Preparation of [1'- C]citric acid as a probe in a breath test to evaluate tricarboxylic acid cycle flux.

[1'- C]Citric acid (1) was efficiently prepared from dimethyl 1,3-acetonedicarboxylate in two steps as a probe for a breath test. The synthetic method was selected because of the yield and reproducibility. Compound 1 was orally administrated to rats, and the time course of the increase of CO / CO ratios (Δ CO ) in their breath was successfully followed, indicating the metabolism of 1.