Vitamin D: an overview

Vitamin D has been all over the medical news recently. Once recommended rarely and with caution, this vitamin is on the verge of becoming a household staple, right up there with vitamin C and the B vitamins.

Sunrise over the Black Sea, Neptun, Romania. Image by Bogdan, CC 3.0. Accessed 5/27/2012.

What changed? First, it’s important to understand the structure of this vitamin and its use by the body. Vitamins A, D, E, and K are fat-soluble. The fat-soluble vitamins have the potential for overdose because quantities in excess of the body’s need are stored in the fat. This storage capability creates the possibility of damage caused by long-term over-supplementation of a nutrient.

In contrast, most vitamins are water-soluble, meaning that excess quantities are excreted by the kidneys. Have you heard the saying that taking vitamins gives you very expensive urine? It refers to the water-soluble vitamins. (We’ll discuss the validity of this statement in a later issue).  It’s difficult to take a dangerous overdose of a water-soluble vitamin, because the body will not store the excess.

Not so very long ago, amounts in excess of 400-800 IU of vitamin D were considered dangerous. Recent studies have overturned this belief, demonstrating no ill effects with amounts up to 100,000 IU daily. That’s quite a difference. In Europe, the recommendation for safe daily consumption now stands at 2000 IU or less. In the UK, a more conservative level of 1000 IU is recommended. In the US, the current recommended adequate intake is 200 IU for adults up to age 50, 400 IU for those 51-70, and 600 IU for those older than 70.

Just as the potential toxicity of vitamin D has been re-evaluated, other studies have shown a correlation between low levels of vitamin D and a host of chronic illnesses, chief among these being osteoporosis and related issues with bone formation, multiple sclerosis, rheumatoid arthritis, and mood disorders. Patients with these conditions are commonly found to have low levels of vitamin D. The usual method to replenish vitamin D is with a short series of mega doses (for instance 50,000 IU), given at intervals of a few days to a month in order to replenish the body’s stores. One of the current issues in the medical use of vitamin D is whether it is more effective to treat with periodic mega doses, or to give a lower dose daily.

Another issue in the use of vitamin D lies in the definition of insufficiency. Different studies use different ranges of normal, and two units of measurement are in common usage {nanomoles per liter (nmol/l) and nanograms per liter (ng/l)}. Even if you don’t know your mole from your femtoliter, you can appreciate the difficulty of trying to compare sets of numbers that use different units. Like translating inches to centimeters, the potential for misunderstanding and miscommunication is great. Doctors are faced with interpreting conflicting definitions of “normal” and “insufficient” ranges of vitamin D when making clinical decisions.

There is one area where agreement seems to have been reached: most current studies rely on a blood test that measures serum 25(OH)D (calcefidiol), which is the “storage form” of vitamin D. Normal ranges of this blood test differ, as I stated above. One researcher states that insufficiency occurs below 75 nmol/l (30 ng/ml). Other studies have found this level to be oversensitive, and question claims that vitamin D insufficiency is reaching epidemic levels.

Natural sources of vitamin D include salmon, cod liver oil, egg yolks, and the sun. We form our own vitamin D when the skin is exposed to ultraviolet light. Sunscreen blocks this process, and if you spend all day inside, you’re blocking it as well. Darker skin may also obstruct the absorption of vitamin D.


There are two types of vitamin D: ergocalciferol (D2) and cholecalciferol (D3) are obtained from food and supplements. D3 is also produced by the skin on contact by sunlight. D2 and D3 move through the bloodstream to the liver, where they are converted, with the help of parathyroid hormone (PTH), to 25(OH)D, the storage form measured by the blood test. This form circulates to the kidneys, where it undergoes another conversion to the biologically active form 1,25(OH) 2 D (calcitriol). Calcitriol is vital to calcium regulation, as well as other processes. If levels of dietary calcium are low, the release of PTH converts vitamin D from calcefidiol to calcitriol. Calcitriol triggers the breakdown of bones, releasing skeletal calcium into the blood for use.

Complicated conversion pathways in the body are like any complex manufacturing process: the more steps involved, the more places for problems to arise. In the case of vitamin D, any damage to the liver, the parathyroid gland, and (especially) the kidneys will influence this pathway and interfere with the regulation of calcium by the body. (By the way, this is a simplified version of this very complex pathway. If you’d like more detail, please help yourself to the nearest Physiology text.)

In addition to its role in calcium balance, vitamin D has local effects on cell growth, change, and immune function. Scientists and doctors are now considering whether low vitamin D is implicated in such disparate conditions as colon cancer, lupus, depression, and chronic pain. We see correlations—for instance , a study may show that elderly women complaining of chronic back pain are more likely to have low levels of 1,25(OH)D than their pain-free counterparts.  What we don’t know is if causation exists: does Aunt Genevieve have chronic pain because she is lacking in vitamin D, or is she lacking in vitamin D because her pain has kept her from outdoor activities for many years? (Or because she lives in an assisted living facility and has little access to the outdoors?)


We are also lacking definitive proof of the value of the beneficial effects of vitamin D supplementation. We can give Aunt Genevieve large doses of vitamin D weekly, or ask her to supplement a moderate dose daily, but we cannot promise that the vitamin will improve her pain.

Still, with the knowledge that vitamin D supplements are relatively safe, and that many people are deficient in vitamin D, many more practitioners are recommending supplementing this vitamin, especially for post-menopausal women and anyone with chronic pain. D3 (cholecalciferol) is the supplement of choice. It is relatively inexpensive, with high-quality 1000 IU capsules costing less than $10 for a two-month supply. For those desiring healthier bones, vitamin D is now commonly bundled with calcium in bone-building and osteoporosis-prevention formulas.

Here’s what it boils down to: it is very important to have sources of vitamin D in your diet. If you don’t eat fish or eggs, consider taking a supplement. And remember to get some sun every day. Vitamin D is produced by as little as 15-20 minutes of sun exposure on the forearms or face (without sunscreen or other barrier). A short walk in the morning or a trip outside on your lunch break will get you what you need. Take a break from whatever you’re working on; get away from the computer, out of the studio, and into the fresh air. You’ll be doing your bones, your immune system, and your emotional wellbeing a favor.

If you suspect you are deficient in vitamin D, and certainly if you suffer from any of the conditions mentioned above (chronic pain, MS, rheumatoid arthritis, mood disorders, bone or colon issues), you may want to ask your doctor about testing for vitamin D sufficiency and whether to consider taking a supplement. These simple actions might make a world of difference.

A multitude of studies can be found in any medical journal or on Medscape which focus on vitamin D. I found these three to be especially useful in the preparation of this article:

J Midwifery Womens Health.  2008;53(5):440-446.  ©2008 Elsevier Science, Inc.

J Am Geriatr Soc.  2008;56(5):785-791.  ©2008 Blackwell Publishing

Nat Clin Pract Rheumatol. 2008; ():. ©2008  Nature Publishing Group

If you have more questions about vitamin D, please contact me or ask your doctor.

©2009 Stephanie J. Draus. All rights reserved.

Picture credits:
  1. Sunshine: Accessed 5/27/2012
  2. Ergocalciferol: Created by Mysid in ChemDraw on September 13, 2005. GFDL. Accessed 5/27/2012.
  3. Cholecalciferol: Created by Pjetter, Public Domain. Accessed 5/27/2012.

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