Fiber and the Microbiome

What it is

Fiber is dietary carbohydrate that resists human digestion. It includes:

• Soluble fiber — oats, beans, psyllium, fruit pectins; viscous, lowers LDL, slows glucose absorption.
• Insoluble fiber — wheat bran, vegetables; stool bulking.
• Fermentable fiber — inulin, beta-glucans, resistant starch; feeds colonic bacteria producing short-chain fatty acids (SCFAs).

Why it matters

• Each ~8 g/day increment in fiber associates with ~5–19% reductions in coronary heart disease, type-2 diabetes, colorectal cancer, and all-cause mortality (Reynolds 2019).
• SCFAs (acetate, propionate, butyrate) regulate colonic epithelial health, modulate systemic inflammation, and feed enteroendocrine GLP-1/PYY signalling.
• Microbiome diversity is strongly fiber-dependent; modern Western diets reduce diversity within generations.

How much

• US/UK recommendations: ~25–30 g/day.
• Typical Western intake: ~15 g/day.
• Hunter-gatherer / traditional diets often delivered 50–100+ g/day.

How to get it

• Legumes (lentils, chickpeas, beans): ~15 g per cup cooked.
• Whole grains (oats, barley, whole-grain bread).
• Berries, apples, pears (with skin).
• Cruciferous and leafy vegetables.
• Nuts and seeds (especially chia, flax).
• Resistant starch from cooked-and-cooled potatoes, rice.

Caveats

• Sudden large increases cause bloating; ramp up over weeks and drink water.
• In small-intestinal bacterial overgrowth (SIBO) or active IBD flares, high fermentable fiber may worsen symptoms.

Related entries

Dysbiosis, Mediterranean diet, Type 2 diabetes.