Maybe you’ve heard that as many as 3/4ths of Americans are deficient in vitamin D.1 Maybe you’ve seen the numbers that show vitamin D deficiency to be prevalent amongst the elderly and you’re concerned for yourself or a family member. Maybe you’re just curious what everyone means when we’re talking about vitamin D deficiency. After all, how can the majority of Americans be deficient in this basic “sunshine vitamin” – and if the majority of Americans are deficient, how are we defining deficient?
25[OH]D blood tests: What exactly are we testing when we test vitamin D levels?
First, it’s important that you understand that there are a variety of forms of vitamin D. Often, when we refer to vitamin D we’re talking about the precursor forms, ergocalciferol (D2) and cholecalciferol (D3). These forms enter the blood stream after synthesis in the skin or following ingestion. The liver then metabolizes these initial forms of vitamin D into 25[OH]D2 and 25[OH]D3. Those forms also circulate in the bloodstream, and are what we test for.
This is why the main vitamin D blood-serum test is referred to as a 25[OH]D (or calcidiol) blood test. Over the years, it was found that circulating levels of 25[OH]D are generally the best predictor of vitamin D status. They provide the most reliable assessment of a person’s risk of vitamin D deficiency conditions such as rickets or osteomalacia.
What about 1,25[OH]D?
25[OH]D is not the final form of vitamin D that is required for our critical biological functions. Rather, our kidneys metabolize 25[OH]D to 1,25[OH]D, a process that is very carefully regulated by the parathyroid, and it is actually this form that is hormonally active.
Renal and Parathyroid Patients
It possible to experience the effects of vitamin D deficiency (or toxicity) even if the standard vitamin D test returns normal, namely if you suffer from some forms of parathyroidism or kidney disease. If these are problems you suffer from, your doctor should have already had a discussion with you about vitamin D. You may even have a prescription for calcitriol, or 1,25[OH]D3. If you do, and you are reading this because you have further questions about your vitamin D levels, I strongly recommend you bring those questions to your doctor immediately. Without careful regulation, 1,25[OH]D levels can easily become fatally high or low.
If you are trying to discern whether you are receiving the optimal levels of vitamin D, the 25[OH]D test is much better than the 1,25[OH]D test. If you have a healthy parathyroid and kidney, the metabolism of 25[OH]D to 1,25[OH]D will be carefully regulated. The amount of circulating 1,25[OH]D varies significantly – for example, if you have a calcium-rich diet, your parathyroid may lower 1,25[OH]D levels in order to prevent hypercalcemia (a potentially fatal condition). However, we want to ensure that we have the right amount of 25[OH]D in our blood stream so that our parathyroid can most easily regulate 1,25[OH]D levels.
How does the 25[OH]D blood test work?
When you are screened for vitamin D deficiency, a blood sample will be taken and tested for 25[OH]D concentration. There are a variety of forms of this test, some which require only a finger prick and some which involve drawing more blood.
There are also a wide variety of ways the blood may be analyzed. There are different detection methods, from immunoassays to liquid chromatography and mass spectrometry. Much of the 25[OH]D in your blood stream is bonded to proteins, and cannot necessarily be detected in this state. There are different methods for releasing the 25[OH]D from the proteins, some of which are more effective than other methods. Unfortunately, all of these differences mean that there can be some significant variation in test results between labs. Additionally, labs may report test results as sufficient or deficient, rather than providing only the actual 25[OH]D concentration. These labs do not have standardized cutpoints for what is and isn’t sufficient.
To some extent, this variation reflects the state of our knowledge of vitamin D. Most of the research regarding vitamin D receptors has only been done in the last ten to fifteen years. However, we’ve known about the relationship between 25[OH]D levels and bone health for decades.
The IOM’s “Recommendations” for Vitamin D Levels
Though the Institute of Medicine (IOM) has given us guidelines on what they believe to be good vitamin D levels, these levels are based primarily on bone health. Arguably, these aren’t even “recommendations.” Rather, these are the values upon which they base their recommendations for vitamin D consumption.
The IOM requires a very high threshold of evidence before they make a recommendation. As such, they are waiting for a larger body of research regarding the efficacy of higher vitamin D levels before they can modify their statements. Therefore, many consider the IOM recommendations to be a minimum guideline for vitamin D levels. We will review those guidelines before moving on to recommendations given by other organizations.
30 nmol/L (12 ng/mL)
The IOM believes that persons with vitamin D levels below 30 nmol/L are “at risk of deficiency relative to bone health[.]”
30 – 50 nmol/L (12 – 20 ng/mL)
The IOM believes that “some, but not all, persons are potentially at risk for inadequacy” at this level.
≥ 50 nmol/L (20 ng/mL)
At this level, the IOM considers “practically all persons [to be] sufficient[.]”
The IOM also objects specifically to guidelines suggesting vitamin D levels of 75 nmol/L (30 ng/mL) or higher, as they do not believe there is currently sufficient evidence of further benefits at this level. They do not believe there is any risk of vitamin D toxicity, however, in blood levels below 125 nmol/L (50 ng/mL).
The Endocrine Society’s Recommendations for Vitamin D Levels
The Endocrine Society is a bit more comfortable with setting higher clinical guidelines. Below are their recommendations in regards to vitamin D levels.
≤ 50 nmol/L (20 ng/mL)
At this level, the Endocrine Society considers vitamin D levels to be deficient.
52.5 – 72.5 nmol/L (21 – 29 ng/mL)
The Endocrine Society believes vitamin D levels in this range qualify for insufficiency.
Approximately 75 nmol/L, or 30 ng/mL, is considered to be a healthy vitamin D level by the Endocrine Society. However, they recommend targeting a level of 100 nmol/L (40 ng/mL) in order to ensure blood levels stay above 75 nmol/L.
Other Recommendations
Other organizations or researchers recommend different vitamin D levels. This is primarily a question of what effects we’re considering. Though a child with vitamin D levels above 50 nmol/L (20 ng/mL) is unlikely to experience rickets, we do not yet have a solid grasp on which vitamin D levels may provide the most benefits for prevention or treatment of conditions such as diabetes, depression, alzheimers, or multiple sclerosis.
So, what about the 3/4ths of Americans who have vitamin D deficiency? What does that statistic mean? That number references an article published in 2009 that found vitamin D levels in the nation had decreased such that only 23% of the US population had vitamin D levels of 30ng/mL or higher – the Endocrine Society’s cutpoint for adequate vitamin D levels. That puts over 3/4ths of the US population at insufficient (or worse) levels, according to the Endocrine Society’s guidelines.
If, however, you prefer to stick with the IOM’s more conservative figures, that same study still has some startling news. They found that 6% of the population has 25[OH]D levels below 10 ng/mL — a rather serious deficiency, even by the IOM’s standards.4
Many doctors will prescribe a high-dose vitamin D supplement such as Decara if your levels fall below 12 ng/mL (30 nmol/L). If your levels are above that but you still believe you are suffering from vitamin D deficiency, you may need to have a longer conversation with your doctor. There are even additional tests that can be used to determine if you are suffering from some of the standard effects of vitamin D deficiency, such as testing for parathyroid hormone levels.
Be sure to communicate all of your symptoms and concerns with your doctor, and avoid taking a high-dosage vitamin D supplement without your doctor’s supervision.