Sensitivity of a 13C-protein oxidation breath test investigated with a protein restricted diet in healthy subjects

RATIONALE: Disturbed protein metabolism may result in malnutrition. A non-invasive low cost clinical tool to measure protein metabolism is lacking. Explorative research (n=1) with a newly developed non-invasive 13C-protein breath test suggested a decrease in protein oxidation after a protein restricted diet. Now, we aimed to test the effect of protein restriction in more subjects, to assess sensitivity of the test.
METHODS: In this exploratory study, 14 healthy male subjects (23±3 y) participated. Habitual intake was assessed by a 4-day food diary. Next, subjects were instructed to use a 4-day isocaloric protein restricted diet (0.25 g protein/kg bw/day). After an overnight fast, a 30 g naturally enriched 13C-milk protein test drink was consumed, followed by collection of breath samples up to 330 min. Protein oxidation was analyzed by Isotope Ratio Mass Spectrometry. 24-h urine was collected on day 4 of the habitual diet, and on every day of the 4-day protein restricted diet, to assess actual change in protein intake.
RESULTS: After the protein restricted diet, 30.2%±7.7 of the 30 g 13C-milk protein was oxidized over 330 min, compared to 30.6%±6.2 (NS) after the subject’s habitual diet (1.4±0.3 g protein/kg bw/day). Within subjects, both increase and decrease in oxidation was found. During the 4-day protein restricted diet, urinary urea:creatinine ratio decreased by 56%±10, consistent with a reduction in protein intake of 44%±15 (g/day) and 53%±12 (g/kg bw/day), based on urea and food diary, respectively.
CONCLUSIONS: The breath test shows variation within subjects and between diets, which could be related to the sensitivity of the test. We cannot explain the variation by the measured variables. Alternatively, our results may implicate that in some of our subjects, protein intake did not sufficiently decrease to levels that could alter protein metabolism.