Selenite metabolism in total parenteral nutrition (TPN)

Patients on long-term TPN commonly receive selenite to prevent selenium (Se) deficiency. Little information is available concerning the effect of chronic selenite supplementation on Se metabolism. In this study, we have used {sup 74}Se to examine selenite metabolism in 2 home TPN patents, one on selenite and one on no supplementation. Afte rcollection of baseline blood and urine samples, 80 {mu}g of selenite enriched with {sup 74}Se was added to the TPN formula, and infused over 12 hrs. Daily urine output was collected for 10 d. Inductively coupled plasma mass spectrometry was used to determine the isotope ratios of {sup 74}Se to {sup 77}Se, and {sup 74}Se to {sup 82}Se (added in vitro and an internal standard) in urine. Cumulative {sup 74}Se retention and an apparent selenite exchangeable pool size were calculated using standard isotope dilution equations. The unsupplemented TPN patient had biochemical Se deficiency, with decreased plasma Se (1 ng/ml) urine Se (1 ug/d) and red cell and plasma glutathione peroxidase activity (GSH-Px). Retention of {sup 74}Se was very high, 93% at 10 d, and the pool size was extremely low, 566 ug at 10 d. The supplemented patent had normal plasma and urine Se levels and plasma andmore » red cell GSH-Px. {sup 74}Se retention was very poor, only 42% at 1 d and 38% at 10 d. The Se pool size increased rapidly over time, reaching 12000 ug at 10 d. In contrast, our previous studies in normal subjects consuming dietary orgaic Se showed a selenite retention of 85-90% at 1 d and 70-80% at 10 d, and a pool size of 6000-8000 ug at 10 day. Conclusions: 1. Using {sup 74}Se, differences in Se retention and pool size can easily be detected in Se deficient vs replete TPN patients 2. Chronic supplementation with selenite appears to result in decreased {sup 74}Se retention and an expanded selenite exchangeable pool in comparison with normals consuming dietary Se.« less