Doubly labeled water with enhanced protocol. One embodiment is a method including providing, to a user, a doubly labeled water (DLW) dose for the user to ingest, the DLW dose including deuterium and oxygen-18, wherein an amount of the deuterium is less than 0.12 grams per kilogram (g/kg) of body water of the user, and wherein an amount of the oxygen-18 is less than 0.18 g/kg of body water of the user. The method also includes receiving, from the user, a non-cooled shipment of urine samples collected in connection with ingestion of the DLW dose, wherein the urine samples remain uncooled after collection and during transit for a period of up to 24 days. The method further includes processing the urine samples with a liquid water isotope analyzer to determine one or more metabolic parameters of the user.