Types of Fiber

Cellulose fiber is the insoluble structural fiber in every plant, from celery stalks to wheat bran, and it mainly works by adding stool bulk. Purified cellulose added to foods like shredded cheese is the same material, and a 2018 EFSA review of ten cellulose additives found no safety concern at normal use levels.

Calcium polycarbophil is a synthetic, water-absorbing polymer sold as a bulk forming laxative in FiberCon and store-brand fiber caplets. In the intestine it soaks up about 70 times its weight in water, softening hard stool and firming loose stool, and the FiberCon label says it usually works within 12 to 72 hours.

Beta-glucan is a soluble fiber in oats and barley that forms a thick gel in the gut and lowers LDL cholesterol: in a 2010 trial of 345 adults, 3 g a day cut LDL by about 5% in four weeks. Yeast and mushroom beta-glucans share the name but are built differently and studied mainly for immune effects.

Arabinoxylan is a fiber in the cell walls of cereal grains, richest in rye and wheat bran. Its best-supported benefit is a gentler blood sugar rise after meals: across five trials, arabinoxylan-rich cereal foods lowered the peak by about 0.56 mmol/L. The rice bran supplement sold for immune support is a different, modified product.

Apple fiber is the mix of soluble pectin and insoluble cellulose in apples, also sold as a powder made from the pulp left after juicing. In a 2013 trial, whole apples and apple pomace lowered LDL cholesterol by about 7 to 8%, while clear apple juice, which loses the pectin, did not.

Acacia fiber is a soluble, prebiotic fiber made from the dried sap of acacia trees, also sold as acacia gum or gum arabic. In a 2008 trial, 10 g a day raised bifidobacteria about 10-fold more than the same dose of inulin, with only minimal changes in gas or bloating even at 40 g a day.

Resistant starch is starch that escapes digestion and feeds gut bacteria like fiber, and how you cook it changes how much you get. Chilling boiled potatoes for 24 hours raised resistant starch from 3.3% to 5.2% of total starch and cut the insulin response by about 28% in one study.

Prebiotic fiber is fiber that selectively feeds specific gut microbes and delivers a measurable health benefit, so not every fermentable fiber qualifies. In one trial, 12 g a day of chicory inulin raised Bifidobacterium and lowered Bilophila, a shift linked to softer stools, and in US diets wheat and onions supply about 95% of the inulin people eat.

Insoluble fiber works mostly through its physical shape: in one trial, bran-like plastic flakes sped whole-gut transit by 24%, while small granules of the same plastic didn’t change it significantly. Across 65 studies, each extra gram of wheat fiber added about 3.7 g to daily stool weight.

Soluble fiber is a family of fibers, and the best studied, beta-glucan, psyllium and pectin, each work a little differently. Across 28 trials, oat beta-glucan lowered LDL cholesterol by about 10 mg/dL, while in one pectin trial the type of pectin decided whether LDL fell by 7 to 10% or by less.