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Last updated July 7, 2020. 8 minute read

Vitamin D blood test: what the results mean

Vitamin D deficiency affects one billion people worldwide and approximately 40% of Americans. Vitamin D testing is via a blood test that measures the levels of 25-hydroxy vitamin D in your bloodstream.

Dr Chimene Richa Md Written by Chimene Richa, MD
Reviewed by Mike Bohl, MD, MPH

What is vitamin D?

Vitamin D, or the “sunshine” vitamin, is not truly a vitamin. While It is an essential nutrient and we do get some of it from food, it’s a prohormone. A prohormone is a substance that the body turns into a hormone; hormones circulate throughout your body, regulating different processes.

Your body makes most of the vitamin D that you need via a chemical reaction that starts with sun exposure. When ultraviolet rays from sunlight hit a specific cholesterol molecule in your skin, your body produces an inert form of vitamin D called cholecalciferol. The liver then converts this to calcidiol (25-hydroxy vitamin D). Then the kidneys convert calcidiol to calcitriol (1,25-dihydroxy vitamin D), the active form of vitamin D. This is why some people refer to sun exposure as “getting some vitamin D.”

Vitals

  • Vitamin D, the “sunshine vitamin,” is a prohormone and not truly a vitamin.
  • The primary role of vitamin D is the regulation of calcium levels in your body.
  • You get most of your vitamin D from sun exposure, with only 10% coming from food like milk or fatty fish.
  • Vitamin D deficiency affects one billion people worldwide and approximately 40% of Americans.
  • Vitamin D testing is done via a blood test that measures the levels of 25-hydroxy vitamin D in your bloodstream.

This process accounts for most of the vitamin D in your body. Few natural foods contain vitamin D and only about 10% of the vitamin D that your body comes from food intake (HHN, 2018). Fatty fish (like salmon, tuna, and mackerel) and fish liver oils have the highest amounts. Beef liver, cheese, and egg yolks also contain some vitamin D; these foods and the fatty fish typically have vitamin D in the form of vitamin D3. Some mushrooms and fortified foods, like milk, have vitamin D in the form of vitamin D2 (NIH, n.d.).

Blood tests to check vitamin D

In some cases, your healthcare provider may recommend checking your vitamin D levels; it is not healthy to have levels that are too high or too low. However, the current guidelines do not recommend vitamin D testing for everyone—especially as testing can be expensive, and various laboratories may test or report the levels of vitamin D in different ways. 

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Experts recommend testing only for people at high risk for vitamin D deficiency, like those populations at high risk for developing low levels, or those with signs suggestive of a vitamin D problem (Kennel, 2010). The best way to check your vitamin D levels is to measure the amount of 25-hydroxy vitamin D (25(OH)D) circulating in the bloodstream (Holick, 2011). 25(OH)D is measured via drawing blood from your arm and sending it to the laboratory for analysis.

The reference ranges are not well established, and there is some argument among experts regarding what levels signify deficiency. According to the NIH’s Office of Dietary Supplements and the Institute of Medicine (IOM), the 25-hydroxy vitamin D level in nanomoles/liter (nmol/L) or nanograms/milliliter (ng/mL) reference ranges are as follows (NIH, n.d.):

Vitamin D deficiency Less than 30 nmol/L (12 ng/mL)*
Inadequate and potential deficiency Between 30 nmol/L (12 ng/mL) and 50 nmol/L (20 ng/mL)
Adequate levels of vitamin D Greater than or equal to 50 nmol/L (20 ng/mL)
High (potential for toxicity) Greater than 125 nmol/L (50 ng/mL)

*The Endocrine Society has a somewhat different measure of vitamin D deficiency. They recommend that vitamin D deficiency be defined as 25-hydroxyvitamin D levels below 50 nmol/L (20 ng/mL) and vitamin D insufficiency as 25-hydroxyvitamin D levels between 52.5-72.5 nmol/L (21-29 ng/mL) (Holick, 2011).

What are Vitamin D’s benefits?

Your healthcare provider may recommend that you get blood tests to check your vitamin D levels, especially if you are at risk for vitamin D deficiency. Vitamin D is an essential nutrient because it is involved in many different processes in the body, including maintaining calcium levels, immune health, and other potential benefits. 

Bone health/osteoporosis prevention

Vitamin D’s primary function is to help your gut absorb calcium and phosphate, thereby working to maintain those levels at appropriate concentrations. Vitamin D and calcium play a vital role in bone growth and remodeling (the constant building and resorption of bone). When calcium levels are low, vitamin D triggers increased calcium absorption from the intestinal tract (Veldurthy, 2016). If this process doesn’t provide enough calcium, your body releases hormones that trigger bone breakdown to get more calcium into the bloodstream. Without enough vitamin D, your bones can become thin and brittle (which is called osteomalacia). Lack of vitamin D also seems to play a role in osteoporosis in older adults.

