Preparation of [1′‐13C]citric acid as a probe in a breath test to evaluate tricarboxylic acid cycle flux

[1′‐ 13 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 13 CO 2 / 12 CO 2 ratios ( Δ 13 CO 2 ) in their breath was successfully followed, indicating the metabolism of 1 . Thus, the 13 C‐breath test using 1 is a promising method to evaluate tricarboxylic acid (TCA) cycle flux.