Vitamin D

Globally, over 1 billion people are vitamin D deficient,¹ and in the United States the NHANES III study from 2001 to 2004 indicated that 77% of U.S. adults are insufficient or deficient. Deficiency rates have increased as people have limited their sun exposure due to the risk of skin cancer. People living near the equator who are exposed to sunlight without sun protection have robust levels of vitamin D; however, vitamin D deficiency is found in regions where and where sun protection is promoted to avoid UV injury to skin.

There are several groups at higher risk of vitamin D deficiency including:

• Breastfed Infants: Sufficiency is dependent on the mother’s vitamin D sufficiency level. Mother's milk typically contains about 25 IU/L of vitamin D.
• Older Adults: As people age, the skin is not able to synthesize vitamin D as effectively, and reduced kidney function impacts the ability to convert vitamin D.
• Dark Skinned People: Melanin in darker skin reduces the ability to produce vitamin D from sunlight exposure.
• Limited Sun Exposure: Eliminates one of the two possible sources of vitamin D.
• Obesity: Vitamin D is fat soluble, which does not allow it to circulate as freely.
• Other: Gastric bypass patients have less small intestine available to absorb vitamin D.

Vitamin D Sufficiency Levels

Most experts^3,4,5 agree that vitamin D sufficiency is above 30 ng/mL (75 nmol/L), an insufficient level is between 20 and 30 ng/mL (50 to 75 nmol/L), and a deficient level is any value below 20 ng/mL (50 nmol/L).

There are two major types of vitamin D^1,2:

• Vitamin D₂ (ergocalciferol) – which is synthesized by plants and is not produced by the human body
• Vitamin D₃ (cholecalciferol) – which is made in large quantities in the skin when sunlight strikes bare skin. It can also be ingested from animal sources

Factors that impact the ability of the body to synthesize vitamin D through the skin are geographic latitude, time of year, time of day, presence of clouds and/or smog, skin melanin content, and whether or not sunscreen has been applied. For example, residents at 42° N latitude or higher are unable to synthesize vitamin D via the skin during the winter months (from November through February).  

In supplements and fortified foods, vitamin D can be either D₂ or D₃. The two forms have traditionally been regarded as equivalent based on their ability to cure rickets, but evidence suggests that vitamin D₃ is approximately three times more effective at maintaining serum concentrations because the binding protein has a higher affinity to vitamin D₃ than vitamin D₂. This allows vitamin D₃ to reside in the circulatory system longer and increase the concentration to sufficient levels more quickly. The major preparations of vitamin D for prescription use in North America are in the form of vitamin D₂, while more over-the-counter vitamin / multivitamin preparations use vitamin D₃. 

Whether it is absorbed through unprotected skin or ingested then absorbed by the intestines, vitamin D is bound to the binding protein (both albumin and vitamin D binding protein) and carried to the liver via the bloodstream. From there it begins two hydroxylation processes. Beginning in the liver it is transformed into 25(OH)vitamin D (calcidiol), which is the primary circulating form of Vitamin D and the most commonly measured form in serum. Then in the kidneys it is transformed into 1,25 dihydroxy-vitamin D (calcitriol), which is the biologically active form of vitamin D.

Measuring Total Vitamin D

Vitamin D levels can be measured separately or as a total value, but not all immunoassays have the same reactivity to vitamin D₂ and D₃. Some immunoassays only detect one type of vitamin D and others may not fully detect the entire amount. No matter which assay you use, the most important value is the final total value, since it represents the total amount of 25-hydroxy-vitamin D (both D₂ and D₃) in the blood. This ensures patients have the most accurate result regardless of level and whether or not they are supplemented over-the-counter or by prescription.
 

Measuring Total Vitamin in Determining Sufficiency

In the case of a true concentration that is just into the sufficiency vitamin D range, if the assay does not detect D₂, it is likely that result will be reported in the insufficient range. This could also happen if the assay detects only a fraction of the D₂ that is present. To get a true reading of the patient’s vitamin D level, an assay should be used that detects both vitamin D₂ and D₃ equally.
 

Measuring Total Vitamin in Determining Toxicity

In the case of a patient that is being treated for malabsorption with a high dose of vitamin D, the different reactivity for D₂ and D₃ also can cause the patient status to be mis-identified. If the supplement is D₂, it is likely to be the largest vitamin D concentration in these patients. Consequently, by not having D₂ detected or partially detected, it may result in the under reporting of the total vitamin D concentration. This can result in missing a patient with toxic levels.

Risk of Vitamin D Toxicity¹

When serum 25-hydroxy-vitamin D levels are consistently > 150 ng/mL (375 nmol/L), it is potentially toxic. This typically occurs due to vitamin D over-supplementation and is observed in patients taking more than the prescribed 50,000 IU per day. Toxicity due to sunlight overexposure and/or diet is unlikely. When vitamin D levels are this high, calcium concentrations rise as well, which can result in nausea, weight loss and constipation. As a result of increased levels of vitamin D and calcium the patient can develop kidney stones.

C3-epi-25(OH)D₃ Can Confound Accurate Measurement

While guidelines specify detection of the primary metabolite 25(OH) vitamin D₂ and D₃, it is important to know the amount of cross-reactivity an assay has to C3-epimer. The percentage of C3-epimer varies but its presence can be found in both children and adults. Unless an assay reports the specific fraction/quantity of the C3-epimer, it is not possible to determine if vitamin D is overestimated. Overestimation can create therapeutic errors, since patients who are deficient or insufficient may appear sufficient and toxicity may be reported in patients with high-normal levels.

In 2008 Phinney stated "the most widely used indicator of vitamin D status is the measurement of 25-hydroxyvitamin D [25(OH)D] in either serum or plasma. Because circulating 25(OH)D can arise from hydroxylation of either vitamin D₂ or vitamin D₃, measurement of total 25(OH)D [both 25(OH) D₂ and 25(OH) D₃] is essential for accurate assessment of vitamin D status."²

References:
1. Dietary Supplemental Fact Sheet: Vitamin D. Office of Dietary Supplements. National Institutes of Health. Updated 11/13/2009. Accessed 08/17/2010.
2. Phinney KW. Development of a standard reference material for vitamin D in serum. Vitamin D and Health in the 21st Century: an Update American Journal of Clinical Nutrition. Vol. 88, No. 2. 511S-512S. August 2008.