Effect of feeding or low temperature on mitochondrial efficiency. To compare data from the two experimental protocols, oxygen consumption rates were calculated at the highest common ATP synthesis rate for both substrates (94.2 nmol ATP min−1 mg−1 protein) by using individual linear relation curves. Then, the resulting effective ATP/O ratios for each respiratory substrate were expressed as a percentage of the corresponding fasted ATP/O ratio measured at 38°C. Black bars, feeding; grey bars, low temperature.