Measurement of fatty acid oxidation: validation of isotopic equilibrium extrapolation

Measurement of whole body substrate oxidation requires prolonged isotope infusion to attain plateau specific activity (SA) of expired CO2. We have investigated in 13 hospitalized patients a technique whereby plateau 14CO2 SA is extrapolated using computer curve fitting based upon the early exponential rise. A primed-constant infusion of albumin-bound 1-14C-palmitate was continued for 260 minutes with isotope priming of the secondary bicarbonate pool at 70 minutes. Plasma free fatty acid (FFA) SA reached steady state by 40 minutes and was 91% +/- 4% (SE) of values obtained at 190 to 260 minutes. At 70 minutes 14CO2 SA reached only 44% +/- 1% of the 190 to 260 minute values, which were consistently at plateau. The predicted steady state 14CO2 SA from the 40 to 70 minute curves and the FFA oxidation rates calculated from those values were 94% +/- 2% and 102% +/- 4%, respectively, of values measured at steady state (190 to 260 minutes). The relationship between predicted and measured values approximated the line of identity for 14CO2 SA (y = 0.90x + 0.14, r = .98, P less than .001) and FFA oxidation (y = 1.02x, r = .98, P less than .001). The results suggest that FFA oxidation can be accurately calculated using a short infusion of labeled FFA without bicarbonate pool priming, thus avoiding overpriming or underpriming and possibly allowing multiple studies and diminished radioisotope exposure.