Fat-soluble vitamin K is a critical part of the activation of coagulation factors II (prothrombin), VII, IX, and X. Compared with adults and older children, newborns have much lower levels of vitamin K: approximately 40% to 60% of normal adult values, which are not typically reached until approximately 6 months of age. This phenomenon is multifactorial, driven in part by low transplacental transfer of vitamin K, low levels of vitamin K excreted in human milk, and a physiologic delay in the acquisition of gut microflora that synthesize vitamin K, which is actually active in several forms. Vitamin K1, or phylloquinone, is present in green leafy vegetables and is the most common source of vitamin K for humans. Vitamin K2, or menaquinone, is synthesized by bacteria and is found in some animal and fermented products.

Vitamin K was first discovered in 1929 by Henrik Dam, who observed...

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