Fibers are made of chains of at least 10 sugar molecules, but with a different structure than starches that can’t be broken down by our digestive enzymes, so we don’t actually absorb the sugars. However, some bacteria digest it in our large intestine and produce beneficial compounds like butyrate which our intestinal cells use for energy. Classical nutrition advice recommends high fiber intakes, though zero fiber diets work best for some people. Soluble fiber and insoluble fiber are both found in foods, though soluble appears to have more benefits. Soluble sources include beans, vegetables, and fruits, and insoluble sources include grains, legumes, and some fruits and vegetables.
Quick Facts
-Made of chains of at least 10 monosaccharides (sugars), but with a different structure than starches
-Can’t be broken down by our digestive enzymes, so we don’t actually absorb any sugars, however some bacteria digest it in our large intestine
-Helps control blood cholesterol, blood pressure, blood sugar, body weight, and may improve gastrointestinal health
-Grouped into water soluble and water insoluble fibre [14]
-The main benefit of fiber appears to be that it feeds the gut microbiome and increases microbiome diversity. Scientists are unsure which types of fiber do this, but they do know that more highly fermentable fibers, like prebiotic fiber, are one group that does increase certain beneficial bacteria species [116].
Soluble Fiber
-Absorbs and retains water, increases size of poop
-Dissolves and forms a gel that slows nutrient absorption [15]
-Feeds microbiome because it is easily fermented by good bacteria in the large intestine, and when the fiber is broken down bacteria release short chain fatty acids that reduce risk of colon cancer [16]
Insoluble Fiber
-Increases size of poop by attracting water, and makes digestion move faster, but slows digestion of sugars, which may help stabilize blood sugar [17]
-Generally not broken down by bacteria, so acts more as a broom to sweep out waste [18]
RDI
28 g [128]
14 grams / 1000 cals
Males < 50 yrs: 38 grams Males > 50 yrs: 30 grams
Females < 50 yrs: 25 grams Females > 50 yrs: 21 grams
[115]
Soluble appears to have more benefits, so focus on soluble instead of insoluble fiber.
Deficiency
None, though some disagree.
Classical nutrition advice argues that fiber is necessary for a healthy microbiome, pooping well, lowering cholesterol, and reducing risk of colon cancer, but others say there is no scientific evidence to support fiber as being good for human health [118].It appears humans can survive without any intake of dietary fiber, evidenced by the fact that many tribes such as Inuits consumed very little fiber, and that many individuals on the carnivore diet currently experience no digestive or other health issues.
See more in our article Do We Really Need Fiber?
Toxicity
Too much fiber, especially certain types, can cause bloating, abdominal pain, flatulence, loose stool/diarrhea, constipation [117].
Sources
Soluble
Black beans, lima beans, avocados, sweet potato, broccoli, turnips, pears, kidney beans, figs, apricots, carrots, nectarines, guava, apples, flax seeds, sunflower seeds, hazelnuts, oats, barley
[141]
Insoluble
Wheat bran or wheat germ, oat bran, beans, lentils and legumes (ex: kidney, black, garbanzo, edamame, split peas, lima, navy, white), blackberries, blueberries, raspberries, strawberries, whole grains, turnips, green peas, okra, spinach, radishes, rutabaga, coconut, apples with skin
[142]
