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Selenium

Supplement mineral immunegeneral nutrition
Aka: selenomethionine, sodium selenite
Brands seen in Singapore: NOW Foods, Life Extension, California Gold Nutrition, Thorne, Swanson Vitamins
Third-party tested: Thorne certifies other products with NSF Certified for Sport, but nothing for selenium.Certification is granted per product, so check the tub you actually own. Not being listed is not the same as failing.
Not medical advice. Check with your doctor or pharmacist before you start, stop or change the timing of anything you take.
How to use
What it’s for
Thyroid hormone metabolism, DNA synthesis, antioxidant role
When to take
Timing not addressed by NIH ODS · no food rule given
Take with
Water · up to ~90% absorbed from the common supplement forms
Pros & cons
Pros
  • Small daily need: RDA 55 mcg for adults
  • HSA allows thyroid, nail and antioxidant claims
  • One brazil nut carries 68–91 mcg
Cons
  • UL 400 mcg a day; excess: hair loss, brittle nails
  • Trials don't back it for cancer, heart or cognition
  • HSA caps SG supplements at 0.2 mg (200 mcg) a day
Sources
  1. NIH ODS — Selenium Fact Sheet for Health Professionals (Updated: September 4, 2025) — Introduction + Table 1 RDAs · read 2026-08-27
    what it says
    “[selenoproteins that play critical roles in] thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection … [RDA, 19–50 years] 55 mcg (male), 55 mcg (female), 60 mcg (pregnancy), 70 mcg (lactation)”
  2. NIH ODS — Selenium Fact Sheet for Health Professionals (Updated: September 4, 2025) — Dietary Supplements + Sources of Selenium · read 2026-08-27
    what it says
    “selenomethionine, selenium-enriched yeast … sodium selenite, and sodium selenate … up to about 90% of selenium from selenomethionine, selenium-enriched yeast, selenite, and selenate … Brazil nuts contain particularly high amounts of selenium … 68–91 mcg per nut”
  3. NIH ODS — Selenium Fact Sheet for Health Professionals (Updated: September 4, 2025) — Selenium and Health — cancer (SELECT) + cardiovascular disease · read 2026-08-27
    what it says
    “selenium supplementation did not reduce the risk of prostate cancer … clinical trial evidence does not support the use of selenium supplementation for reducing the risk of CVD”
  4. NIH ODS — Selenium Fact Sheet for Health Professionals (Updated: September 4, 2025) — Selenium and Health — thyroid disease + cognitive decline · read 2026-08-27
    what it says
    “more research is needed to understand the role of selenium supplements in thyroid disease … clinical evidence does not support the use of selenium supplementation … for reducing the risk of cognitive decline”
  5. NIH ODS — Selenium Fact Sheet for Health Professionals (Updated: September 4, 2025) — Health Risks from Excessive Selenium + Table 3 ULs · read 2026-08-27
    what it says
    “garlic odor in the breath and a metallic taste in the mouth … hair loss and nail brittleness or loss … skin rash, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities … [UL, 19–50 years] 400 mcg”
  6. NIH ODS — Selenium Fact Sheet for Health Professionals (Updated: September 4, 2025) — Interactions with Medications + Groups at Risk of Selenium Inadequacy · read 2026-08-27
    what it says
    “Cisplatin, an inorganic platinum chemotherapy agent, is used to treat ovarian, bladder, lung, and other cancers … hemodialysis removes some selenium from the blood … People living with HIV often have low selenium concentrations”
  7. FDA — Daily Value on the Nutrition and Supplement Facts Labels, Reference Guide: Daily Values for Nutrients (content current as of 03/05/2024) · read 2026-08-27
    what it says
    “DVs are the recommended amounts of nutrients to consume or not to exceed each day … Selenium 55mcg”
  8. HSA Singapore — Health Supplements Guidelines GL-CHPB-3-001 Rev. No. 020 (January 2026), clause 4.8 + Table 5 Minerals Limits · read 2026-08-27
    what it says
    “4.8 HS for use by the general adult population should not contain vitamins and minerals exceeding the stipulated limits stated in Tables 4 and 5 … Table 5: Minerals Limits … Selenium 0.2 mg”
  9. HSA Singapore — Health Supplements Guidelines GL-CHPB-3-001 Rev. No. 020 (January 2026), List of Health Claims (vitamins and minerals) · read 2026-08-27
    what it says
    “7. Maintain/support healthy thyroid gland function • Iodine, selenium … 12. Contribute to maintenance of normal nails • Selenium, zinc … 33. Contribute to protection of cells from oxidative stress • Copper, selenium, manganese, zinc”
  10. HSA Singapore — Guidelines on Prohibited and Restricted Ingredients in Health Supplements and Traditional Medicines, GL-CHPB-1-012 Rev. No. 001 (October 2025), Annex A · read 2026-08-27
    what it says
    “93. Selenium — Restricted use in HS and TM — Maximum Daily Dose: When for oral use by general adult population, the maximum recommended daily dose should not provide more than 0.2mg”
  11. iHerb Singapore — Selenium category page (Supplements > Minerals > Selenium), default order, 79 results — positions 1, 2, 4, 5, 7 · read 2026-08-27
    what it says
    “NOW Foods, Selenium, 100 mcg, 100 Tablets … Life Extension, Super Selenium Complex & Vitamin E … California Gold Nutrition, Selenium, Yeast-Free, 200 mcg, 180 Veggie Capsules … Thorne, Selenium, 200 mcg, 60 Capsules … Swanson Vitamins, Albion® Selenium, 200 mcg”
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Updated 2026-08-30. Every line above traces to a numbered source. Kozen-X is not a medical device and does not give medical advice. It lists what published sources have written down — nothing more. A pair not listed is unrecorded, not cleared. Before you start, stop or re-time anything, talk to your doctor or pharmacist.