Vitamin D3 (cholecalciferol) is naturally produced by the human body in response to sunlight on skin. It is also available through dietary sources like fish. When it is mass produced for use in vitamin supplements, manufacturers typically use ultraviolet irradiation of 7-dehydrocholesterol extracted from the lanolin found in sheep’s wool.

Another form of vitamin D (vitamin D2 or ergocalciferol) was long thought to be equivalent to D3 but studies have shown it is not as potent or long-lasting as D3. Vitamin D2 is not actually produced in the human body, but the body can take it and make D3 from it. D2 is created by exposing certain plant-derived sources like mushrooms to ultraviolet light. While D2 is plant-based, the animal-based D3 most closely resembles the active form of vitamin D in the human body so cholecalciferol is the form the GRM reviews in our vetting process. Finding a reliable source, however, can be a significant problem.

Vitamin D Potency

A research letter published in the Journal of the American Medical Association (JAMA) Internal Medicine, 11 Feb, 2013, documents the experience of Kaiser Permanente scientists in Portland, OR who were studying the effects of Vitamin D on postmenopausal women. When they sought to verify the potency of compounded vitamin D3 pills used in the study, they found the levels in different bottles varied significantly. Only one-third of the pills fell within 10% of the stated dose. Intrigued, they checked other compounded vitamin D pills and found potency ranged from 23% to 146% of the stated dose advertised on the label. They then purchased 55 bottles of over-the-counter vitamin D3 ranging from 1,000 IU to 10,000 IU from 12 suppliers in Portland stores. They reported,

“The cholecalciferol (vitamin D3) content of Over-The-Counter (OTC) and compounded vitamins was highly variable; potency ranged from 9% to 146%. In our test, just over one-half of OTC pills and only one-third of compounded pills met USP Convention standard….As more people take vitamin D supplements, it is critical that health care providers and patients understand that cholecalciferol potency may vary widely.”

Consumer Reports also published an analysis of vitamin D supplements and found unacceptable levels of lead in nine out of 12 vitamin D-calcium combination pills. reported finding almost 20% of the vitamin D products failing quality standards. In its study of multivitamins, found nearly one-third of the 38 products analyzed not meeting acceptable criteria.

Most consumers assume they can safely rely on label information. They believe that someone like the Food and Drug Administration (FDA) monitors the quality and potency of supplement products. But this is not the case. Consumers must rely on the integrity of the manufacturer and third-party laboratories like the NSF or to ensure their supplements are worth taking. If you would like vitamin sources recommended by the GRM, go to to request resources.


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