Everybody's talking about vitamin D's role in maintaining bone health and preventing certain types of cancer. You probably even know that depending on your age you should be getting upward of 600 IU per day from sun exposure or—in winter—supplements. But read the fine print: There's more than one type of this essential nutrient.

Known as vitamin D3, cholecalciferol is the form found naturally in food, lanolin (the oil in sheep's wool), and fish-liver oil. It's also what your body produces when you stand outside on a sunny day. Vitamin D2, or ergocalciferol, is found in fungi that have been treated with ultraviolet light—a process that converts sterols into vitamin D. And it does matter which you take: Vitamin D2 was shown to be less than a third as potent as D3, according to a 2004 study published in The Journal of Clinical Endocrinology and Metabolism.

To get the most from your D supplement, read the label when you restock your supply. Ideally, supplements should contain 200-600 IU of vitamin D3. Those who want nonanimal-based vitamin D2 should consider taking a higher dose to achieve the same effect.