Following the ingestion of only $5.1\mathrm{mL}$ of ${\mathrm{D}}_2\mathrm{O}$ , a mid-infrared laser spectrometer determines the D/H isotope ratio increase in exhaled water vapor for the first time, to the best of our knowledge. This increase is still detectable several weeks after the heavy water intake. Collected breath samples are directly transferred into a high-temperature multipass cell operated at $373\mathrm{K}$ . No breath sample preparation is required. Aside from the capability to hinder unwanted condensation, measurements at elevated temperatures offer other advantages such as a lower temperature dependence of the delta value or the possibility to vary the intensity of absorption lines. We lay the foundation for many laser-based clinical applications. As an example, we measure a total body water weight of $55.2\%\pm 1.8\%$ with respect to the total body weight, in agreement with the normal value of the male population.