An oxycalorimeter is a laboratory instrument that measures the gases exchanged when a food sample is completely burned — the oxygen taken up and the carbon dioxide released — data from which the food’s energy content is calculated. The principle rests on the consistent relationship between oxygen uptake and heat release during combustion of organic matter. Historically, bomb calorimetry, burning food in pressurised oxygen and measuring the temperature rise, was the direct method for determining the energy values that underpin food composition tables. Modern nutrition labelling more often uses the Atwater system, calculating energy from protein, fat, carbohydrate, and alcohol content, but calorimetry remains the reference method for validating those factors and for analysing novel foods. The instrument embodies the first law of thermodynamics applied to nutrition: energy in food becomes measurable heat.