supplements of specific type of fiber for specific benefits. Soluble fibers which are very viscous seems to have a larger number of benefits.