Vitamin K aids with blood clotting; helps build and strengthen bones; and prevents calcium buildup in arteries.

How Much: Recommended dietary allowance (RDA) = 120 micrograms (mcg) for men; 90 mcg for women.

Too Much: A tolerable upper limit (UL) for vitamin K has not been determined. 

Too Little: Vitamin K deficiency is rare. Too little causes problems with blood clotting and can cause bruising and bleeding gums.

Foods: Spinach; kale; parsley; romaine lettuce; and fermented soy foods, such as soy sauce and miso.

Interactions: Antibiotics, blood-thinning drugs.

Research Note: Animal and human studies suggest vitamin K destroys inflammatory cells that contribute to rheumatoid arthritis (RA). Other research links low levels to an increased risk of hand and knee osteoarthritis (OA). Vitamin K also reduces fracture risk; eating a serving of romaine or other dark green, leafy vegetable every day may cut the risk of hip fractures in half.