Dietary fiber can be defined as ” the plant polysaccharides resistant to hydrolysis by the digestive enzymes in the human intestinal tract”. In simple words, it means that they are compounds of plant origin which cannot be digested in the human body.

In the human body, they serve two important purposes, namely providing bulk to facilitate defecation of unwanted or unabsorbed food residues, and also for acting as a blotting paper to absorb some of the fats and oils in our diet and thus help to maintain their levels in blood within accepted healthy limits. High levels of fatsĀ and oils in blood can clog the blood vessels and lead to high blood pressure, heart attack, stroke and gangrene of limbs.

Types of fiber

There are two types of fiber:

  • Soluble fiber : These include pectins, gums, mucilages and hemicelluloses. Sources of these are oats, legumes, barley, guar gum, apple, carrot and citrus fruits. Soluble fiber helps in lowering the blood glucose and cholesterol levels.
  • Insoluble fiber : These consist of celluloses, lignin and some of the hemicelluloses. Sources of these are whole-wheat flour, bran, vegetables, whole grains and fruits with edible seeds. This type of fiber is helpful in constipation and weight loss.

Most of the fiber-containing foods include both the types, but usually one type often predominates the other and this in turn determines the characteristic texture of that food.

What are the important physiological effects of dietary fiber?

There are various physiological effects of dietary fiber. Some of the important effects can be enlisted as follows-

  • Stimulates chewing and thus saliva flow and gastric juice secretion.
  • Provides a sense of satiety.
  • Insoluble fibers increase theĀ  fecal bulk by absorbing water thus decreasing the pressure on the walls of the colon. This helps in preventing constipation and diverticulosis (small herniations in the mucous membrane of the colonic wall).
  • Normalizes intestinal transit time.
  • Soluble fiber delays gastric emptying and slows the rate of digestion and absorption of nutrients.
  • Soluble fibers such as guar gum, oat bran, legumes etc. may help in reducing the total cholesterol, specifically the low density lipoprotein (LDL) cholesterol.
  • Soluble fibers may also help in controlling blood glucose levels. They reduce the efficiency of enzyme hydrolysis in the gut thus slowing the rate of absorption of glucose in the process.

How much fiber I should take?

Age Fiber intake
2-6 years old 10-12 gm/day
6-10 years old 13-18 gm/day
10-14 years 15-20 gm/day
>14 years 20-30 gm/day

What are the important physical properties of dietary fiber?

Some important physical properties of dietary fiber are-

  • Bacterial degradation– Dietary fiber can not be digested by the enzymes of human digestive system but they are fermentable to varying degrees in large bowel. In this way they provide the suitable substrate for bacterial degradation. By this fermentation process short-chain fatty acids are produced which can induce some physiological response.
  • Water holding capacity– Hydration of fibers result in the formation of a gel matrix. This can result in the higher viscosity of the small intestine content and have critical effect on nutrient absorption. This property can also be related to increased fecal bulk.
  • Adsorption of organic molecules-It has been observed that adsorption of bile acids, cholesterol, toxic compounds is affected by dietary fiber components. Some dietary fiber have cholesterol lowering effect also.
  • Cation exchange capacity- Some dietary fiber sources has a unique property of binding with minerals and electrolytes.

Fiber content of various foods