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Energy metabolism is the quantification of energy flows in the body. Indirect calorimetry (Weir equation) estimates resting energy expenditure (REE) by measuring O consumption and CO production: these gases reflect the oxidation of substrates (fat: RQ=0.7; carbohydrates: RQ=1.0; proteins: RQ0.8). The RQ also allows us to infer what fuel the body is oxidizing.

Oxidative phosphorylation in the mitochondrial respiratory chain converts the proton gradient generated by NADH and FADH into ATP (P/O 2.5 and 1.5 respectively). Aerobic glucose yields ~3032 ATP vs. only 2 ATP in anaerobic glycolysis: this explains why lactate accumulates rapidly when O supply is insufficient (ischemia, high intensity exercise).

Kleiber's law (BMR M^0.75) is one of the most robust biological regularities: basal metabolism does not scale with mass (M^1) but with M^0.75, probably reflecting the fractal geometry of the blood distribution network. This implies that large animals are more metabolically efficient per unit mass but require more total energy.

VO_max (Fick equation: CO_max × maximum a-v difference) is the upper limit of aerobic capacity. In untrained subjects, the limit is mainly the maximum cardiac output (CO_max 20 L/min); In elite athletes, lung diffusion and muscle extraction capacity also matter. VO_max is the best predictor of longevity and cardiac functional capacity.

METs allow us to quantify the energy expenditure of physical activities in a practical way: walking at 5 km/h 3.5 METs; run at 10 km/h 10 METs. In ICU, the REE of critically ill patients can be 1.22× that predicted by Harris-Benedict (hypermetabolism), and indirect calorimetry guides artificial nutrition to avoid overfeeding (hyperglycemia, hypercapnia, difficulty weaning from the ventilator).

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ID:('ky', 40)

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ID:(0, 40)