Why Blood Testing Is Important for Vitamin D?

By Amy Brownstein, April 28, 2023

Man sitting on a dock soaking up the sun

Vitamin D plays an essential role in the body. This fat-soluble vitamin supports bone health, balances calcium levels, impacts mood, strengthens immunity, and promotes longevity.

Vitamin D is measured through a blood test, but it’s not regularly measured as part of preventative bloodwork at yearly physicals (especially in the United States), although it’s estimated that 50% of the world's population, has insufficient vitamin D levels. InsideTracker's Ultimate plan includes vitamin D, and data show that 40% of first time customers have insufficient levels. You may be curious about checking your vitamin D levels and learning how to maintain optimal vitamin D status to reap the benefits associated with this vitamin. [1,2]

Here’s what you need to know about blood testing for vitamin D, how to understand your test results, and the factors that impact vitamin D status. 

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What is vitamin D?

Vitamin D, also known as the “sunshine vitamin” or calciferol, is a fat-soluble vitamin involved in many physiological processes. Vitamin D plays a crucial role in calcium homeostasis and contributes to insulin production, immune responses, and cardiovascular functioning. [3] 

Vitamin D is obtained from food and supplements and is produced by the body when the skin is exposed to ultraviolet (UV) rays from the sun, initiating vitamin D synthesis. Vitamin D comes in two forms: D2 (ergocalciferol) or D3 (cholecalciferol).

Vitamin D acquired from food sources can be in the form of D2 or D3, but vitamin D made by the skin when it is exposed to sunlight is always in the form of cholecalciferol. [3]


Vitamin D must be converted into its active form to be used

Regardless of whether it is D2 or D3, ingested or synthesized vitamin D must go through a two-step hydroxylation process (a series of chemical reactions within the body) to become activated.

The first hydroxylation occurs in the liver. Vitamin D is converted to 25-hydroxyvitamin D [(25(OH)D], also known as calcidiol. The second hydroxylation is in the kidneys, where 25(OH)D is converted to 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol. This form is biologically active and carries out vitamin D’s functions. [4] 

The active form of vitamin D is a hormone and is involved in countless physiological processes. Once activated, vitamin D helps regulate calcium and phosphorus levels, contributes to bone growth and remodeling, reduces inflammation, and participates in other bodily processes, such as cell growth, neuromuscular and immune function, and glucose metabolism. [4]


What does a vitamin D test measure?

A vitamin D test measures 25-hydroxy to assess how much vitamin D is available for conversion to the active form. Extra 25(OH)D is stored in fat cells until the body needs it. Ensuring adequate stores of vitamin D helps prevent health issues from arising when limited vitamin D is available for physiological functioning. [3]

The 25-hydroxy test reflects endogenous (created within the body) vitamin D production and vitamin D obtained from food and supplements. Moreover, because 25-hydroxy has a half-life of about 15 days (the amount of time it takes for a molecule or substance to be eliminated from the body), it more accurately reflects vitamin D status. [4] 

In comparison, 1,25(OH)2D is not an accurate measurement of vitamin D status. Calcitriol has a shorter half-life (hours) and circulating levels are tightly regulated by other nutrients and hormones in the body. Because of this, levels of 1,25-dihydroxy do not decrease until there is severe vitamin D deficiency. [4] 


Can vitamin D2 or D3 be measured individually?

Vitamin D test results may display D2 or D3 separately or as one value.

Regardless, all vitamin D—whether it originates as D2 or D3—is converted into 25-hydroxy, which is then used to evaluate vitamin D status. Mass spectroscopy (an analytical testing method that sorts chemicals based on their mass-to-charge ratio) separates 25-hydroxy into D2 or D3 for reporting. [4]

Measuring 25-hydroxy is beneficial for diagnosing and monitoring vitamin D deficiency. Alternatively, measuring D2 or D3 may provide a better, more direct method for tracking the response to supplementation. [1]


Benefits of optimizing vitamin D levels



How do you check your vitamin D levels?

Vitamin D levels are measured with a blood test. Levels shift—positively or negatively—in response to different lifestyle habits. So while many people only get a blood test yearly at physicals, routinely assessing vitamin D about two to four times per year helps you track any changes to vitamin D with those changes you may experience throughout the year. 

