Agents that alter the pH of the stomach may decrease the absorption of selenium.
Concern has been raised that antioxidants may interfere with radiation therapy or some chemotherapy agents (such as alkylating agents, anthracyclines, or platinums), which themselves can depend on oxidative damage to tumor cells for anti-tumor effects. Studies of the effects of antioxidants on cancer therapies yield mixed results, with some reporting antagonistic effects (interference), others noting synergism (benefit), and most suggesting no significant interaction. This remains an area of study and controversy. In particular, selenium may reduce toxic side effects associated with chemotherapy drugs including cisplatin, doxorubicin, irinotecan (Camptosar®), or bleomycin. However, until better evidence is available, selenium supplementation is not recommended during chemotherapy or radiation therapy due to potential interference. Patients considering the use of selenium during chemotherapy or radiation therapy should discuss this choice with their medical and radiation oncologists.
High-dose steroid therapy may decrease plasma selenium levels.
Selenium has been suggested to increase the effects of erythropoietin in hemodialysis patients.
Chronic high selenium levels may decrease sperm motility, although effects on fertility are not known.
Taking selenium in combination with beta-carotene and vitamins C and E appears to decrease the effectiveness of the combination of simvastatin (Zocor®) and niacin, although long-term effects are not known. This may be due to antioxidant effects associated with selenium use. Theoretically, selenium could reduce the effectiveness of other HMG-CoA reductase inhibitors such as atorvastatin (Lipitor®), fluvastatin (Lescol®), lovastatin (Mevacor®), and pravastatin (Pravachol®).
Selenium levels may vary in the female life cycle and may be related to estrogen status. Selenium levels may be increased in patients taking birth control pills.
Selenium is a component of glutathione peroxidase, which possesses antioxidant activity and demonstrates antioxidant properties in humans. Long-term clinical benefits remain controversial. Selenium may add to the effects of other antioxidants in the body, such as vitamins A, C, and E, lycopene, green tea, soy, grape seed extract, or melatonin. The antioxidant activity of selenium may be affected by n-3 polyunsaturated fatty acids (n-3 PUFA).
There is preliminary evidence that vitamin C may be necessary for maintaining selenium levels in the body. Vitamin C appears to increase the absorption of natural selenium (found in foods) but not sodium selenate (found in supplements).
Selenium supplementation may affect the absorption of calcium and magnesium.
This information is based on a systematic review of the medical literature, edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com): Ethan Basch, MD (Memorial Sloan-Kettering Cancer Center); Wendy Chao, PhD (Natural Standard Research Collaboration); Dawn Costa, BA, BS (Natural Standard Research Collaboration); Shaina Tanguay-Colucci, BS (Natural Standard Research Collaboration); Catherine Ulbricht, PharmD (Massachusetts General Hospital); Wendy Weissner, BA (Natural Standard Research Collaboration).