Vitamin K1 (phylloquinone) and K2 (menaquinone-4) supplementation improves bone formation in a high-fat diet-induced obese mice.
Several reports suggest that obesity is a risk factor for osteoporosis. Vitamin K plays an important role in improving bone metabolism. This study examined the effects of vitamin K1 and vitamin K2 supplementation on the biochemical markers of bone turnover and morphological microstructure of the bones by using an obese mouse model. Four-week-old C57BL/6J male mice were fed a 10% fat normal diet group or a 45% kcal high-fat diet group, with or without 200 mg/1000 g vitamin K1 (Normal diet + K1, high-fat diet + K1) and 200 mg/1000 g vitamin K2 (Normal diet + K2, high-fat diet + K2) for 12 weeks. Serum levels of osteocalcin were higher in the high-fat diet + K2 group than in the high-fat diet group. Serum OPG level of the high-fat diet group, high-fat diet + K1 group, and high-fat diet + K2 group was 2.31 ± 0.31 ng/ml, 2.35 ± 0.12 ng/ml, and 2.90 ± 0.11 ng/ml, respectively. Serum level of RANKL in the high-fat diet group was significantly higher than that in the high-fat diet + K1 group and high-fat diet + K2 group (p<0.05). Vitamin K supplementation seems to tend to prevent bone loss in high-fat diet induced obese state. These findings suggest that vitamin K supplementation reversed the high fat diet induced bone deterioration by modulating osteoblast and osteoclast activities and prevent bone loss in a high-fat diet-induced obese mice.
Vitamin K is related to blood coagulation, assisting the promotion of OC carboxylation of γ-glutamic acid, which is produced by osteoblasts, and aiding in bone formation by coupling carboxylated OC with phosphine.(15) Many studies have demonstrated that low intake of vitamin K decreases bone density, and that this is a factor that increases osteoporosis and bone fracture.(16) In the study by Booth et al.,(17) low intake of vitamin K1 led to low bone density, and was a factor for increased risk of bone fracture. When vitamin K1 was administered to human bone marrow culture, osteoclast formation was inhibited.(13) After administering vitamin K2 to osteoblasts, real-time gene expression analysis found that the OC, OPG, and RANKL genes were expressed, demonstrating that vitamin K2 has an influence on osteoblasts and osteoclasts.(18) In addition, vitamin K2 supplementation in patients with osteoporosis necessitated by the administration of glucocorticoids inhibited OPG decrease, and had effects of bone loss prevention.(19) Vitamin K2 supplementation in patients with rheumatoid arthritis accompanied with osteoporosis decreased RANKL levels and inhibited osteoclast activation.(20) Therefore, vitamin K affects bone condition both in healthy adults and in patients with specific diseases.
The results of the bone density analysis revealed an increase with the vitamin K1 and K2 supplementation in high-fat diets. Studies on the relationship between bone density and vitamin K generally have used dual-energy x-ray absorptiometry or ultrasonic densitometry,(32) but this study used high-resolution 3D micro-CT to analyze the morphologic microstructure of trabecular bone. In the study by Fujikawa et al.,(24) the Tb.N increased when vitamin K2 and calcium were fed to ovariectomized mice, and the Tb.Sp decreased. Yamaguchi et al.,(33) also fed vitamin K2 to ovariectomized rats, and reported that it prevented bone loss. These two studies used osteoporosis-induced animals, and the methods differed from those in this study, in which obesity-induced mice were fed vitamin K supplements. In this study, even though there was no significantly statistical difference in the microstructure analysis between the groups, but BV, Tb.N, and Tb.Sp were seemed to be better in the vitamin K2-supplemented group than those in the HF group, indicating that vitamin K2 may play a role in protecting the structures of trabecular bone.
The effects of vitamin K1 and K2 supplementation in normal diet on bone metabolism were not statistically significant. However, vitamin K1 and K2 supplementation in a high-fat diet could prevent a decrease in bone density, and vitamin K2 had a greater effect on this parameter. Therefore, vitamin K2 increases OPG, a marker related to bone density and the metabolism of osteoclasts and osteoblasts, and it decreases RANKL, and thus has an influence on bone metabolism. This study has showed the effects of vitamin K on bone density and metabolism in animals, but further studies are needed to determine whether the same holds true for obese humans. Future studies would need to perform bone measurement and biochemical examinations on the bone microstructures and metabolism in humans.