Vitamin K is a fat soluble dietary nutrient that is essential for the binding of proteins to calcium. Vitamin K deficiencies can lead to bone weakening and blood coagulation problems. Vitamin K is rarely used as a Naturopathic treatment for acne. There is no reliable evidence that indicates Vitamin K supplementation has a significant effect on acne symptoms in otherwise healthy people.
Vitamin K is generally obtained through the dietary intake of leafy green vegetables. After consumption, bacteria in the digestive tract convert Vitamin K into several structurally similar molecules with important functions. Due to the ability of vitamin K to increase protein binding to calcium, connective tissues in skin may be more flexible and softer with an adequate supply of the vitamin. Because of this effect, some people think that Vitamin K supplements can help improve the appearance of acne scars, but this has not been proven.
Overall, Vitamin K supplements are safe when consumed in recommended concentrations, but are unlikely to be effective treatments for acne.
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Vitamin K @ Wikipedia
Uses of vitamins A, C, and E and related compounds in dermatology: A review. Keller, et al. 1998.