Unfortunately, even if you do get yearly bloodwork, vitamin D is not covered as part of routine preventative screenings (check out Cigna’s and United Health Care’s medical coverage policy for vitamin D testing).

But there are direct-to-consumer blood testing options to get your vitamin D measured in a lab or with a finger-prick blood sample at home, including InsideTracker's Ultimate Plan.  


What’s the purpose of a vitamin D blood test?

A vitamin D blood test evaluates whether you have enough vitamin D to support normal physiological processes. It can also monitor vitamin D status, enabling you to maintain optimal levels and take action before serious problems arise. 

Low vitamin D is associated with increased mortality (death) as well as age-associated conditions. And evolving research continues to show the role of vitamin D in numerous other health conditions, including age-related concerns like heart disease, fractures, falls, moods, and susceptibility to the flu. [2] 

Vitamin D affects many important physiological processes including the metabolism of calcium, phosphorus, and bone. Maintaining optimal vitamin D levels helps prevent any downstream effects that could result from changes to metabolism in these other nutrients. So, screening for vitamin D deficiency and monitoring nutrient status benefits health and longevity. [2]





What do vitamin D blood test results mean?

The National Institute of Health separates the results of a vitamin D test into the following categories: [4]

  • Deficient: <30 nmol/L or <12 ng/mL
  • Low or insufficient: 30-50 nmol/L or 12-20 ng/mL
  • Adequate: ≥ 50 nmol/L or ≥20ng/mL
  • High: >125 nmol/L or >50 ng/mL

*nmol/L (nanomoles per liter), ng/mL (nanograms per milliliter)

A 25-hydroxy level greater than 30 ng/mL is required to maintain an adequate vitamin D status. A healthcare provider should always make a diagnosis of vitamin D deficiency. 

InsideTracker takes this analysis a step further and sheds light on whether or not you have optimal levels of blood biomarkers like vitamin D.

Based on the available peer-reviewed scientific literature as well as data from a large database of hundreds of thoughts of Americans (including demographic information such as age, sex ethnicity, and activity), InsideTracker developed these ranges for vitamin D.      

  • At risk low: <20 mg/mL (contact a healthcare practitioner about these results)
  • Suboptimal: < 30 ng/mL
  • Optimized: 32-100 ng/mL
  • At risk high: > 100 ng/mL

While the At risk high threshold is much higher than the upper end of the NIH’s designated range, 100 ng/mL aligns with vitamin D safety and toxicology reports as well as Quest Diagnostics ® vitamin D reference range. [5]


How do you know if you have a vitamin D deficiency or insufficiency?

A blood test will show if you have suboptimal levels of vitamin D. 

People with limited exposure to sunlight or certain health conditions may be at risk of a vitamin D deficiency. The time of day, season, latitude, altitude, cloud coverage, smog, clothing, skin pigmentation, and the use of sunscreen can affect the ability to synthesize vitamin D from sunlight. [2]

Additionally, certain conditions—such as gastrointestinal malabsorption, kidney issues or disease, or inadequate nutrient intake—can influence the likelihood of a vitamin D deficiency.


What causes low vitamin D levels?

Many factors influence vitamin D status, including some that are uncontrollable.


Low sun exposure

Approximately 50-90% of your vitamin D needs come from the sun. [6] So inadequate sun exposure is a significant risk factor for low vitamin D levels. 

Vitamin D synthesis from sunlight is most effective without the use of sunscreen. Current guidelines from the NIH recommend people get 5-30 minutes of sun exposure between 10 am and 4 pm to the face, arms, hands, and legs without sunscreen daily or at least twice a week to achieve sufficient vitamin D status. [4] 

Despite the benefits of not wearing sunscreen for short outdoor exposure to vitamin D synthesis, it is still recommended to use sunscreen with a sun protection factor (SPF) of at least 15 whenever outside and exposed to sunlight. Ultraviolet (UV) radiation from the sun is a carcinogen that can cause skin damage, increase the risk of skin cancer, and lead to faster aging and wrinkles. [4]



Aging-related processes affect the amount of active vitamin D in the body. 

