How does fiber affect health?

Dietary fiber intake is linked to a lower risk of cardiovascular disease, lower obesity rates, and improved bowel health. In addition, intake of fiber may lower blood pressure, modulate blood glucose, and enhance satiety. Different fibers have different effects, so dietary guidance on fiber typically recommends consumption of a range of dietary fibers.

Dietary fiber intake is essential to digestive health. Fiber’s effect on gut health primarily occurs in the large intestine. When fiber reaches the colon, certain types of fiber are fermented by the microflora, resulting in the production of short-chain fatty acids and intestinal gas.

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This also results in changes in the proportions of more and less healthy colonic bacteria. In a high-fiber diet, more short-chain fatty acids are produced, which lowers the pH of the colon. This is thought to be protective against digestive disorders, including colon cancer. 

Certain fibers, especially oligosaccharides including inulin and fructo-oligosaccharides, are extensively fermented by the gut microbiota.2 This fermentation increases the amount of bacteria, including bifidobacteria and lactobacillus. These bacteria are considered “healthy,” and this change in microbiota is called the “prebiotic effect.”

Prebiotics are similar to probiotics, except that the change in gut microflora is made within the digestive tract — the fermentable fiber increases the healthy microbiota in the gut, rather than consuming the microbiota in a food or supplement, or probiotic.

The consumption of prebiotics may also make the gut environment more hospitable for the survival of increased numbers of probiotic organisms ingested from probiotic foods or from probiotic supplements; synbiotics are a combination of probiotics and prebiotics taken together. Oligosaccharides are known prebiotics, although certain foods are also known prebiotics (Table 1).

Table 1: Examples of prebiotic fiber-containing foods

Chicory Root Onion
Jerusalem artichoke Garlic
Agave Asparagus
Leek Wheat

Short-chain fatty acids include acetate, butyrate, and propionate. Butyrate is especially important because it is the preferred fuel for the colonic mucosa cells. Short-chain fatty acid production has been related to lowered serum cholesterol and decreased risk of cancer. Many types of undigested carbohydrates speed up intestinal transit.