Dose-response relationship between fat ingestion and oxidation: quantitative estimation using whole-body calorimetry and C-13 isotope ratio mass spectrometry

Objective: To determine dose-dependent relationship between ingested fat an d its oxidation in the immediate post-prandial period in humans. Design: Subjects were randomly selected for the study at the Dunn Clinical Nutrition Centre, Cambridge, UK. Subjects ingested naturally enriched C-13 corn-oil doses (range 20-140 g) in a whole-body indirect calorimeter, and w ere studied for 8 h. Ingested fat oxidation was estimated from the subject' s breath C-13 enrichment and total carbon dioxide production. Total far and carbohydrate oxidation were estimated from non-protein oxygen and carbon d ioxide exchanges. Endogenous fat oxidation was estimated as the difference between total fat and ingested fat oxidation. Results: The amount of fat dose oxidized was nonlinearly related to the amo unt ingested. On average, 25.6 +/- 2.7% of the mean fat dose was oxidized. A significant (r = -0,72, P < 0.001) inverse correlation was found between the amount of fat dose and the proportion oxidized. Endogenous carbohydrate oxidation was negatively and significantly correlated to fat dose oxidized (r = -0.61, P < 0.01), but it was not correlated to endogenous fat oxidati on. Conclusions: There was a nonlinear relationship between amount of fat dose and its quantity that was oxidized in the immediate post-prandial period. T he inverse relationship between the size of the fat load and the proportion that was oxidized post-prandially implies increased dietary fat storage be yond about 50 g in a normal resting adult. This has important implications for (CO2)-C-13-based studies. Sponsorship: This study was funded by the Nestle Foundation, Switzerland an d the British Medical Research Council, UK.