A vitamin D test measures 25(OH)D, the form of vitamin D in your blood that shows whether you have enough [2]. Your clinician may order one if you have limited sun exposure, stay indoors, cover up outside, use sunscreen, or live where there is little sunlight [2]. One UK study of 6,433 South Asian adults aged 40 to 69 found 92% with levels below 50 nmol/L and 55% below 25 nmol/L [3]. That is UK data, not US data. No published US prevalence figure exists for vitamin D deficiency in South Asian adults, so the honest answer to how common this is among US South Asians is that this guide cannot document it from the sources available.

What does the vitamin D test actually measure?

A vitamin D test measures the level of 25(OH)D in your blood [2]. The abbreviation stands for 25-hydroxyvitamin D, and it is the most accurate way to see if you have enough vitamin D [2]. Your body makes vitamin D when your skin is exposed to sunlight, and you also get it from food and from things you take by mouth. The test shows the total amount circulating in your blood at the time of the test.

Your clinician interprets the result against a reference range, which varies by lab. The test itself is straightforward: a blood draw, sent to a lab, a number back. What that number means for you is a conversation with your clinician, not something this guide can predict.

Why would a clinician order this test?

MedlinePlus lists the reasons a clinician may order a vitamin D test [2]. Limited sun exposure is one: a person stays indoors, covers up outside, uses sunscreen, or lives where there is little sunlight. Other reasons include certain medical conditions, medicines that affect how your body uses vitamin D, and a family history of bone disease. Your clinician decides whether the test fits your situation.

The test is not routine screening for everyone. It is ordered when there is a reason to check, and the result guides the next step, which is a conversation with your clinician about what it means for you.

What does the UK data show?

Darling and colleagues analyzed 6,433 UK South Asian adults aged 40 to 69 in the UK Biobank cohort [3]. They found 92% had 25-hydroxyvitamin D below 50 nmol/L and 55% below 25 nmol/L [3]. This is UK data, not US data. The UK Biobank is a research cohort in the United Kingdom, and the people in it are UK residents.

That number is striking. It describes a population where the vast majority had levels below a threshold that public health agencies consider adequate. But it describes the UK, not the United States. The people in the UK Biobank live in a country with less intense sunlight than much of the United States, and they may have different patterns of sun exposure, clothing, diet, and skin tone than South Asian adults in the US. The UK figure cannot be assumed to describe US South Asians.

Is there a US prevalence figure?

No. The published data do not include a US prevalence figure for vitamin D deficiency in South Asian adults. The MASALA study, which follows US South Asian adults for heart disease risk, does not publish vitamin D prevalence. The MESA study, which includes South Asian participants, does not publish a South Asian-specific vitamin D figure in the sources available here. The CDC and NIDDK do not publish vitamin D deficiency prevalence by ethnicity or ancestry in their public materials.

This is a gap in the published data. It means that if you want to know how common vitamin D deficiency is among US South Asian adults, the honest answer is that no one has published that number. The UK figure is the strongest number in the literature, and it is UK data. Using it to estimate US prevalence would be guessing.

What is the connection to diabetes and heart disease?

South Asian adults in the United States have elevated type 2 diabetes risk: 23% prevalence among US South Asian adults aged 44 to 84, against 6% in white adults [1]. South Asians also have higher proportional mortality rates from atherosclerotic cardiovascular disease compared with other Asian groups and non-Hispanic whites [4]. These are measured facts in the published data.

Vitamin D is one of many factors that clinicians consider when screening and managing cardiometabolic risk. The published data do not establish that vitamin D deficiency causes diabetes or heart disease. Vitamin D levels are one piece of a much larger picture that includes family history, weight, physical activity, blood pressure, cholesterol, glucose, and many other factors. Your clinician weighs all of them together.

What happens after a vitamin D test?

If your clinician orders a vitamin D test and the result comes back, the next step is a conversation with your clinician about what it means for you. The result is a number, and the interpretation depends on your full clinical picture: your symptoms, your other test results, your medical history, your medicines, and your clinician's judgment.

This guide cannot tell you what your result means or what to do about it. That conversation belongs with your clinician, who knows your history and can explain the result in the context of your care. If you do not understand the result or the recommendation, ask your clinician to explain it again. You are entitled to understand what a test means before you decide what to do next.

Understanding screening and risk

When you read that 92% of UK South Asian adults had vitamin D below a certain threshold, it is easy to think that means you will too [3]. It does not. That number describes a population in the UK. You are one person in the United States with your own sun exposure, your own diet, your own skin tone, your own medical history, and your own clinician. A population number is not a prediction about you.

The same is true for the elevated diabetes and cardiovascular risk measured in South Asian adults overall [1][4]. Group risk is real and measured. It is also not destiny. It is not in the blood. It is not a fate. It is information that helps your clinician decide whether screening makes sense for you, and it is information that helps you understand why your clinician might recommend certain tests or conversations. But it does not tell you what will happen to you personally.

What to do next

  1. Ask your clinician whether a vitamin D test makes sense for you, based on your sun exposure, your symptoms, and your medical history.
  2. If your clinician orders the test, ask what the reference range is at your lab and what your result means.
  3. If your result is outside the normal range, ask your clinician what the next step is. Do not assume you know what to do until you have had that conversation.
  4. Write down your result and the date of the test so you can track it over time if your clinician recommends repeat testing.
  5. If you are interested in your broader cardiometabolic risk as a South Asian adult, read the metabolic blood tests explained guide and the South Asian diabetes risk guide to understand what screening looks like and why.
  6. Check the guides hub for other tests and risk factors your clinician might discuss with you.

Sources

  1. Kanaya and colleagues, MASALA and MESA diabetes prevalence, type 2 diabetes in US South Asian adults, accessed 2026-08-06, https://pubmed.ncbi.nlm.nih.gov/24705613/
  2. MedlinePlus, vitamin D test, what the test measures and reasons for ordering, accessed 2026-08-06, https://medlineplus.gov/lab-tests/vitamin-d-test/
  3. Darling and colleagues, vitamin D deficiency in UK South Asian adults, UK Biobank prevalence in adults aged 40 to 69, accessed 2026-08-06, https://pmc.ncbi.nlm.nih.gov/articles/PMC7844605/
  4. American Heart Association scientific statement on atherosclerotic cardiovascular disease in South Asians, cardiovascular mortality in South Asian populations, accessed 2026-08-06, https://www.ahajournals.org/doi/10.1161/CIR.0000000000000580