The brilliant sunshine streaming through my window make me want to spend more time outdoors before it’s too late. The gray winter doldrums, especially in northern latitudes, say above the axis between Boston and northern California, lead to a silent, invisible epidemic: vitamin D deficiency. This is a major component of widespread osteoporosis and resultant hip and wrist fractures that orthopaedists are called upon to treat.
You see, as important as calcium intake is to your bone health and integrity, it can’t get into your bloodstream and your bones without vitamin D. This essential vitamin is necessary to allow calcium ions in food to cross the intestinal wall.
Vitamin D seems to prevent and even treat a variety of cancers
Vitamin D is really not a vitamin though in the usual sense of the word. In fact, it is a steroid hormone, with an amazing array of bioactive properties. [2,4] More recently discovered functions of this exotic vitamin include:
- Regulation of a number of genetic functions that help to reduce cell proliferation (dangerous cell growth).
- Promotion of cell differentiation (transformation of different cell types).
- Prevention of cancerous transformation and modulation of apoptosis (cell death). This means that vitamin D seems to prevent and even treat a variety of cancers, including breast, colon, prostate, lung, and lymphoma. 
And that’s in addition to its regular functions like maintaining serum calcium and phosphate levels, promoting the normal mineral growth of bone, and preventing tetany (spastic contraction of muscles due to low levels of calcium).
Vitamin D prevents the “soft bone” diseases — rickets (in children) and osteomalacia (in adults). It promotes bone growth and bone remodeling, and along with calcium, it prevents osteoporosis in older adults. [1,4]
Unfortunately, dietary sources of vitamin D are restricted to comparatively few foods like fatty fish (salmon, mackerel, tuna, sardines) or fish livers (i.e., cod liver oil), milk and dairy products, egg yolks, and beef liver. Other dietary sources are from fortified foods, to which vitamin D has been added.
The major source for humans is sunlight, specifically ultraviolet B (UVB) radiation4 which synthesizes vitamin D in our skin. [3,4]
How much vitamin D should you get?
Whether you get plenty of sunlight, eat a lot of fish, eat fortified foods, or take dietary supplements, how can you tell if you’re taking the right amount of vitamin D — or even too much?
The answer to that question can vary depending on with whom you consult and your age. Most health care professionals suggest daily intake from dietary or supplement sources at doses of between 200 IU and 1200 IU. And because it’s a fat-soluble substance, you can take too much, but only at very high doses. (Consult with a doctor if you want to supplement with more than 2000 IU per day.) Oddly enough though, excessive sunlight does not cause vitamin D toxicity.
Toxic symptoms include nausea, excessive levels of calcium and phosphate in the blood, and possibly, heart rhythm abnormalities and kidney stones. The key is to take the right amount for you.
So here’s what I recommend. If you live in a year-round warm climate that permits sunbathing, the easiest thing to do is expose your body to the sun daily, for about 20 to 30 minutes on each side, front and back, between 10 AM and 2 PM. You must NOT use a sun block or tanning lotion, which will block the vitamin D generating effects.
It has to be direct sunlight, too — UVB does not penetrate glass walls or windows. And if you have a darker complexion, this method is less effective, as melanocytes (pigment cells) in the skin block much of the necessary UVB radiation. 
An alternative is a tanning parlor for the same daily exposure. Do not exceed these exposures without protection or else you may invite the proven skin cancer-causing effects of solar radiation. Above all, avoid burning. If you prefer to use supplements, try 400-1000 IU vitamin D per day. [1-3]
You can also ask your doctor to test your levels of 25-hydroxy vitamin D (25 (OH) D) to ensure you are not deficient.
If you do decide to have your vitamin D level checked, be advised that no one knows exactly what the “correct” level of this indicator is.  Results of less than 20 ng/mL indicate significant deficiency and must be treated. The range of 20-30 ng/mL reflects inadequate levels and should be treated. Over 30, and up to the 60 ng/mL range is optimal (and in sun worshipers, even 100 is considered normal by Quest Diagnostics and Lab-Corp laboratories).  Levels consistently above 150-200 ng/mL are considered toxic. 
Remember to take sufficient calcium with your vitamin D. These two nutrients work together to strengthen your bones. For middle-aged and older folks, a minimum intake of about 1200 mg/day of calcium is recommended.
I’d recommend testing your vitamin D levels every few months, especially during the winter if you live in a colder northern climate. I’d also recommend a calcium test every few months. By following your serum 25 (OH) D levels and knowing what you’re ingesting, you and your doctor can determine what the “correct” dosage is for you.
1. Dietary Supplement Fact Sheet — Vitamin D: National Institutes of Health, Office of Dietary Supplements, last updated 12/11/2008.
2. Does Vitamin D Treat Cancer? in The Vitamin D Newsletter. Jan. 2008.
3. Can Vitamin D Prevent Cancer? American Cancer Society News Center. 12/29/2005.
4. Kochupillai, N. Indian J. Med Res. 127, March 2008, pp 256-262.
[Note: Dramatically reduce your risk of heart disease, cancer and diabetes… by spending just a few minutes each day in the sun? Yes you can! Achieving optimal vitamin D levels is one of the most important (not to mention one of the cheapest and easiest) ways to ensure great health. Learn everything you need to know in THB’s groundbreaking book, Your Best Health Under the Sun.]
Dr. Bill is the online handle for William T. Stillwell, MD, FACS, FICS, FAAOS, FAANAOS, FAAPGS, a highly credentialed, board-certified orthopaedic surgical specialist, now President & CEO of Dr. Bill's Clinic, Inc. and author of How to Avoid Knee Surgery.