Our objective was to quantify plasma protein synthesis after a single meal with a bolus administration of deuterium oxide. Labeled body water rapidly equilibrates with amino acids such as alanine prior to incorporation into newly synthesized proteins. We utilized a gas chromatograph combustion isotope ratio mass spectrometer (GCC‐IRMS) to measure very low levels of deuterium isotope incorporation with high precision. We compared the rate of plasma protein synthesis in CD‐1 mice in four groups: water treated control or three amounts of whey protein (low, medium and high). After food deprived overnight, all mice received an intraperitoneal injection of deuterium oxide four hours prior to treatment. Enrichment of plasma body water, free alanine and alanine bound to plasma proteins was determined at 5–8 hours after control or protein treatment. The fractional rate of plasma protein synthesis was 17.5%, 29.6%, 37.5% and 43.9% for control, low, medium and high protein treatment respectively. These finding show that the GCC‐IRMS with deuterium labeling of body water can be used to detect differences in protein synthesis after a single meal.