Alteration of CO2 production during nonfasting isotopic CO2 breath tests: concise communication.

Carbon isotope breath tests are often interpreted assuming a constant endogenous production of CO2 (some including calculations assuming a specific production of 9 mmol CO2/body weight per hour). We have evaluated the endogenous-CO2 production following ingestion of caloric meals varying with the range of most currently available carbon isotope breath tests. On three separate test days, fasting basal CO2 production was 8.08 +/- 0.55, 8.00 +/- 0.47, and 8.23 +/- 0.48 mmol/kg-hr (mean +/- s.e.m.), with a range 6-11 mmol/kg-hr. Administration of zero and 100 kcal led to no significant change from the basal CO2 production. In contrast, administration of 200 kcal or more led to significant elevation of endogenous CO2 production both by normal subjects and by subjects with nutrient malabsorption. This phenomenon could influence interpretation of some nonfasting isotopic CO2 breath tests; it deserves further evaluation.