We will evaluate the reproducibility of the selenite-exchangeable metabolic pool (Se-EMP) in home-TPN patients and in normal subjects. Each test will consist of replacement of the subject's selenium intake on the test day (TPN patients) with labeled selenite (stable isotope) or infusion of the selenite label in saline (normals). Urine and in selected tests, feces, will be collected over 10 days and analyzed for excretion of the label. Based on this, the time course of the size of the Se-EMP will be calculated.