09/27/2010 04:17 pm ET | Updated Nov 17, 2011

To Sun or Not To Sun? A Rascally Question

Is it good or bad to spend time in the sun? The answer is: Yes.

You probably remember all those horrible but eminently preventable diseases we used to suffer from until scientists discovered vitamins. Scurvy, rescued by vitamin C. Night blindness, vitamin A. Pellegra, vitamin B3.

And rickets, a debilitating bone disease in children, vanquished by an adequate amount of vitamin D. So why, if we've known about vitamin D for more than 100 years and tackled insufficiency before, is too little vitamin D a growing problem again?

First, Some D Factoids

Interestingly, vitamin D isn't actually a vitamin. It's technically considered part of a group of fat-soluble hormones because we produce them naturally (as do plants and other animals) in the presence of ultraviolet sunlight. (Who knew all your mom's entreaties to "eat your vitamins" were really instructions to "eat your hormones"?)

Since all it takes is a little time in the sun, you'd think we'd have plenty of vitamin D. Alas, no. Case in point, rickets.

The Rise and Fall of Rickets

Rickets (along with its adult form osteomalacia) has been around for a long time.

Reportedly first documented in 1645 [pdf] by Dr. Daniel Whistler, the disease became endemic by the late 19th and early 20th centuries in industrialized countries such as England and the United States, in part because of poor diet and in part because of less sunlight for the swarms of new city dwellers from the rural-urban migration. Children in northern, industrialized cities, where child factory labor was probably also a contributing factor [pdf], were particularly prone to the disease.

But things started changing in the 1930s. Scientists established the role of vitamin D in preventing rickets [pdf]. Diets changed, and kids began receiving daily doses of cod liver oil (remember the kids on the Little Rascals TV series receiving spoonfuls of the oil, with noses pinched and lips pursed?). In 1938, the Fair Labor Standards Act set limits on child labor. And in the 1940s vitamin D-enriched milk became the norm, probably sinking the cod liver oil market and, more importantly, reducing the incidence of rickets in children by 85 percent. End of problem, or so it seemed.

Vitamin D Insufficiency and Its Connection to a Host of Diseases

Rickets generally occurs when a person's level of 25-hydroxyvitamin D, a form of vitamin D in the blood, falls below about 20 nanograms per milliliter (levels below that threshold indicate vitamin D deficiency). For decades scientists thought people with levels of 25-hydroxyvitamin D above that threshold were okay vis-a-vis vitamin D. But that thinking is changing.

In recent years, evidence has been mounting that low levels of vitamin D (vitamin D insufficiency as opposed to the more severe vitamin D deficiency) are correlated with a host of other maladies, some life-threatening, including cardiovascular disease, type 2 diabetes, cancer, and autoimmune conditions.

Now, the studies linking vitamin D insufficiency with these diseases are statistical and therefore do not establish a cause-and-effect link. Some experts believe that the correlation is in fact not causative but that other factors causing these diseases are common in people whose lifestyles lead to low vitamin-D levels. (More on that later.)

So, is vitamin D insufficiency, wherein your 25-hydroxyvitamin D level is between 29 and 20 nanongrams per milliliter, a problem? The answer brings us to the bloody part of the story.

The Feds Are Collecting Our Blood

Did you know that government scientists have been taking American blood for years? Yep, in a program called the National Health and Nutrition Examination Survey, or NHANES, the Centers for Disease Control and Prevention have been taking blood samples from a large cohort of Americans since the 1960s. (How au courant are they?! You'll be relieved to know their interests are not vampiric but scientific.) The collected samples have been chemically analyzed to document the presence of bad stuff like toxins as well as good stuff like 25-hydroxyvitamin D.

A Drop in Blood D

The results of recent analyses of vitamin D data in the NHANES show that:

  • the levels of 25-hydroxyvitamin D in Americans' blood are declining, and
  • the number of people with a vitamin D insufficiency has almost doubled since the previous survey from 1994.

Individual findings with respect to the general population are that:

  • roughly 25 to 35 percent of Americans have vitamin D deficiencies, and

  • more than 90 percent of darker skinned Americans (e.g., blacks, Hispanics, and Asians) and almost 75 percent of white Americans have vitamin D insufficiencies.
  • With respect to children: 

    Why the Decline in D?

    Another analysis of the NHANES data, published in the American Journal of Clinical Nutrition by Anne Looker of the National Center for Health Statistics and colleagues, points to three factors contributing to the decline:

    1. We're spending less time outdoors, and when we are outside, we dutifully slather on sunscreen to protect against skin cancer, as per our doctors' advice -- advice predicated on the assumption that we get enough vitamin D from our diet and so needn't worry about giving up time in the sun.

  • Americans are drinking less milk (including vitamin D-enriched milk).
  • American obesity is on the rise. What's the connection there? We don't know for sure, but one possibility is that because vitamin D is fat-soluble, it tends to be pulled out of circulation (literally) by our bodies' fat cells. The more fat, the theory goes, the less vitamin D in the blood.
  • What Can You Do If You Are Concerned About Your Own Levels of Vitamin D?

    • First of all, see your doctor and request a blood test to determine if you have an insufficiency.
    • Because many experts think the U.S. daily recommended intake is too low to maintain  25-hydroxyvitamin D levels of 30 nanograms per milliliter, try to eat and drink more vitamin-D rich foods, preferably in the form of D3. In addition to milk and other fortified foods, consider foods naturally rich in vitamin D like oily fish (salmon, mackerel) and fish oils and, to a lesser extent, eggs. Also, consider the recommendation of the American Academy of Dermatology and take a supplement.
    • Try adjusting your diet and exercise regimen to lose weight if appropriate..

  • Finally, you might consider spending some time in the sun. This has got to seem like a huge medical about-face. For years we've had it beat into our heads that because sunlight causes skin cancer, we should avoid the sun and, when in it, use sunscreen. Now we're getting somewhat more nuanced advice: because of concerns about vitamin D deficiency and insufficiency, the American Medical Association now recommends that you spend time on the order of 10 to 15 minutes in direct sunlight (without sunscreen) several times a week.
  • So thanks to good ole vitamin D, it's once again safe to spend some unprotected (aka sunscreenless) time outside in the broad daylight. Enjoy, but don't overdo it.

    Cross-posted with