Malic acid

Malic acid is an intermediate in the citric acid cycle.

Malic acid is found in apples.

Fibromyalgia
Non-blinded study of malic acid + magnesium http://www.tandfonline.com/doi/abs/10.3109/13590849208997961

''While these results are promising, a subsequent double-blind crossover study of primary fibromyalgia patients who received 300 mg magnesium and 1200 mg malic acid or placebo in random order for four weeks, each with a two-week washout period in between, failed to find significant improvements in pain, tenderness, and functional or psychological measures'' http://www.altmedrev.com/publications/5/2/93.pdf