Vitamin D blood test: reading your result in ng/mL and nmol/L and what to do next

What does the vitamin D test actually measure?

When your doctor orders a vitamin D test, they are measuring the level of a compound called 25-hydroxyvitamin D in your blood, abbreviated as 25(OH)D. This is the main storage form of vitamin D in the body and the one that can be measured reliably. The test does not measure the active form (1,25-dihydroxyvitamin D) directly because that fluctuates rapidly and does not reflect your actual stores.

Vitamin D is not a vitamin in the classical sense but closer to a hormone. The body makes it in the skin when exposed to ultraviolet B (UVB) radiation from sunlight, and small amounts are found in oily fish, egg yolk and fortified dairy products. It plays a central role in absorbing calcium from the gut and maintaining bone health, immune function and muscle strength.

Many people test their level after noticing fatigue, muscle aches or frequent illness, and vitamin D deficiency may be a partial explanation or not. The real value of the test is establishing your actual number so the right supplementation dose can be chosen. The dose needed to correct a level of 10 ng/mL is very different from what is needed at 22 ng/mL, and guessing the dose without knowing the level wastes time and risks either underdosing or overdosing.

Two units: ng/mL versus nmol/L

One of the most common sources of confusion in vitamin D results is that laboratories across the region do not use the same unit. Some report in ng/mL (nanograms per millilitre) and others in nmol/L (nanomoles per litre). Both describe the same thing but the numbers look very different, and mixing them up is easy and dangerous.

The conversion is simple: multiply ng/mL by 2.5 to get nmol/L, or divide nmol/L by 2.5 to get ng/mL. For example, 20 ng/mL equals 50 nmol/L. This matters because information from different sources uses different units, and a patient comparing their result to something they read online can easily misinterpret whether they are deficient.

Always check the unit printed on your laboratory report before comparing your result to any reference. A result of 20 in ng/mL means deficiency. A result of 20 in nmol/L means severe deficiency. The numbers are not interchangeable and no assumption should be made about which unit the lab is using.

Why is deficiency so common in one of the sunniest regions on earth?

This question surprises many people. A comprehensive review published in Bone Reports in 2018 covering Middle Eastern and North African countries demonstrated that vitamin D deficiency affects large proportions of the population in Saudi Arabia, the UAE, Lebanon, Syria, Egypt, Morocco and other countries, including those living outdoors.

The reasons are several and they overlap. Many women cover most of the skin with clothing, which prevents UVB from reaching the skin. Extreme daytime heat drives everyone indoors during the hours when UVB intensity is highest. Glass windows block UVB entirely, so sitting next to a sunny window does not produce vitamin D. Darker skin pigmentation requires longer sun exposure to produce the same amount of the vitamin. Air conditioning has extended indoor hours across the Gulf. The combination of these factors means that skin synthesis is minimal for much of the population for much of the year.

A 2019 study of people in Abu Dhabi found that over 80 percent of participants had levels below the target range. Research published in BMC Public Health in 2024 found that awareness of vitamin D deficiency remained limited across a broad cross-section of Syrian adults despite the high prevalence of the problem. The gap between the sun outside and the deficiency inside is one of the clearest examples of how the modern indoor lifestyle transforms a region's health profile.

Who is most at risk of deficiency?

Women who cover most of the body with clothing are at higher risk, and this does not mean that clothing should change. It means that monitoring the level and supplementing where needed becomes more important, not optional. Older adults are more vulnerable because the skin's ability to synthesise vitamin D from sunlight naturally decreases with age, and some commonly used medications including certain blood pressure and cholesterol drugs are associated with lower levels.

Exclusively breastfed infants need their own supplement because breast milk contains very little vitamin D. People with obesity tend to have lower circulating levels because the vitamin is captured by fatty tissue rather than remaining in the bloodstream. People with gut conditions including Crohn's disease and coeliac disease absorb less vitamin D even when their intake appears adequate.

Patients with diabetes, kidney disease or liver disease need closer attention because both the kidneys and the liver are needed to convert vitamin D into its active form. If you belong to any of these groups, a vitamin D level is not an optional extra on your annual blood test but a necessary part of managing your condition.

What does a very high result mean?

Vitamin D toxicity is real and occurs with prolonged use of very high doses. It cannot happen from sun exposure alone, because the skin regulates its own production, but it can occur from aggressive supplementation without monitoring. A level above 100 ng/mL (250 nmol/L) warrants reassessment of the dose being taken.

