Fumaric acid sits inside the Krebs cycle, the chain of reactions your cells use to produce energy. Industrial and supplement makers frequently bind it to minerals, creating salts like ferrous fumarate for iron or magnesium fumarate.
By itself, fumaric acid has no clear purpose as a dietary supplement. When people use it as part of a mineral salt, any benefit comes from the attached mineral, not from the fumarate portion.
Large doses can upset the stomach. No standard dose exists for pure fumaric acid.