The purpose of this investigation was to determine the effects of fat content and frequency of meals on the oral bioavailability of menatetrenone (2-methyl-3-all-trans-tetraprenyl-1,4-naphthoquinone), a vitamin K2 with four isoprene units. In the first series of studies, menatetrenone (15 mg) was administered at breakfast time to 18 healthy male volunteers after meals with three different fat contents (meals A, B, and C) on three occasions in a crossover design. The three types of meals had almost the same calorie content (721-746 kcal) with varied fat contents (A, 8.8 g; B, 20.0 g; C, 34.9 g). The area under the plasma menatetrenone concentration-time curve within the first 24 h (AUC0-24) increased with increase of fat content: 371 +/- 194, 485 +/- 150, and 1024 +/- 341 ng.h/mL (mean +/- SD, n = 18) after meals A, B, and C, respectively. On the fourth occasion, the same dose of menatetrenone was administered to all volunteers after taking meal B, but in this case the lunch 5 h after drug administration was omitted from the protocol. The time profile of plasma menatetrenone showed a single peak when lunch was not taken, whereas it showed two peaks with lunch. On the fifth occasion, 12 out of 18 volunteers took the same dose of menatetrenone after a meal with the highest fat content (53.8 g of fat and 789 kcal; meal D), showing that AUC0-24 was almost the same as that for meal C, 1027 +/- 389 and 991 +/- 392 ng.h/mL (n = 12) for meals C and D, respectively. The oral bioavailability of lipid-soluble vitamin K was influenced by the fat content of a meal, although the increase in bioavailability seemed to reach a peak when the lipid content of the meal was > 35 g.