Selenium Excretion and Plasma Life
Daily selenium excretion in healthy adults averages 15-20 μg per day, representing approximately 30% of a typical dietary intake, with a plasma half-life reflected in the erythrocyte lifespan of approximately 120 days. 1, 2
Selenium Excretion Patterns
Daily Excretion Rates
- Average urinary excretion in healthy adults:
- Women: 17.7 ± 4.2 μg Se/day
- Men: 19.0 ± 9.0 μg Se/day 2
- After supplementation with 50 μg selenium (as sodium selenate), approximately 31% of the administered dose is excreted within 24 hours 3
- With higher supplementation (200 μg/day), excretion varies by selenium form:
- Selenate: 57% of dose (123 μg/day) at week 2
- Selenomethionine: 27% of dose (66 μg/day) at week 2, increasing to about 100 μg/day by week 16 4
Factors Affecting Excretion
- Renal clearance of selenium is approximately 0.18 mL/min in healthy individuals 2
- Excretion varies by selenium form:
- Selenomethionine: 0.4 mL/min renal clearance
- Selenate: 0.8-1.1 mL/min renal clearance 4
- Selenium excretion positively correlates with:
- Urea excretion (reflecting dietary protein intake)
- Creatinine excretion (reflecting muscle mass)
- Potassium excretion
- Glomerular filtration rate
- Urine volume 2
Selenium Metabolism and Plasma Life
Plasma Selenium
- Plasma selenium exists in multiple forms:
- Selenoprotein P: accounts for >50% of plasma selenium
- Extracellular glutathione peroxidase (GPX-3): accounts for approximately 20%
- Other forms including albumin-bound selenium 1
Plasma Half-Life
- Red cell glutathione peroxidase (GPX-1) reflects selenium intake over a longer period, corresponding to erythrocyte lifespan (approximately 120 days) 1
- Different forms of selenium have different plasma kinetics:
- Selenomethionine: reaches plateau at 190 ng/mL after 16 weeks of supplementation
- Selenate: reaches earlier plateau at 110 ng/mL after 7 weeks of supplementation 4
Metabolism and Excretion Forms
- Unmetabolized selenate is the main elimination product (approximately 30% of administered dose) 3
- Some individuals (TMSe eliminators) metabolize selenium to trimethylselenium ion
- Selenium-containing carbohydrates (SeSug1, SeSug3) are minor metabolites in most individuals 3
Clinical Implications
Monitoring Considerations
- Inflammation significantly affects plasma selenium levels:
- CRP 10-40 mg/L: reduces plasma selenium by 15-25%
- CRP 41-80 mg/L: reduces plasma selenium by about 35%
- CRP >80 mg/L: reduces plasma selenium by about 50% 1
- For accurate assessment, always measure CRP simultaneously with selenium levels 1
Special Populations with Altered Excretion
- Burns patients have high selenium losses requiring supplementation of around 375 μg/day 1
- Patients on continuous renal replacement therapy have increased losses 1
- Patients with kidney dysfunction may have altered selenium excretion patterns, limiting the usefulness of urinary selenium as a status marker 2
- Intensive muscular activity increases selenium excretion 2
Understanding selenium excretion patterns and plasma kinetics is essential for appropriate supplementation strategies, especially in clinical scenarios with increased losses or altered metabolism.