A leading health expert is calling vitamin D deficiency the world's most common medical condition... with 50 percent of the planet lacking the right amount of this essential nutrient.

Boston University's Dr. Michael Holick has gotten into some hot water over his view – he's suggested that maybe we've been a little too cautious when it comes to sun exposure, and should get out more. After all, the best source of D is the sun – they don't call it the sunshine vitamin for nothing.

But that's heresy in the world of dermatology... so Dr. Holick was kicked out of the school's dermatology department.

It would be a mistake to dismiss Dr. Holick because he has a point – and he shouldn't be ignored. We spend too little time in the sun, and now we're paying for it.

At the same time, sunlight isn't the only answer. It may not even be the best answer, because it's so hard to figure out if you're getting the right amount of the right kind of sunlight, and on a consistent basis.

For example, the earth's winter tilt means millions of people in places like New York, Philadelphia, Boston, Minneapolis, and Seattle get almost no real D from the sun for part of the year – even if they do spend plenty of time outside.

People in other cities may get some better sun – but in most cases, not enough to generate what your body really needs.

Want proof? Check the Archives of Internal Medicine, which found that 77 percent of Americans aren't getting enough vitamin D.

Since it's hard to know exactly how well you're doing when it comes to this nutrient, you could visit your doctor and order up a regular blood test to check your levels... or save yourself the time and use your co-pay to buy a quality supplement instead.

It's one of the few vitamins that I can recommend for nearly everyone, everywhere, in all health conditions. Even Harvard University is urging people to take a D supplement – and researchers at that institution rarely recommend vitamins of any kind.

The U.S. government says you need 400 IUs each day, but ignore that – that's the barest of bare minimums. Many leading health experts suggest around 2,000 IUs each day. And if you have darker skin or get very little sunlight, you may need up to 4,000 IUs daily.

In return, you'll get an amazing array of benefits that seem too good to be true – especially when you consider that they come from a single inexpensive and widely available vitamin.

One new study confirms previous research, which found that vitamin D prevents premature death from all causes in seniors. Another recent study shows that D can help improve insulin resistance and sensitivity, which could lower your risk for diabetes.

And that's in addition to its many well-established benefits, which range from immune system function and calcium absorption to increased muscle strength and a decreased risk of falls and fractures. Some studies have shown that vitamin D can fight diseases like cancer and osteoporosis.