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Electronic letters published:
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About the codon of selenocysteine and the synthesis of selenoproteins
Antioxidant Confusion
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Rubens Pazza, Dr. Universidade Federal de Viçosa, Campus Rio Paranaíba.
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rpazza{at}ufv.br Rubens Pazza
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Nowadays we are flooded with new researches that are putting down the idea of nutritional supplements as a font of health. One of these cases is related to an article recently published in this journal, about the correlations between long term selenium supplementation diet and diabetes in humans (Bleys et al., 2007). The article is very informative, but one information makes me concerned. Authors saids that “selenium is incorporated into selenoproteins as selenocysteine through a complex genetic mechanism encoded by UGA codon”. Some explanations become necessary after that. Selenium is an essential nutrient of fundamental importance for human biology, witch has an anti-oxidant function as a component of selenoproteins with enzymatic function, besides their role in cancer and immune response (Rayman, 2000). Furthermore, selenium supplement is being adopted in animal nutrition, especially in dairy cows, as a resistance factor for clinical mastitis (Smith et al., 1983; Paschoal et al., 2003). The incorporation of selenium in selenoproteins is due to substitution of sulfur in the cysteine and methionine aminoacids, becoming selenocysteine and selenomethionine and these aminoacids are incorporated into selenoproteins through the same codon of original aminoacids – AUG for selenomethionine; UGU and UGC for selenocysteine. The mechanism of selenoproteins synthesis is the same as other proteins, through the translational process drived by ribosomes. Therefore, the affirmation of Bleys et al. (2007) must be revised to avoid misunderstanding. References: Bleys, J.; Navas-Acien, A.; Guallar, E. (2007). Selenium and Diabetes: More Bad News for Supplements. Ann Intern Med 147(4): 271-273. Paschoal, J. J.; Zanetti, M. A.; Cunha, J. A. (2005). Clinical mastitis in dairy cows supplemented with selenium and vitamin E. Pesq Agropec Bras 40(10): 1043-1046. Rayman, M. P. (2000). The importance of selenium to human health. The Lancet 356: 233-241. Smith, K. L.; Harrison, J. H.; Hancock, D. D.; Todhunter, D. A.; Conrad, H. R. (1983). Effect of Vitamin E and Selenium Supplementation on Incidence of Clinical Mastitis and Duration of Clinical Symptoms. J Dairy Sci 67: 1293-1300. Conflict of Interest:None declared |
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Neil E Levin, CCN, DANLA
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neil.levin{at}nowfoods.com Neil E Levin
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It is unfortunate that synergistic nutrients are tested individually, like drugs. This is a pharmaceutical practice, not human nutrition, and shouldn’t imply that comprehensive mixtures of nutrients (like a multivitamin) would have the same effects. I dispute that “randomized, controlled clinical trials have shown that ß-carotene and vitamin E supplements, which were widely believed to be safe, increase mortality and morbidity”. The choices, increasing variables, and manipulation by statistical models in meta-analyses are often questionable. In this report, prediabetic symptoms were not even considered as a variable. Buijsse noted that high carotenoid intake, confirmed by measures of blood levels, was associated with lower mortality rates among the elderly over a ten-year period, countering the claim that ß-carotene dangers are proven. The Miller meta-analysis, cited as proof of vitamin E’s dangers, was heavily criticized in published responses, and its conclusions were NOT replicated when the same data was re-analyzed (Hathcock). It should therefore not be cited as proof of the vitamin’s danger. Another meta-analysis cited (Bjelakovic) reported that antioxidant vitamins may increase death rates. But the authors did not determine a dose-dependent or cause-and-effect relationship between antioxidants and deaths (from all causes) of study participants. The researchers pooled 68 published trials, excluding 405 published studies with no deaths reported. Too wide a range of potencies (Vitamin A 1333 IU - 200,000 IU, vitamin E 10 IU to 1000 IU), and durations (28 days to 12 years) were lumped together. The statement, “No dietary supplement, including selenium, has proven useful so far for the prevention of cardiovascular disease or cancer in the general U.S. population,” is questionable. The Alpha-Tocopherol, Beta- Carotene Cancer Prevention (ATBC) Study published by the National Cancer Institute demonstrated a 32% reduction in prostate cancer incidence in response to daily alpha-tocopherol supplementation. The Women’s Health Study (JAMA) reported a significant 24% reduction in cardiovascular death with supplemental vitamin E. The NIH reports, “Taking a daily supplement containing 200 μg of selenium … significantly reduced the occurrence and death from total cancers. The incidence of prostate cancer, colorectal cancer, and lung cancer was notably lower in the group given selenium supplements.” Yet you suggest lowering consumption below the Daily Value (70 μg) used in multivitamins and far below the current upper limit. On the contrary, the Lewin Group reports that the use of antioxidants could save the vision and independence of many senior citizens, saving billions of dollars in healthcare costs. REFERENCES: Bjelakovic G, et.al. Mortality in Randomized Trials of Antioxidant Supplements for Primary and Secondary Prevention: Systematic Review and Meta-analysis. JAMA 2007. 297(8):842-857 Buijsse B, et al. Plasma carotene and alpha-tocopherol in relation to 10-y all-cause and cause-specific mortality in European elderly: The Survey in Europe on Nutrition and the Elderly, a Concerted Action (SENECA). Am J Clin Nutr 2005;82:879–886. DaVanzo JE, et al. An Evidence-Based Study of the Role of Dietary Supplements in Helping Seniors Maintain their Independence. The Lewin Group Inc. January 20, 2006 Hathcock JN, et al. Vitamins E and C are safe across a broad range of intakes. Am J Clin Nutr. 2005 Apr;81(4):736-45. Review. PMID: 15817846 Weinstein SJ, et al. Serum α -Tocopherol and γ-Tocopherol in Relation to Prostate Cancer Risk in a Prospective Study. J. Natl. Cancer Inst. 2005 97: 396-399; doi:10.1093/jnci/dji045 Conflict of Interest:Nutrition educator for a dietary supplement manufacturer, but the opinions expressed are my own. |
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