What is the best fiber supplement for both constipation and diarrhea?

The best fiber supplement for both constipation and diarrhea is psyllium powder, which is found in products such as Metamucil, according to clinical trial results discussed by WebMD. Metamucil is a soluble fiber that absorbs water and increases the bulk in stool, which causes the movement of the intestines.

WebMD advises combining psyllium power with a glass of water and drinking it up to three times a day. The increase in fiber amount should be gradual since ingestion of the drink may cause a sense of bloating while the body becomes accustomed to the supplement. Plenty of liquid should also be ingested daily.