Menaquinone-4 supplemented diet enhances testosterone production in rat testis. Male Wistar rats were fed with a control (Cont) or a MK-4-supplemented (MK-4 sup) diet for 5 weeks. Testosterone concentrations in the plasma from the tail vein (A) and testis (B) and luteinizing hormone levels in plasma from the abdominal aorta (C) were measured by ELISA. Data are represented as means ± SE (n = 7 or 8). Values are significantly different from those of the control group at *p < 0.05. †Plasma testosterone levels in the MK-4 sup group were significantly increased when compared with the control group (p < 0.05).