The 13CO2 breath test for liver glycogen oxidation after 3-day labeling of the liver with a naturally 13C-enriched diet

Objective: When naturally 13C-enriched carbohydrate is used to label hepatic glycogen, 13C-liver glycogen oxidation can be monitored subsequently by measuring the 13C enrichment of breath CO2 during a sedentary fast. In our previous breath test studies, we used a 1-d labeling protocol to enrich liver glycogen. Others found that after 3 d of labeling the liver glycogen 13C enrichment is identical to the dietary carbohydrate 13C enrichment. Methods: We compared a diet protocol in which naturally 13C-enriched carbohydrate was given for 3 d before the breath test with our previously applied 1-d labeling design. The 13CO2 breath test was combined with indirect calorimetry. The results were compared with those from our previous studies. In addition, we compared liver glycogen oxidation rates with those from our present technique and different techniques as used in other published studies. Results: Six healthy volunteers were included in this study. The 13C enrichment of breath CO2 at plateau excretion level did not differ after 1 or 3 d on a labeling diet. However, the end of plateau time tended to be later after the 3-d diet, 14.3 h versus 12.5 to 13.5 h postprandially in the 1-d labeling studies. Also, the return to baseline time was later in the 3-d study, at 25.8 h versus 19.0 to 23.2 h postprandially after 1 d of labeling. The liver glycogen oxidation rate was similar in both techniques until 17 h postprandially. After this time the 3-d labeling protocol showed a higher level of liver glycogen oxidation. Conclusion: The results indicated that the labeling of liver glycogen is slightly less complete after 1 d on a 13C-enriched diet as compared with 3-d labeling. Our 13C breath test results compared rather well with studies from the literature using the 13C-NMR technique, the D2O technique, or the 13CO2 breath method to measure liver glycogen oxidation.

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