Some dieters say that fiber helps suppress their appetites.

Unlike laxatives, fiber can truly help regulate bowel patterns. If you choose to take a fiber supplement, be sure you don’t inadvertently purchase a laxative supplement instead. The labels on both types of supplements may say something like “regulates bowel patterns.” While the featured ingredient of fiber supplements will likely be an ingredient such as psyllium, the featured ingredient of laxatives tend to be herbal-based. Such supplements are designed only for short-term constipation.

While there is weak evidence that fiber may promote a feeling of fullness, it seems to be necessary to use it in conjunction with a diet and exercise program in order to be effective for contributing to weight loss.

The best way to get fiber is from food. However, if you don’t include enough fiber-rich food in your diet and choose to use a fiber supplement instead, choose a product that has different types of fiber in it—both soluble and insoluble. When taking a fiber supplement, be sure to stay well hydrated.

Fiber supplements are one way to add fiber to a weight-loss diet. Several trials have shown that supplementation with fiber from a variety of sources accelerated weight loss in people who were following a low-calorie diet.^{1 , 2 , 3 , 4} Other researchers found, however, that fiber supplements had no effect on body weight, even though it resulted in a reduction in food intake.⁵ Supplementation with 3 to 4 grams per day of a bulking agent called glucomannan, with or without a low-calorie diet, has promoted weight loss in overweight adults,^{6 , 7 , 8} while 2 to 3 grams per day was effective in a group of obese adolescents in another controlled trial.⁹ However, guar gum, another type of fiber supplement, has not been effective in controlled studies for weight loss or weight maintenance.^{10 , 11 , 12}

The most common adverse effects of dietary fiber are minor gastrointestinal symptoms. There have been several reports of allergic reactions to psyllium. In rare cases, obstruction of the large or small intestine has occurred in people consuming wheat bran or bran cereal.

Beans, a good source of soluble fiber, also contain special sugars that are often poorly digested, leading to gas. 

People with scleroderma (systemic sclerosis) should consult a doctor before taking fiber supplements or eating high-fiber diets. Although a gradual introduction of fiber in the diet may improve bowel symptoms in some cases, there have been several reports of people with scleroderma developing severe constipation and even bowel obstruction requiring hospitalization after fiber supplementation.¹³

Fiber reduces the absorption of many minerals. However, high-fiber diets also tend to be high in minerals, so the consumption of a high-fiber diet does not appear to impair mineral status. However, logic suggests that calcium, magnesium and multimineral supplements should not be taken at the same time as a fiber supplement.

Bran, which contains insoluble fiber, reduces the absorption of calcium enough to cause urinary calcium to fall.¹⁴ In one study, supplementation with 10 grams of rice bran twice a day reduced the recurrence rate of kidney stones by nearly 90% in recurrent stone formers.¹⁵ However, it is not known whether other types of bran would have the same effect. Before supplementing with bran, people should check with a doctor, because some people—even a few with kidney stones—do not absorb enough calcium. For those people, supplementing with bran might deprive them of much-needed calcium.

Whole grains are particularly high in insoluble fiber. Oats, barley, beans, fruit (but not fruit juice), psyllium, chia seed, and some vegetables contain significant amounts of both forms of fiber and are the best sources of soluble fiber. The best source of lignan, by far, is flaxseed (not flaxseed oil, regardless of packaging claims to the contrary).