fiber
Meaning of fiber: Two kinds behave quite differently in the gut: one dissolves into a gel, the other keeps its structure and adds bulk.
Definition of fiber
Fiber is the part of plant food that human digestive enzymes cannot break down, so it reaches the large intestine largely intact. It comes from fruit, vegetables, legumes, nuts and whole grains. The same word carries an unrelated anatomical meaning, a long thread like cell or strand, as in muscle fiber and nerve fiber.
How do soluble and insoluble fiber differ?
| Soluble | Insoluble | |
|---|---|---|
| Behavior in water | Dissolves and thickens into a gel | Does not dissolve; holds water and adds bulk |
| Effect in the gut | Slows stomach emptying and the absorption of sugar | Speeds material through the intestine |
| Found in | Oats, barley, beans, lentils, apples, citrus | Wheat bran, whole grains, nuts, vegetable skins |
Almost every plant food contains both, in different proportions, so the two are separated for explanation rather than found apart on a plate. Soluble fiber also traps bile acids and carries them out of the body, and because the liver draws on cholesterol to replace them, that loss lowers the amount circulating in the blood.
What happens once fiber reaches the colon?
The bacteria living there ferment much of the soluble fraction and produce short chain fatty acids, which the cells lining the colon use as their own fuel. Gas is a byproduct, which is why a sudden jump in fiber intake brings bloating that eases as the bacterial population adjusts. The insoluble fraction holds water, so stool stays softer and bulkier and moves through more quickly, giving more regular bowel movements. Population studies link higher fiber diets with lower rates of colorectal cancer and cardiovascular disease, though these are associations drawn from observing what people eat rather than proof that fiber alone accounts for the difference.
Why does the same word describe muscle and nerve?
The anatomical sense is older and completely separate. A muscle fiber is one long muscle cell, running the length of the bundle it belongs to and shortening to produce movement. A nerve fiber is the axon, the drawn out extension of a nerve cell that carries an electrical signal over distance, sometimes for a meter or more. Both are named for their shape rather than their composition, which is all they share with the material in a bowl of oats.
Used in a sentence
The dietary consultation focused on increasing the patient's daily fiber intake.
Good to know
Dietary fiber and the fiber of muscle and nerve tissue are unrelated ideas sharing a name, one describing plant material passing through the gut and the other a shape of cell.