The catabolism rates of a medium chain fatty acid (octanoic acid), an even-numbered fatty acid (palmitic acid), and odd-numbered fatty acids (pentadecanoic acid and heptadecanoic acid) in mice were compared using stable isotope (13C) labeled fatty acids and isotope-ratio MS (IRMS). The catabolism rates of respective fatty acids were evaluated by the ratio of 13C and 12C in carbon dioxide expired from mice. The results show that the catabolism rate of octanoic acid is three times faster than that of palmitic acid. This result is in agreement with previous knowledge that medium chain fatty acids are easily beta-oxidized as compared to long chain fatty acids. The catabolism rates of odd-numbered fatty acids such as pentadecanoic acid and heptadecanoic acid were significantly lower as compared to those of even-numbered fatty acids such as palmitic acid. This finding supports our previous report that odd-numbered fatty acids are easily accumulated into body fat. The high accumulation of odd-numbered fatty acids in body fat would be a direct result of their low beta-oxidizability.
Practical applications: 13C-labeled fatty acids were administered to mice and the rates of 13CO2 formation were compared among medium chain, even-numbered, and odd-numbered fatty acids using IRMS. We found that the catabolism rates of odd-numbered fatty acids such as pentadecanoic acid and heptadecanoic acid were significantly lower in comparison to those of even-numbered fatty acids such as palmitic acid. These findings could be valuable for the development of the lipid metabolism field.