“Vitamin K2 is crucial for osteoporosis prevention,” claims Mercola.com (which will be happy to sell you a 30-day supply of K2 for $28). “There are 11 forms of vitamin K,” says Sarah Booth, director of the Vitamin K Laboratory and of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. Leafy greens are good sources of K1. The other 10 forms, collectively called K2, are made by bacteria. “We like to refer to a K2-rich diet as the Oktoberfest Diet,” says Booth. “Cheese, cured meat, sauerkraut. And natto”—fermented soybeans—“is also loaded with K2.” Is vitamin K essential for healthy bones?