Fiber

Soluble fiber dissolves in water and forms a gel, and gut bacteria ferment part of it. Insoluble fiber does not dissolve and mainly adds bulk to stool.^[2]

• Soluble: pectin, gums, mucilage, fructans and some resistant starches. Oats, barley, beans, lentils, apples and citrus fruit are good sources.^[2]
• Insoluble: lignin, cellulose and hemicellulose. Whole wheat, brown rice, nuts, green beans and leafy greens are good sources.^[2]

The effects differ too. Soluble fiber lowers LDL cholesterol.^[6] In irritable bowel syndrome, soluble fiber helped and bran, an insoluble fiber, did not.^[9] In observational studies, however, insoluble fiber tended to be linked to lower death rates more than soluble fiber.^[28]

Only plant foods contain fiber: fruit, vegetables, whole grains, legumes, nuts and seeds.^[2] Good sources include:

• Grains: oats, barley, rye, whole wheat, brown rice and bulgur.^[2]
• Legumes: beans, lentils, chickpeas and peas.^[2]
• Fruit: apples and pears with skin, berries, citrus fruit, plums and bananas.^[2]
• Vegetables: carrots, broccoli, sweet potatoes, green beans, spinach and kale.^[2]
• Nuts and seeds: almonds, walnuts, flaxseed and chia seeds.^[2]

Removing the peel from fruit and vegetables lowers the amount of fiber. Raw and cooked foods both offer the benefits.^[4] For cholesterol, 3 servings of oatmeal (28 g each) give about 3 g of soluble fiber.^[29]

Yes. Soluble fiber lowers LDL cholesterol, by a small to moderate amount. A 2023 review of 181 trials found a fall of 8.28 mg/dL, and each extra 5 g a day added about 5.57 mg/dL.^[6] A Cochrane review found that its trials were too short to show whether this means fewer heart attacks.^[15]

Often not. More fiber tends to raise the chance of bloating, as in a trial of 412 adults where a high-fiber diet raised the risk by 41%.^[8] Soluble fiber such as psyllium does improve overall symptoms of irritable bowel syndrome, and the NHS suggests soluble fiber when a bloated person is also constipated.^[9][24][19]

Yes. A 2022 review of 16 trials with 1,251 people found that 66% responded to fiber, against 41% on control treatment.^[7] Psyllium, doses above 10 g a day and at least 4 weeks of use appeared to work best, and gas was more common with fiber.^[7]

Only a little. A 2020 review of 62 trials with 3,877 people found that viscous (gel-forming) fiber lowered body weight by 0.33 kg and waist size by 0.63 cm, with no calorie limit. Body fat did not change significantly. Overweight people and people with diabetes or metabolic syndrome lost more.^[12]

Viscous fibers such as pectin, beta-glucan and guar gum reduced appetite in 59% of comparisons, and less viscous fibers in 14%. The effects on food intake and weight were relatively small.^[30]

In people with diabetes, yes, by a modest amount. In 29 trials with 1,517 adults with type 2 diabetes, soluble fiber supplements lowered HbA1c, a blood test of average blood sugar, by 0.63% and fasting glucose by 0.89 mmol/L.^[10] A review of more fiber in diabetes found HbA1c 2.00 mmol/mol lower in 33 trials. These results varied widely between trials.^[10][31]

In healthy adults, fewer than 20% of interventions with isolated fibers lowered blood glucose.^[32]

The European Medicines Agency says people with diabetes should use psyllium only under medical supervision, because their diabetes medicine may need to change.^[1] Health Canada says not to use it when blood sugar is hard to control.^[26]

Bacteria in the large intestine ferment fiber, and this makes gas.^[1] The steps are:

1. Fiber reaches the large intestine undigested.^[3]
2. Bacteria ferment it and make carbon dioxide, hydrogen and methane.^[1]
3. Gas stretches the bowel, and the belly feels full and uncomfortable.^[33][19]

The effect is strongest with highly fermentable fibers such as inulin, resistant starch and some oligosaccharides, and when intake rises too quickly.^[2] People with irritable bowel syndrome feel it more. In one study, 13 of 29 people with the condition reached a set level of symptoms after 40 g of inulin. Their gas levels were similar to those of healthy people.^[33]

This problem often goes away when the gut bacteria get used to the increase, so sources suggest a slow start over a few weeks.^[4] One review suggests adding 2 to 3 g every few days.^[2]

Psyllium can delay the absorption of other medicines, so it is taken apart from them. The European Medicines Agency (EMA) names:^[1]

• minerals and vitamin B12^[1]
• cardiac glycosides, such as digoxin^[1]
• coumarin blood thinners^[1]
• carbamazepine, a seizure medicine, and lithium^[1]

The time gap differs by source. The EMA says 30 to 60 minutes, MedlinePlus says 3 hours for digoxin, aspirin and nitrofurantoin, and Health Canada says a few hours.^[1][16][26]

Other medicines need care for other reasons. Doses of diabetes medicine and thyroid hormone may need to change, and opioids slow the gut, which raises the risk of a blockage.^[1] Anyone who takes a medicine every day should ask a doctor or pharmacist before starting psyllium.^[16]

Trials have given psyllium together with a statin, and LDL cholesterol fell further than with the statin alone. In 68 patients, 10 mg of simvastatin plus 15 g of psyllium a day lowered LDL by 63 mg/dL at 8 weeks, and 10 mg of simvastatin plus placebo lowered it by 55 mg/dL. The combination did as much as 20 mg of simvastatin alone, and it was well tolerated.^[34]

A 2018 review of 3 trials, lasting 4 to 12 weeks, reached a similar result. Three of its five authors worked for the company Procter & Gamble Health Care, so this review is not independent.^[35] A 2022 review of people with heart disease or high blood pressure found benefits from more fiber with or without heart medicines.^[36]

These trials were small and short, and they measured blood fats, not heart attacks.^[34][35] A person on a statin should talk to a doctor before adding fiber or changing the dose, because the European Medicines Agency says this use needs medical supervision.^[1]

No fiber is a clear winner. These are the LDL falls in separate reviews, each at a different dose and in different people, so they cannot be ranked directly:

• Konjac glucomannan: 0.35 mmol/L at about 3 g a day, from only 370 people.^[23]
• Psyllium: 0.33 mmol/L at about 10.2 g a day.^[20]
• Barley beta-glucan: 0.25 mmol/L at about 6.5 g a day.^[22]
• Oat beta-glucan: 0.19 mmol/L at about 3.5 g a day.^[21]

A 1999 review of 67 trials found no significant difference between soluble fiber from oats, psyllium and pectin.^[29]

Viscous fiber, which makes the gut contents thicker, appears to matter. In healthy adults, interventions with viscous fibers and mixed plant fibers lowered cholesterol more often than others.^[32] In one trial, oat beta-glucan lost half of its effect when its molecular weight was low.^[27]

Regulators back some of these fibers. EFSA, the EU food safety authority, found that at least 3 g of oat beta-glucan a day lowers LDL cholesterol.^[37] The US Food and Drug Administration allows a heart disease claim for soluble fiber from certain foods, with rules for oats, barley and psyllium husk.^[38]

Psyllium appears to cause less gas than inulin, based on small studies. In 19 people with irritable bowel syndrome (IBS), inulin made the most gas in the colon on MRI scans. Psyllium alone did not raise breath hydrogen, a measure of fermentation, while inulin did.^[39] No study read for this page compared all common fibers.^[39][40]