Vitamin D: The Wonder Vitamin
By Jennifer Koslo, MS, RD, CSSD, ACE-CPT
Full-time faculty member, Kaplan University School of Health Sciences

Vitamin D, specifically vitamin D3 (cholecalciferol), has received positive press lately and may be one of the most underrated vitamins.

Medical journals have published recent research on vitamin D and the implications it can have on personal health. But what exactly do the findings mean for your health?

To discover the answer to this question and more, pull up a chair in the sun, pour yourself a tall glass of a vitamin D-fortified beverage, and read on!

Just the facts: What is vitamin D?
Vitamin D is technically not a vitamin but a secosteroid hormone, similar in structure to a steroid, that interacts with almost 1,000 genes in the body.

The fact that vitamin D functions as a hormone and interacts with so much of our DNA makes it easy to see how it can be a factor in the pathology of heart disease; stroke; hypertension; autoimmune diseases, such as multiple sclerosis, diabetes, depression, chronic pain, osteoarthritis, and osteoporosis; birth defects; periodontal disease; and at least 17 varieties of cancer.1

There are three forms of vitamin D: cholecalciferol, calcidiol, and calcitriol. Cholecalciferol is the naturally occurring form, which is made in large quantities when your skin is exposed to the sun. It is also referred to as vitamin D3 and is the form that can be taken as a supplement.

Calcidiol is the form that exists in your blood and is made from cholecalciferol. To measure the amount of vitamin D in the body, a blood test will determine how much calcidiol is circulating in the blood. This test is known as 25-hydroxyvitamin D.

Calcitriol, made from calcidiol in the kidneys and other tissues, is the most potent form of vitamin D and has anticancer properties. Calcitriol is often referred to as the active form of vitamin D, but it should not be used to determine if you are vitamin-D deficient.

What are the current recommendations?
Vitamin D recommendations are given in micrograms (ug). For adult males and females age 19 to 50, the suggested vitamin D intake is 5 ug. For ages 51 to 70, it is 10 ug.

You may also see recommendations that use the new International Units (IU), in which 1 ug equals 40 IU. The current recommended dietary allowance (RDA) for men and women age 50 or younger is 200 IU.

How much vitamin D does my skin make?
Just 20 to 30 minutes of full skin exposure to the midday sun or to the lights in a tanning booth will result in the production of 10,000 to 50,000 units of vitamin D, which is more than enough to meet your needs.

However, there are many reasons that people don’t get 20 to 30 minutes of sun exposure. Sunscreen, clothing, and glass all block the UVB rays that make vitamin D. In addition, many people live in areas where there is a lack of sunlight due to cloud cover or colder temperatures.

Why is vitamin D crucial to health?
The most important part of vitamin D metabolism occurs after cholecalciferol is made by the skin, or taken as a supplement, and is transported to the liver and converted into calcidiol.

Calcidiol is important for maintaining calcium blood levels. For the majority of people, this regulation of calcium by way of the kidneys is what uses up the typical vitamin D intake.

However—and this is key—if you take in enough vitamin D to satisfy the first pathway of maintaining calcium blood levels and have enough left over, then the excess vitamin D takes a second pathway that leads to the tissues. It is in the tissues that all of the health benefits discovered in the last 10 years occur.

Having extra calcidiol in your body is important because it can then go into the many cells that are able to make their own calcitriol, which is a potent product for fighting cancer. Tissue calcitriol levels also continue to increase with vitamin D intake, turning on more genes that are essential to prevent diseases such as cancer.

Do I need protection from toxicity?
Vitamin D may not be toxic in high levels as formerly believed. The body has natural protection against excessive tissue calcitriol levels. The more calcitriol that is made, the more that is metabolized and excreted in bile.

Under natural conditions, such as sun exposure, your skin produces upwards of 10,000 IU, and research has indicated that vitamin D is safe when used in doses similar to those produced naturally, i.e. up to 4,000 IU.1

What is the bottom line?
Research findings warrant an increase in the current 200 IU suggested daily vitamin D intake. Ask your doctor for a 25-hydroxyvitamin D test if you suspect you are deficient.

Your blood levels should be between 50 to 80 ug/ml all year. You can make sure that you are getting adequate vitamin D by spending at least 20 to 30 minutes in the sun, and taking a vitamin D supplement.


1. Vitamin D Council (n.d.) Understanding vitamin D cholecalciferol. Retrieved September 14, 2008, from

Jennifer Koslo is a full-time faculty member with Kaplan University’s School of Health Sciences and teaches courses such as Vitamins, Herbs, and Nutritional Supplements and Contemporary Diet and Nutrition.  Ms. Koslo is a Registered Dietitian (RD) and one of the few Certified Specialists in Sports Dietetics (CSSD) in the country.  In addition to teaching online, Ms. Koslo teaches wellness and weight loss classes in the local community, and is a sports nutrition consultant on a private consultation basis.

Ms. Koslo received her Bachelor of Science in Biology from Juniata College, and earned a dual Master of Science in Human Nutrition and Exercise Science from Colorado State University. She received a Registered Dietitian certification from the American Dietetic Association and Colorado State University, and is currently working on her doctorate degree in education with an emphasis in instructional design for online learning.

In addition to serving in the US Peace Corps in Sierra Leone, West Africa, for 2-1/2 years, Ms. Koslo also worked as a cardiac rehabilitation dietitian and at the Arizona Department of Health Services as the chronic disease nutritionist.  She is also an American Council on Exercise Certified Personal Trainer. 


Kaplan Higher Education Corporation is a division of Kaplan, Inc., a subsidiary of The Washington Post Company.

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