VITAMINS

Vitamin K is an essential cofactor for the activation (γ-glutamylcarboxylation) of vitamin K-dependent proteins including blood coagulation factors and bone matrix proteins. On the other hand, several novel functions of vitamin K other than γ-glutamylcarboxylation have been reported: inhibition of tumor cell growth, anti-inflammatory effect in macrophage, and the ligand activity of nuclear receptor PXR/SXR. Vitamin K is found not only in liver and bone, but also pancreas, brain and gonadal tissues. However, the function of vitamin K in testis has not been fully elucidated. Therefore, we explored a novel function of vitamin K in rat testis and found that testosterone production in testis is positively correlated with dietary level of vitamin K. Menaquinone-4, one of vitamin K_2, not vitamin K_1, enhanced testosterone production in mouse testis-derived tumor cells (I-10). Protein kinase A (PKA) is a crucial factor for steroidogenesis in testis Leydig cells. PKA was activated (or phosphorylated) by vitamin K treatment in I-10 cells. A PKA inhibitor H-89 abolished the enhancement of testosterone production by menaquinone-4. These results indicated that vitamin K_2 enhances testosterone production in testis via activation of PKA.

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Antioxidant micronutrients, such as vitamins and carotenoids, exist in abundance in fruits and vegetables. It is known that they contribute to the body's defense against reactive oxygen species. Recent numerous epidemiologic studies have demonstrated that a high dietary consumption of fruits and vegetables rich in carotenoids or a high level of blood carotenoids results in lower risks of certain cancers, diabetes, and cardiovascular disease. These epidemiologic studies have suggested that carotenoids possessing antioxidant action may have a protective effect against several lifestyle-related diseases. beta-Cryptoxanthin is a carotenoid pigment found especially in Japanese mandarin (Citrus unshiu Marc.) fruit, which is mainly produced in Japan. Our nutritional epidemiologic survey, Mikkabi Study, utilized the data obtained in the health examination of inhabitants which had been performed in the town of Mikkabi in Shizuoka, Japan. In this survey, we measured serum beta-cryptoxanthin as a specific bio-marker to estimate the consumption of Japanese mandarin fruit. From the cross-sectional analyses in the Mikkabi Study, we found out the inverse associations of serum β-cryptoxanthin levels with the risks for atherosclerosis, insulin resistance, liver dysfunctions, metabolic syndrome, low bone mineral density, and oxidative stress. This review has outlined recent epidemiologic studies on the associations between serum beta-cryptoxanthin levels with the risks of several lifestyle-related diseases in our country and other countries.

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Soy isoflavones such as genistein and daidzein are naturally occurring phytochemicals and exert estrogen-like activities through binding to two isoforms of the estrogen receptor (ER), ERα and ERβ. Estrogens including 17β-estradiol (E2) function not only in reproductive tissues, but also in peripheral tissues. Both ERα and ERβ are expressed in skeletal muscle. E2 decreases skeletal muscle mass in young females. These findings suggest that soy isoflavones and E2 have physiological roles in skeletal muscle. In this paper, we discuss the mechanism of E2-induced decrease in skeletal muscle mass as well as the beneficial effect of daidzein on skeletal muscle mass through ERα and ERβ in young females. Furthermore, we briefly review the present situation about the effects of isoflavones on the quantity and quality of skeletal muscle in postmenopausal females.

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