Fiber

Fiber is the part of plant foods that humans can't digest. It comes in two broad types — soluble (dissolves in water, forms a gel: oats, beans, psyllium, fruit pectin) and insoluble (passes through largely intact: wheat bran, vegetable skins, nuts, whole grains). Both matter, and most fiber-rich foods contain a mix.

Average fiber intake in many Western countries is roughly half the recommended amount (US guidance: ~25g/day for women, ~38g for men). Higher intake is associated with lower cardiovascular disease, lower colon cancer risk, better blood-sugar control, better cholesterol profile, and more regular bowel habits.

Soluble vs insoluble

Soluble fiber slows digestion, helps lower LDL cholesterol, blunts blood-sugar spikes, and is fermented by gut bacteria into short-chain fatty acids that nourish the colon lining. Insoluble fiber adds bulk to stool and accelerates transit — particularly helpful for constipation, but can worsen diarrhea in some people. For IBS, soluble fiber (especially psyllium) is generally better tolerated than insoluble bran.

Building tolerance

The biggest mistake is ramping fiber too fast. Going from 10g to 35g overnight reliably causes bloating, gas, and cramping until the gut microbiome adapts — usually over 2–4 weeks. Increase by ~5g/day each week, drink plenty of water, and lean on diverse plant foods rather than one fiber supplement. People with IBS or active GI conditions should work up more slowly and under guidance.