Background: Nutrition studies in the intensive care unit (ICU) have shown that adequate enteral nutrition (EN) support has clinical benefits. However, the course of amino acid concentrations in plasma has never been investigated in patients admitted with shock receiving EN. We hypothesized that plasma concentrations, when deficit, increase during EN and that persistent deficiency is associated with poor outcome. Methods: In 33 septic or cardiogenic shock patients receiving EN, plasma amino acid concentrations were measured during 5 days. Changes in amino acid concentrations, correlations with clinical outcome variables, and regression analyses were studied. Results: On ICU admission, several plasma concentrations were deficient. Plasma concentrations of almost all amino acids increased. In contrast, taurine decreased by >50%, from 47.6 μmol/L on admission to 20.0 μmol/L at day 1, and remained low at day 5. Taurine (admission) correlated with time on mechanical ventilation (R = -0.42, P =.015). Taurine decrease within 24 hours correlated with Acute Physiology and Chronic Health Evaluation II predicted mortality (R = 0.43, P =.017) and Sequential Organ Failure Assessment score (R = 0.36, P =.05). Regression analyses confirmed correlations. Conclusions: Several amino acids were deficient in plasma on ICU admission but increased during EN. Taurine concentrations declined and were associated with longer periods of mechanical ventilation and ICU support. Fast taurine decline correlated with severity of organ failure. These findings support the role of taurine during ischemia, reperfusion, and inflammation. Taurine may be an essential candidate to enrich nutrition support for critically ill patients, although more research is required.