A system for metabolic monitoring comprising: interface appliance configured to collect exhaled gas from nasal airways of a subject; a mixing chamber operatively connected with the interface appliance, the mixing chamber configured to receive at least a portion of the exhaled gas from the interface appliance; a plurality of sensors configured to output signals related to one or more gas parameters related to inhaled gas and exhaled gas by the subject during one or more breaths; and one or more physical computer processors operatively connected with the sensors to receive the output signals, the one or more physical computer processors configured by computer readable instructions to: determine a flow rate of gas in the interface appliance during the one or more breaths based on the output signals from the sensors; determine pressure changes near a mouth of the subject during the one or more breaths based on the output signals from the sensors to detect mouth breathing of the subject; determine concentration measurements of 02 and CO2 of the portion of the exhaled gas in the mixing chamber based on the output signals from the sensors, and discard concentration measurements corresponding to mouth breathing; and determine a rate of oxygen consumption VO2 and a carbon dioxide production VCO2 of the subject based on the determined flow rate of gas, the determined carbon dioxide concentration of the exhaled gas in the mixing chamber, and the determined oxygen concentration of the exhaled gas in the mixing chamber. interface appliance configured to collect exhaled gas from nasal airways of a subject; a mixing chamber operatively connected with the interface appliance, the mixing chamber configured to receive at least a portion of the exhaled gas from the interface appliance; a plurality of sensors configured to output signals related to one or more gas parameters related to inhaled gas and exhaled gas by the subject during one or more breaths; and one or more physical computer processors operatively connected with the sensors to receive the output signals, the one or more physical computer processors configured by computer readable instructions to: determine a flow rate of gas in the interface appliance during the one or more breaths based on the output signals from the sensors; determine pressure changes near a mouth of the subject during the one or more breaths based on the output signals from the sensors to detect mouth breathing of the subject; determine concentration measurements of 02 and CO2 of the portion of the exhaled gas in the mixing chamber based on the output signals from the sensors, and discard concentration measurements corresponding to mouth breathing; and determine a rate of oxygen consumption VO2 and a carbon dioxide production VCO2 of the subject based on the determined flow rate of gas, the determined carbon dioxide concentration of the exhaled gas in the mixing chamber, and the determined oxygen concentration of the exhaled gas in the mixing chamber. determine a flow rate of gas in the interface appliance during the one or more breaths based on the output signals from the sensors; determine pressure changes near a mouth of the subject during the one or more breaths based on the output signals from the sensors to detect mouth breathing of the subject; determine concentration measurements of 02 and CO2 of the portion of the exhaled gas in the mixing chamber based on the output signals from the sensors, and discard concentration measurements corresponding to mouth breathing; and determine a rate of oxygen consumption VO2 and a carbon dioxide production VCO2 of the subject based on the determined flow rate of gas, the determined carbon dioxide concentration of the exhaled gas in the mixing chamber, and the determined oxygen concentration of the exhaled gas in the mixing chamber.