Nearly every cell in the body contains a vitamin D receptor. Research shows vitamin D plays an important role in skeletal, immune, developmental and cardiovascular health. Thus, maintaining optimal vitamin D levels is vital for overall health.
A heated debate surrounds the recommended vitamin D dosage and levels. While the Vitamin D Council recommends that adults take 5000 IU of vitamin D3 daily, the Institute of Medicine (IOM) established 4000 IU daily as the tolerable upper level of intake, meaning the highest level unlikely to cause harm in nearly all adults. On the other hand, the Endocrine Society Practice Guidelines state that up to 10,000 IU of vitamin D3 daily is safe for most adults. These conflicting recommendations can leave the public confused.
So, what’s the truth? How much vitamin D do our bodies need? How much vitamin D is too much? Researchers conducted a study in search of the answers.
A total of 3,882 participants were included in the study. The average age of the participants was 60 years old. Less than 1% of the participants were considered underweight, 35.5% had a normal BMI, 37.0% were overweight and 27.5% were obese.
At the beginning of the study, 55% of the participants reported taking vitamin D. The average dose of vitamin D increased from 2,106 IU daily at the beginning of the study to 6,767 IU daily about one year later. The average vitamin D levels increased from 34.8 ng/ml to 50.4 ng/ml during this time (p < 0.001).
The researchers wanted to determine the dosage needed to achieve healthy vitamin D levels, as defined by levels of 40 ng/ml or higher. They also wanted to determine the incidence of side effects, including hypercalcemia. Here is what the researchers found:
- Change in vitamin D levels were influenced by vitamin D dosage, body mass index (BMI) and vitamin D levels at the beginning of the study.
- Participants who were vitamin D deficient (< 20 ng/ml) at baseline experienced greater increases in vitamin D levels than those with insufficient or sufficient vitamin D levels at baseline.
- Participants without vitamin D deficiency at baseline experienced a blunted response to the same vitamin D dose compared to someone with vitamin D deficiency.
- Response to vitamin D supplementation was less with increasing BMI. In other words, obese individuals required the greatest supplementation to achieve sufficient levels, and normal weight or underweight required the least supplementation to achieve sufficient levels.
- Vitamin D3 intakes of at least 6,000 IU daily were required for those with a normal BMI to achieve a vitamin D status above 40 ng/ml.
- Overweight participants required vitamin D3 intakes of at least 7,000 IU daily to achieve a vitamin D status above 40 ng/ml.
- Obese participants required vitamin D3 intakes of at least 8,000 IU daily to achieve a vitamin D status above 40 ng/ml.
- Average calcium levels did not differ from the beginning of the study to the end of the study.
- A subgroup of participants (285) did not experience significant increases in vitamin D status despite reporting taking substantial intakes of vitamin D (> 4,000 IU daily). The researchers determined that gastrointestinal issues were likely attributed to the malabsorption but non-compliance undoubtedly played a role as well. (For example, the rate of non-compliance with antihypertensive medication is about 30%.)
- Twenty new cases of hypercalcemia occurred between the beginning of the study and the end. Those with vitamin D levels below 40 ng/ml were more likely to experience hypercalcemia than those with vitamin D levels of 40 ng/ml or higher.
The researchers were also interested in whether vitamin D supplementation led to an increased incidence of hypercalciuria, high calcium levels in the urine. It is thought that excessively high vitamin D levels may cause hypercalciuria. However, the researchers found that the incidence of hypercalciuria actually declined after vitamin D supplementation, starting with a total of 67 hypercalciuric cases but at follow-up 67% were no longer hypercalciuric.
Furthermore, it’s important to point out that none of the participants developed any evidence for clinical vitamin D toxicity, which consists of hypercalcemia and 25(OH)D> 200 ng/ml, fatigue, anorexia, abdominal pain, frequent urination, irritability, excessive thirst, nausea and sometimes vomiting.
Biochemical vitamin D toxicity consists of 25(OH)D >200 ng/ml, hypercalcemia and a suppressed PTH level with no clinical symptoms, but none of the participants had that either.
Because most labs identify the normal range for 25(OH)D at 30-100 ng/ml, some physicians believe any 25(OH)D above 100 ng/ml is toxicity. It is not; it is usually just hypervitaminosis D.
The researchers concluded,
“Doses of vitamin D in excess of 6,000 IU/d were required to achieve serum 25(OH)D concentrations above 100 nmol/L [40 ng/ml], especially in individuals who were overweight or obese without any evidence of toxicity.”
One thing that the authors didn’t mention is the role genetics may play in this. For example, the gene that codes for the 25-hydroxylase has a genetically determined variation in its transcription. Some people have more 25-hydroxylase than others and thus will obtain higher 25(OH)D levels than others.
While the authors did not stress it, these subjects had repeated vitamin D levels drawn. Given these findings, keeping the genetics in mind, the only way to be sure you are > 40 ng/ml is to get a 25(OH)D test.
Tovey, A. & Cannell, JJ. How much vitamin D is needed to achieve optimal levels? The Vitamin D Council Blog & Newsletter, May 10, 2017.
Kimball, S. Mirhosseini, N. & Holick, M. Evaluation of vitamin D3 intakes up to 15,000 international units/day and serum 25-hydroxyvitamin D concentrations up to 300 nmol/L on calcium metabolism in a community setting. Dermato-Endocrinology, 2017