  • Aging reduces the ability to produce vitamin D from sunlight
  • Age-related declines in kidney function impact the conversion of 25-hydroxy to 1,25-dihydroxy

These changes as well as changes to calcium absorption, which influence and are influenced by vitamin D status, can make it more challenging to maintain optimal vitamin D levels in older age. [7] This is particularly concerning given the role vitamin D plays in maintaining optimal health throughout the lifespan. Sufficient vitamin D levels are linked to decreased risk of falls and fractures and improved balance and physical functioning in older adults—which are all critical components to maintaining independence and vitality. [8] 


Drug interactions

Certain medications can contribute to low vitamin D levels. And it is always best to consult your medical provider before starting a supplement to ensure it will be safe and effective for you. [9]

  • Statins: Vitamin D produced in the body originates from cholesterol. So statin medications (used to reduce cholesterol) can lower vitamin D. 
  • Steroids: corticosteroids (such as prednisone) used to help inflammation can reduce calcium absorption, which negatively affects vitamin D metabolism.
  • Antiepileptic drugs: Many drugs for treating epilepsy affect bone metabolism and can lead to bone disease, which in turn decreases 25-hydroxy D levels. 
  • Bisphosphonates: Vitamin D deficiency may accompany insufficient calcium levels, as bone metabolism and vitamin D status are closely linked. Bisphosphonates (medications for bone loss) appear most effective when optimal vitamin D levels are maintained. Routine vitamin D testing when taking bisphosphonates helps prevent additional bone loss.


What causes high vitamin D levels?

It is possible to have too much vitamin D in your system. High vitamin D levels are often associated with excessive supplementation for prolonged periods, as food provides only a small amount of vitamin D. Likewise, it is unlikely that increased sun exposure alone causes high levels of vitamin D, but significantly elevated vitamin D levels can occur with frequent use of tanning beds. [4] 



Food sources of vitamin D



What are natural sources of vitamin D?

The sun is the best natural source of vitamin D. Vitamin D2 is mostly found in UV-treated mushrooms. In contrast, vitamin D3 abounds in fatty fish (mackerel, salmon, and sardines) and exists in smaller quantities in other animal products, such as beef liver, egg yolks, and cheese. [4] 

Fortified foods—like milk and orange juice in addition to some breads and cereals—account for most of the food-related vitamin D intake in the United States. [3] For example, cow’s milk and plant-based milk alternatives are usually fortified with 3 micrograms per cup (120 International Units) of vitamin D. [4] 


Do you need to supplement with vitamin D?

As few foods naturally contain vitamin D, supplementation can be necessary to ensure optimal vitamin D levels, especially in the winter when the sun is less potent. And evolving research shows the importance of vitamin D in other areas of health, such as supporting immune function and certain hormone levels

The Recommended Dietary Allowance (RDA), established based on vitamin D intake from foods, is 15 micrograms (600 International Units) daily for men and women ages 19-70. [4] But many healthcare providers recommend regular supplementation with 1000 IU of vitamin D to maintain adequate levels. [2] 

For a more targeted approach, InsideTracker can help establish a supplement regimen based on your specific vitamin D levels, dietary intake, and sun exposure. 


Key takeaways:

  • A vitamin D test measures 25-hydroxy to assess vitamin D status. Blood sample results can be shown at D2 or D3 or total 25(OH)D. 
  • Vitamin D is vital for several physiological processes, including maintaining calcium balance and supporting bone health.
  • Most vitamin D comes from skin exposure to sunlight. But some foods—especially fortified milk and juice—can provide vitamin D. 
  • Low sun exposure, aging, and interactions with certain medications can reduce vitamin D levels. 
  • Routinely testing vitamin D can help you take appropriate steps to optimizing your levels, but coverage of vitamin D blood tests through insurance may be challenging. 
  • Vitamin D analysis through InsideTracker can determine the best actions you can take to optimized your vitamin D levels. 


[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912737/

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3356951/

[3] https://www.ncbi.nlm.nih.gov/books/NBK441912/

[4] https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

[5] https://pubmed.ncbi.nlm.nih.gov/18689406/ 

[6] https://www.ncbi.nlm.nih.gov/books/NBK532266/

[7] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3782116/

[8] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4399494/

[9] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3427195/


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