Vit D Reference Ranges and How They Were Determined
The last blog, Vitamin D Levels: How Much Can We Get Naturally, reviewed natural synthesis of vitamin D through the skin, the many variables that affect this process, and ways to optimize vitamin D production via natural sunlight. The optimal daily dose of vitamin D varies by individual. This blog will review serum reference ranges and how these ranges were chosen, and the upper limit of Vitamin D Supplementation.
Normal Reference Ranges for Vitamin D Levels
Vitamin D levels were not readily measured until the late 1980’s. Since then “there is no absolute consensus on what normal vitamin D ranges should be.”
To date, the levels used to define deficiency, insufficiency, normal and toxicity are not generally agreed upon. In a review concerning vitamin D reference ranges, there are differing opinions:
“There is no agreement on the normal ranges for serum total 25(OH)D:
- the U.S. Institute of Medicine (IOM) suggested a minimal concentration of 20 ng/mL,
- the Endocrine Society (ES) suggested a minimal concentration of 31 ng/mL.”
However, those are not optimal levels for treating chronic disease.
The following summarizes vitamin D serum ranges commonly accepted by labs today:
- Normal: 30-100 ng/mL
- Insufficiency: <30 ng/mL
- Deficiency: <20 ng/mL
- Toxicity: >150 ng/mL
How Were “Normal” Reference Ranges for Vitamin D Determined?
Usually normal reference ranges are determined by obtaining blood from several hundred volunteers who have no medical diagnoses and are on no medication. However, with vitamin D levels this is not the case because it was found 30-50% of European and US population were vitamin D deficient or insufficient so the reported normal range of 10-55 ng/ml became inadequate.
Vitamin D Deficiency: <20 ng/ml
A provocative test was done in a study in which healthy adults with vitamin D levels between 11-25 ng/mL were given 50,000 IU vitamin D once a week for 8 weeks.
There was a 55% decrease in PTH levels among those with vitamin D values between 11-15 ng/mL and a 35% decrease with values 16-19 ng/mL. Those with values over 20 ng/mL showed no significant change in PTH levels, leading to the suggestion vitamin D deficiency should be defined as a level below 20 ng/mL.
Vitamin D Insufficiency: 21-29 ng/ml
Vitamin D Insufficiency is defined as a level of 21-29 ng/ml. The upper limit of insufficiency being > 30 ng/ml was based on a 45-65% increase in the efficiency of intestinal calcium transport when women had vitamin D levels of >32 ng/ml.
Vitamin D Normal Levels: 30-100 ng/ml
A 1997 study showed insight into what a sufficient level of vitamin D is based on parathyroid hormone (PTH) function. They found PTH levels plateaued when vitamin D levels were between 30-40 ng/mL.
An article was published calling for the minimum vitamin D serum levels to be established from a physiology approach rather than a disease avoidance approach. This means levels should reflect what is required for cell and tissue activity. At low serum levels, the cells and tissues are affected but there are no immediate clinical symptoms. As the low levels are prolonged through time, an evolution occurs in which symptoms begin and clinical disease becomes evident. A physiology approach reflects higher serum values are needed to prevent downstream clinical disease.
Vitamin D levels supporting basic cell function are required for development, optimal immune system function, skeletal structure, anti-carcinogenic activity, lactational activity, cardiovascular health and many other benefits. The author concludes serum values supporting basic human physiology range between 40-60 ng/mL. In conclusion, the current lab minimal ranges are too low to support optimal basic physiological function of the body.
The upper limit of the reference range was determined by lifeguards’ vitamin D levels of 100-125 ng/ml. No lifeguard was ever reported to be vitamin D toxic so these levels were accepted as high normal.
Many functional medicine experts have a preference for an optimal range of 50-100 ng/ml based on studies showing this to be the minimum range in the prevention and treatment of various illnesses and chronic diseases.
Vitamin D Toxicity: >150 ng/ml
Vitamin D toxicity is defined as a vitamin D level >150 associated with high blood calcium levels high urine calcium levels, and often high blood phosphate levels.
Upper Limit of Vitamin D Supplementation
The upper intake limit (UL) of vitamin D was determined in 1997 by the Food and Nutrition Board (FNB). The UL means the highest chronic daily dose that poses no risk or adverse effects. It took 13 years for the FNB to increase the UL to 4,000 IU of Vitamin D3 per day, where it remains currently.
The updated UL was published by the FNB in 2010 in which they state the concern of vitamin D toxicity is hypercalcemia. This is however not supported by credible scientific literature. FNB states, “In the absence of well-controlled studies, the serum 25OHD level representing vitamin D toxicity threshold in humans is not readily defined. Similarly, vitamin D intakes required to trigger toxicity symptoms are not precisely known………What the data does suggest is it would be unlikely to observe symptoms of toxicity at daily intakes below 10,000 IU, while it is possible daily intakes above 10,000 IU could be associated with toxicity.” Despite their conclusion, they decided 4,000 IU was the UL for daily intake.
A 2007 study review article assessed the risk of toxicity of vitamin D, and supported a significantly higher UL for vitamin D consumption. Human clinical trials were evaluated showing the absence of toxicity at a daily dose of 10,000 IU. Extreme caution continues in the recommendations of vitamin D not based on credible literature.
“These restrictive practices have served to effectively curtail research efforts and limit the public from deriving the most possible benefit from this nutrient. The conclusion the present UL established by the FNB is lower than justified by the scientific evidence has been echoed by several experts in the field of vitamin D research…..
Newer clinical trial data are sufficient to show vitamin D is not toxic at intakes much higher than previously considered unsafe. This demonstrated safety profile of vitamin D should safely permit increased intakes to achieve additional benefits of this vitamin at higher levels than previously recognized.”
Summary
There is much confusion regarding ranges of vitamin D and dosing guidelines as serum levels are not generally agreed upon and the recommended upper limit is outdated and not based on updated scientific evidence. There has been a push for years by experts to raise the upper intake level of vitamin D as the current upper level is too low to even reach serum ranges for basic human physiology in most people. The next blog will review how much vitamin D you should take.
Have an awesome day! Dr D