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In biochemistry, an ultratrace element is a chemical element that normally comprises less than one microgram per gram of a given organism (i.e. less than 0.0001% by weight), but which plays a significant role in its metabolism.
J. Mann & A. S. Truswell (editors). Essentials of Human Nutrition (3rd edition, 2007). Oxford: Oxford University Press
Yamada, Kazuhiro (2013). "Chapter 9. Cobalt: Its Role in Health and Disease". In Astrid Sigel; Helmut Sigel; Roland K. O. Sigel (eds.). Interrelations between Essential Metal Ions and Human Diseases. Metal Ions in Life Sciences. Vol. 13. Springer. pp. 295–320. doi:10.1007/978-94-007-7500-8_9. ISBN978-94-007-7499-5. PMID24470095.
Nielsen, Forrest H., Ultratrace Elements in Nutrition, Annual Review of Nutrition
Vol. 4: 21-41 (Volume publication date July 1984)
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