An indirect calorimeter for measuring the metabolic rate of a subject includes a respiratory connector configured to be supported in contact with the subject so as to pass inhaled and exhaled gases as the subject breathes, a flow pathway, and a hygiene barrier positioned to block a predetermined pathogen from the exhaled gases. The indirect calorimeter also includes a flow pathway having a first end in fluid communication with the respiratory connector and a second end in fluid communication with a source and sink for respiratory gases. The flow pathway includes a flow tube through which the inhaled and exhaled gases pass, an outer housing surrounding the flow tube, and a chamber disposed between the flow tube and the first end. The indirect calorimeter also includes a flow meter configured to generate electrical signals as a function of the instantaneous flow volume of inhaled and exhaled gases passing through the flow pathway, and a component gas concentration sensor operable to generate electrical signals as a function of the instantaneous fraction of a predetermined component gas in the exhaled gases as the gases pass through the flow pathway. The indirect calorimeter further includes a computation unit operable to receive the electrical signals from the flow meter and the concentration sensor and operative to calculate at least one respiratory parameter for the subject as the subject breathes through the calorimeter.