The Importance of Vitamin D

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Written by Austin Winegar, Ask the Scientists Manager at USANA Health Sciences

Role in the Human Body

Vitamin D is a fat-soluble nutrient essential for bone growth and general health. It is acquired through diet and exposure to sunlight.

Vitamin D is an essential component of bone health in both children and adults. Without vitamin D, bones do not calcify properly, leading to the condition known as “rickets” in children. Vitamin D also plays an important role in tooth development. It is necessary for proper tooth eruption, growth, and strength. Through its role in regulating calcium and phosphorus metabolism, vitamin D plays a continuing role in maintaining a stable nervous system, normal heart activity, and normal blood clotting.

A unique property of vitamin D is that it functions very much like a hormone. It target tissues include the kidneys, intestines, and bones, where it helps regulate calcium and phosphorus homeostasis. Its specific activity in the intestines involves stimulating the synthesis of active transport proteins that mediate absorption of calcium. In bone tissue, vitamin D plays a role in regulating calcium deposition (bone mineralization) and mobilization. A role for vitamin D in immune system modulation is now under investigation.

Deficiency Symptoms

Vitamin D deficiency is most directly related to poor bone health, including rickets and osteomalacia. However, vitamin D deficiency is also associated with an increased risk of other disorders, including certain cancers, type 1 diabetes, multiple sclerosis, tuberculosis, rheumatoid arthritis, muscle weakness and pain, depression, hypertension, and pregnancy complications.

While many of these associations are actively being researched to determine the extent of their connection with vitamin D deficiency, we currently know that vitamin D unquestionably exerts a significant influence on many body systems.


Exposure to the sun is the most important source of vitamin D for most humans. Limited amounts of vitamin D are available from food, including fortified milk, certain types of fish, and fortified breakfast cereals. Larger doses of dietary vitamin D must typically come from supplementation.

Light-induced synthesis occurs in the skin when ultraviolet light reacts with a cholesterol precursor, converting it to vitamin D. This molecule is then altered by the liver and kidneys to form the physiologically active vitamin.

Several compounds have vitamin D or potential vitamin D activity. The most important forms are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The latter is the form normally produced in humans, and it is also the form used in most nutritional supplements and clinical research.


The tolerable upper intake level (UL) for vitamin D in adults is 100 mcg (4000 IU) per day.

There are no published accounts of vitamin D toxicity occurring as a result of sun exposure. Research also suggests that vitamin D toxicity is very unlikely at dietary intake levels below 1,000 mcg (40,000 IU) per day in healthy adults.

Vitamin D supplements do have the potential to interact with certain types of medications (including steroids and cholesterol-lowering drugs). Those taking medications should discuss vitamin D supplementation with their healthcare providers prior to use.


In recent years, many studies have reported that vitamin D deficiencies are surprisingly common worldwide, especially during winter months. As such, many doctors and health professionals have advised supplementing with vitamin D (exact dosage depends on the individual, his/her location, and health history).

Those supplementing greater than the UL of 100 mcg (4000 IU) per day should only do so under the supervision of a medical provider, and undergo regular blood tests to determine vitamin D status.

Conversion – IU to Micrograms

1 microgram vitamin D = 40 IU vitamin D

e.g. 400 IU vitamin D = 10 mcg vitamin D

Find more information about USANA Vitamin D here.

About the Author

Austin Winegar holds a bachelor of science in biology from the University of Utah with an emphasis in molecular biology. Austin’s specialties include sports nutrition, weight loss/maintenance, anatomy, physiology, and biochemistry. In 2011 Austin joined USANA Health Sciences in the Department of Science Information Services. Austin has played an integral role in organizing third-party published research, ensuring the safety and efficacy of products, reviewing externally facing materials for scientific accuracy, researching new ingredients, writing scientific manuscripts, answering customer technical questions, and lecturing internationally on the importance of nutrition for optimal health. But where Austin’s true passion lies and has made the largest impact is managing the very popular website, where users can find science-based answers to questions on health, wellness, and nutritional supplementation. To find out more about the importance of vitamin D, please visit here