Vitamin D

It's suddenly one of the most popular vitamins on the planet, and for good reason. This nutrient has wide-ranging health impacts: More than 1,000 human and lab studies indicate that vitamin D is not only essential for calcium absorption (the two are often paired together for this reason) but may also ward off breast, colorectal, ovarian, and other cancers. 

And getting too little vitamin D could cause premature death from heart disease, according to a recent study. Not to get too laundry list-y, but more research shows that D may also regulate immunity, relieve backaches, lower diabetes risk, and even fight depression.

However, up to 53% of us may not get enough vitamin D from sources such as direct sunlight or food (fish and fortified dairy, for example), reports the National Institutes of Health--and current guidelines have recently been dismissed by experts as insufficient.
How to get it:
Nicknamed the sunshine vitamin, D is the only vitamin our body produces on its own with 10 to 15 minutes of sunlight exposure. But since it's tough to get enough from food and sunlight (especially during non-summer months and if you wear sunscreen), take a supplement to achieve optimal levels.
Vitamin D is found in multivitamins or stand-alone supplements; it's also commonly coupled with calcium.

Good food sources

Fish such as salmon, herring, and sardines; fortified milk and cereals; veal; beef; and egg yolks.
Dosage: Current government guidelines call for 200 IU daily for women under age 51, 400 IU for women ages 51 to 70, and 600 IU for women age 71 and older, but this is a hotly debated area of science right now. Many researchers believe we may need 1,000 IU or more for optimal health.
Precautions: Because vitamin D is stored in the body (what's known as being "fat soluble"), it can potentially reach toxic levels and cause high blood pressure and kidney damage, if you take super-high doses for a long time. People who take estrogen or diuretics may get D from medication side effects, so ask your doctor before starting a supplement. People who take antacids, certain blood pressure meds, and some other drugs may get too little D and require additional supplements.

Did you know...

You can reduce your risk of developing colon, breast, and esophageal cancers by 30 to 50% just by getting enough vitamin D.