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June 2012

High Vitamin D Levels May Be Harmful

A new study suggests that having too much vitamin D may be just as harmful to health as having too little.

Vitamin D is included in most multivitamins, usually in strengths from 50 IU to 1,000 IU (international units), as softgels, capsules, tablets and liquids. The recommended daily allowance for vitamin D is 600 IU daily for people age 1-70 and 800 IU for people over age 71. The recommended serum level is about 50 nanomoles per liter. Toxicity may more likely occur as a result of regular excess intake of synthetic vitamin D, rather than through natural sources such as diet or sunlight.

In a new study, researchers conducted blood tests on a total of 247,574 men and women living in Copenhagen, Denmark, and measured levels of vitamin D, calcium and parathyroid hormone.

The results suggested that people with vitamin D levels between 50 and 60 nanomoles per liter had the lowest mortality risk. Subjects who had very low levels of vitamin D at 10 nanomoles per liter or very high levels at 140 nanomoles per liter had an increased risk of mortality. The scientists also found this association for very low and very high levels of calcium and parathyroid hormone.

Although this study suggests that very high levels of vitamin D may be just as harmful as very low levels, the researchers emphasized that more evidence is needed before firm conclusions can be made.

Vitamin D is found in many dietary sources, such as fish, eggs, fortified milk and cod liver oil. The sun also contributes significantly to the daily production of vitamin D, and as little as 10 minutes of exposure is thought to be enough to prevent deficiencies. The term "vitamin D" refers to several different forms of this vitamin. Two forms are important in humans: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D2 is synthesized by plants. Vitamin D3 is synthesized by humans in the skin when it is exposed to ultraviolet B (UVB) rays from sunlight. Foods may be fortified with vitamin D2 or D3.

The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. It is used, alone or in combination with calcium, to increase bone mineral density and decrease fractures. Recently, research also suggests that vitamin D may provide protection from osteoporosis, hypertension (high blood pressure), cancer and several autoimmune diseases.

For more information about vitamin D, please visit Natural Standard's Foods, Herbs & Supplements Database.


  1. Durup D, Jørgensen HL, Christensen J, et al. A Reverse J-Shaped Association of All-Cause Mortality with Serum 25-Hydroxyvitamin D in General Practice, the CopD Study.J Clin Endocrinol Metab. 2012 May 9. View Abstract
  2. Natural Standard: The Authority on Integrative Medicine.
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