Summary
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What are the health benefits of prebiotics?
Prebiotic supplements provide soluble fiber that is fermentable and acts as food for microbes in the gut, which can turn it into other helpful compounds such as butyrate. There are a variety of sources of prebiotic fiber in supplements isolated from natural sources, such as chicory, acacia, larch, and guar, or synthesized to resemble natural prebiotic fiber. These prebiotic fibers include inulin (which contains fructooligosaccharides (also called FOS or fructans) and other prebiotic fibers), guar gum, and galactooligosaccharides (GOS) made from lactose from milk (see What It Is)
However, you can easily get prebiotic fiber from a wide variety of plant-based foods, including whole grains like oats and wheat, garlic, onions, and asparagus -- which have the benefit of also providing forms of fiber that are nonsoluble and/or nonfermentable fiber and help with stool regularity.
Be aware that that not all soluble fiber is prebiotic fiber. The fiber found in psyllium is predominantly soluble but is nonfermentable and forms viscous gel which helps with laxation. Also, while the beta-glucans in oats and other grains are prebiotic, beta-glucans in mushrooms are less soluble and, likely, offer different benefits. -
How much prebiotic should I take?
Clinical studies have used about 5 to 10 grams of prebiotic ingredient given once to three times per day, often starting with once daily to reduce initial gastrointestinal side effects (see What It Does). -
What did ConsumerLab's tests of prebiotic supplements show?
Our tests showed that you can't easily tell how much prebiotic fiber is actually in a prebiotic supplement: The actual amount of prebiotic fiber in products ranged from as little as 26% of the listed amount of prebiotic ingredient (i.e., 2.6 grams in a product claiming 10 grams) to as much as 96% (see What CL Found).
We also found the cost to get a gram of prebiotic fiber from supplements to range from just 12 cents to 85 cents. 
What is ConsumerLab's Top Pick among prebiotics?
Our two Top Picks among prebiotics passed our tests showing that they contained all or more of the prebiotic fiber that we expected (without heavy metal contamination) at a clinically relevant dose and relatively low cost (see Top Picks).-
Are there side-effects with prebiotics?
Prebiotics can initially cause gastrointestinal discomfort, gas and bloating. This can be an issue for some people with irritable bowel syndrome and/or those following a low FODMAP diet that, by definition, restricts prebiotic fiber. People with liver disease should be cautious in using prebiotics (see Concerns & Cautions).
