Your favorite health food store has plenty of supplements. But trying to figure out which nutrient is the best—decoding them—can be tricky. Here is a codebook for three of the most widely used and most important natural ingredients. Vitamin: K2 What it is: Vitamin K2, or menaquinone, is one of two forms of vitamin K. Where it comes from: Vitamin K2 is found in fish, meat, eggs, cheese, and the traditional Japanese fermented food natto. Main use: Cardiovascular health and bone density. Vitamin K2 activates proteins that transport calcium through the bloodstream and ensure that it absorbs into your bones. K2 also reduces calcium buildup in your arteries and aids in blood clotting. What to look for: Look for a clinically studied menaquinone (K2) derived from natto, like