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Immune system health

Vitamin D receptors exist on immune cells. These cells may respond to vitamin D, especially B cells, T cells, and monocytes (Aranow, 2011). Low levels of vitamin D have been associated with an increased risk of infection and a higher chance of autoimmune diseases (Aranow, 2011). Specifically, vitamin D deficiency has been linked to tuberculosis, upper respiratory tract infections (like the flu), and asthma, but more research is needed in these areas (Chang, 2019).

Protection from certain cancers

Some observational studies have suggested that higher levels of vitamin D are linked to lower levels of certain cancers, especially prostate, breast, and colon cancers (Hansen, 2016). Vitamin D is involved in regulating the genes that control cell proliferation, differentiation, and death—these are essential processes in both healthy and cancer cell development (NIH, n.d.). However, a nationwide study of over 25,000 people did not show a lower incidence of cancer with vitamin D supplementation (Manson, 2019).

Regulates insulin/reduced risk of diabetes

Diabetes is a global health problem that affects millions of people. Scientists are always looking for ways not only to treat the condition but also to prevent the development of diabetes. One possibility is by looking at the role that vitamin D may play in diabetes. Studies show that there are vitamin D receptors on pancreatic cells; these pancreatic cells produce insulin, the hormone responsible for blood sugar regulation.

Also, vitamin D may be linked to insulin sensitivity and inflammation, both of which are affected by diabetes. Some observational studies have suggested that low vitamin D levels increase the risk of developing diabetes. However, a nationwide survey of over 25,000 people showed that people who took vitamin D supplements did not have a lower risk of diabetes than those who took placebo pills (Pittas, 2019).

Cardiovascular health

Heart disease (also known as cardiovascular disease) is the leading cause of death in the U.S., according to the CDC (CDC, 2017). So it comes as no surprise that researchers are continually looking for ways to help people improve their cardiovascular health—and vitamin D may help improve cardiovascular health. Vitamin D receptors exist on heart muscle cells as well as blood vessel cells, suggesting that vitamin D may play a role in cardiovascular function.

One study found that low levels of vitamin D were associated with heart disease and its risk factors, like high blood pressure, coronary artery disease, and cardiomyopathy (enlarged heart) (Vacek, 2012). However, a nationwide study of over 25,000 people did not show any difference in the incidence of major cardiovascular events (like heart attack, strokes, or death due to heart problems) between those who took vitamin D supplementation vs. placebo (Manson, 2019).

Strength

Since vitamin D is so vital to bone health and calcium levels, it is not unreasonable to wonder if vitamin D can help improve your strength. However, the data to date does not support this theory. A clinical trial looking at how vitamin D supplementation can affect people’s bone mineral density (BMD) and strength did not find any improvement in bone strength with the added vitamin D in healthy adults without osteoporosis (Burt, 2019).

Another study reviewed the literature and found that taking vitamin D supplements does not prevent bone fracture or improve bone density and strength (Bolland, 2018).

Vitamin D deficiency

As you can see, vitamin D is vital to keeping your body functioning normally. If you do not get enough vitamin D (also referred to as vitamin D deficiency), you may develop health problems. Approximately one billion people worldwide have vitamin D deficiency, making this a global public health issue (Sizar, 2020).

 In the United States, about 40% of Americans suffer from low levels of vitamin D (Parva, 2018). Specific populations are at higher risk for developing low vitamin D levels, including older adults, people with obesity, nursing home residents, and hospitalized patients (Sizar, 2020).

People become vitamin D deficient as a result of not taking in enough vitamin D, absorption problems, or getting rid of more of the nutrient than you should. For example, people with limited sun exposure or who have kidney disease that prevents them from converting vitamin D to its active form may have lower vitamin D levels (NIH, n.d.).

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Also, those who have dietary restrictions, such as milk allergies or some types of vegetarianism/veganism, or malabsorption issues (like gastric bypass surgery or Crohn’s disease) may not take in enough vitamin D. Lastly, people with darker skin have more pigment (melanin), preventing ultraviolet rays from interacting with the skin to produce enough vitamin D. This can increase the risk of developing vitamin D deficiency (Holick, 2011).

Most people with low vitamin D levels are asymptomatic and never realize that there is a problem. Unfortunately, low levels of vitamin D over a long period of time can lead to low levels of calcium in the blood (hypocalcemia) and high levels of parathyroid hormone (hyperparathyroidism). These abnormalities can lead to symptoms like bone pain, joint and muscle aches, weakness, and fragile bones (osteomalacia and osteoporosis). Older adults with vitamin D deficiency may experience more frequent falls and an increased risk of bone fractures (Holick, 2011).

On the other hand, children with low levels of vitamin D may develop skeletal deformities (also known as rickets) and may have trouble standing and walking (Holick, 2011). Fortunately, since the advent of vitamin D-fortified milk (and other foods), childhood rickets is relatively rare in the United States.

Conclusion

If you are concerned about your vitamin D status, talk to your healthcare provider about whether vitamin D testing is right for you, especially if you fall into one of the high-risk categories.