A method for weight and/or fitness management using a metabolic analyzer that measures metabolic data including oxygen and carbon dioxide. The metabolic analyzer includes integrated collection-detection sensors with for high efficiency and collection, high specificity and simultaneous detection of at least two metabolic signatures, including at least oxygen and carbon dioxide, in breath via a porous membrane with high density of sensing binding sites, where the porous membrane includes sensing materials such that the sensing binding sites are specific to the metabolic signatures, and change colors upon interactions with the metabolic signatures. Weight of the subject is measured using a weight measurement device and a recommendation for food intake and/or physical activity is based on at least the readings of the metabolic analyzer and weight of the subject.