**1. Introduction**

Vitamin K is an important fat-soluble vitamin. The discovery of vitamin K was in Germany in 1929 by Henrik Dam in his research on sterol metabolism, and he suggested the name vitamin K on the basis of its role in coagulation (koagulation in German spelling).

The exact function of vitamin K in the human body was discovered in the 1970s with the discovery of γ-carboxyglutamic acid (Gla), an amino acid found in all vitamin K proteins [1].

Gamma glutamyl carboxylase is an enzyme that located in the endoplasmic reticulum and mediates the posttranslational conversion of glutamyl to γ-carboxyglutamyl residues in vitamin K-dependent proteins; this enzyme needs vitamin K as a cofactor for this conversion; thus the important role of vitamin K appears in tissues that contain vitamin K-dependent protein to make them a functional protein [2].

During the last two decades, the researches have focused on the role of vitamin K in osteoporosis [3], cardiovascular disease [4], diabetes [5], and cancer [6] besides its role on coagulation [7].

In the liver there are several vitamin K-dependent proteins which all play a role in hemostasis. In addition to the hepatic tissue, bone tissue contains vitamin K-dependent proteins such as osteocalcin (bone Gla protein) and matrix Gla protein (MGP). Mineral binding capacity of osteocalcin needs vitamin K for adding mineral to the bone matrix in normal bone growth and development [8].

Studies have reported that vitamin K plays a role in bone metabolism in other mechanisms. There is an evidence that vitamin K positively affects calcium balance and increase of calcium retention [9]. Vitamins K and D work synergistically on bone metabolism, the form of osteocalcin that osteoblasts produce is undercarboxylated osteocalcin, and this process is upregulated by vitamin D, while carboxylation of osteocalcin is mediated by vitamin K [10].

Vitamin K is shown to decrease bone resorption by osteoclasts and inhibits production of bone-resorbing agents such as interleukin-6 [11] and prostaglandin E2 [12].

Osteoporosis is a metabolic bone disease of reduced bone density, fragile bone, and elevated susceptibility to fracture. Genetic factors, age, sex, race, general health, exercise, cigarette smoking, alcohol abuse, hormone replacement therapy, and nutrition are some of the factors that influence an individual's risk of osteoporosis [13].

The aim of the present paper is to summarize the present knowledge on vitamin K and bone metabolism, emphasize the role of vitamin K in bone health, and evaluate vitamin K as a diagnostic and therapeutic marker in osteoporosis.
