The present disclosure describes a system for metabolic monitoring comprising: collecting exhaled gas from nasal airways of a subject; a mixing chamber for receiving a portion of the exhaled gas; a plurality of sensors for outputting signals related to gas parameters related to inhaled gas and exhaled gas during one or more breaths; and processors configured to determine a flow rate of gas in the interface appliance; determine pressure changes near the mouth of the subject to detect mouth breathing of the subject; determine concentration measurements of O2 and CO2 of the portion of the exhaled gas in the mixing chamber, and discard concentration measurements corresponding to mouth breathing; determine a rate of oxygen consumption VO2 and a carbon dioxide production VCO2 of the subject based on the determined flow rate, the CO2 concentration, and the determined O2 concentration.