Levels above 150 ng/mL can cause high calcium in the blood, a condition called hypercalcaemia. Symptoms include nausea, vomiting, excessive thirst, frequent urination, constipation and in severe cases kidney damage. This is the main reason why knowing your level before starting a supplement matters, and why a high-dose correction regimen needs follow-up testing.

If you are on a high weekly dose to correct a documented severe deficiency, your doctor will usually repeat the test after eight to twelve weeks to check the response and adjust the dose downward to a maintenance level. Do not continue on a high correction dose indefinitely without review.

How deficiency is corrected: sunlight, food and supplements

Sun exposure between 10am and 2pm, exposing the forearms and lower legs for 15 to 30 minutes, produces useful amounts of vitamin D especially in summer. In the region, these are the hottest hours and carry the highest risk of sunburn and skin damage. The practical window for safe and productive sun exposure in summer is often only early morning or late afternoon, when UVB intensity may be lower. For most people with a documented deficiency, sunlight alone will not correct the problem within a clinically useful timeframe.

Foods high in vitamin D are limited and many are not staples of the regional diet. Oily fish including salmon, sardines and mackerel contain the highest amounts. Egg yolk and liver contain moderate amounts. Fortified dairy products exist in some countries but the dose per serving is generally too small to correct an existing deficiency. Dietary contribution matters for maintaining an adequate level but is rarely sufficient as the sole source when the starting level is low.

Supplements are the most reliable way to correct a low level. Doses range from 1,000 to 2,000 IU daily for general maintenance, up to 50,000 IU weekly to correct a severe deficiency under medical supervision followed by a repeat test. Vitamin D3 (cholecalciferol) is preferred over D2 because it raises the blood level more effectively and sustains it longer. Taking it with a fat-containing meal such as one including olive oil or eggs improves absorption. For patients who forget daily doses, a larger weekly dose is an acceptable alternative and may improve adherence.

Regional considerations: Ramadan, travel and medications

During Ramadan, eating and supplement-taking happen at defined times. Vitamin D can be taken at suhoor or iftar without any problem, and taking it with a fat-containing meal improves absorption. Do not stop a correction supplement during Ramadan if you are treating a documented deficiency, as this sets back progress considerably.

Frequent travellers who test at different laboratories across the region may find their results reported in different units each time. Keep a clear record of the unit used alongside every result so that comparisons over time are meaningful. A result of 45 nmol/L from one country and a result of 20 ng/mL from another are very different things, even though both numbers look similar.

Several commonly used medications affect vitamin D metabolism. Anti-epileptic drugs including phenobarbitone and phenytoin speed up the breakdown of vitamin D in the liver. The cholesterol drug cholestyramine reduces intestinal absorption. Rifampicin also affects levels. If you take any of these medications, mention this when discussing your vitamin D level with your doctor because you may need a higher maintenance dose than someone on no medication.

When and how to have the test done

The vitamin D test does not require fasting and can be done at any time of day. If you are already supplementing and want to measure your stable level, wait at least six hours after your dose or test early in the morning before taking that day's supplement.

If you are correcting a severe deficiency, the test is usually repeated after eight to twelve weeks to check the response and adjust the dose. For maintenance in a stable situation, once a year is generally sufficient. If you belong to a high-risk group such as those with kidney disease or on medications that affect vitamin D, more frequent testing may be appropriate.

Where possible, try to use the same laboratory each time you test, because different instruments can produce slightly different readings from the same sample. This is a recognised phenomenon and does not mean the lab is unreliable. What matters when tracking your level over time is the trend rather than small differences between individual measurements.

Sources

• Chakhtoura M, Rahme M, Chamoun N et al: Vitamin D in the Middle East and North Africa, Bone reports, 2018
• Al Zarooni AAR, Al Marzouqi FI, Al Darmaki SH et al: Prevalence of vitamin D deficiency and associated comorbidities among Abu Dhabi Emirates population, BMC research notes, 2019
• Anouti FA, Ahmed LA, Riaz A et al: Vitamin D Deficiency and Its Associated Factors among Female Migrants in the United Arab Emirates, Nutrients, 2022
• Alibrahim H, Swed S, Bohsas H et al: Assessment the awareness of vitamin D deficiency among the general population in Syria: an online cross-sectional study, BMC public health, 2024
• Al-Daghri NM, Al-Saleh Y, Aljohani N et al: Vitamin D status correction in Saudi Arabia: an experts consensus under the auspices of ESCEO, Archives of osteoporosis, 2017
• Saleh A, Shibli F, El Masri J et al: Osteoporosis and vitamin D consumption: knowledge and practice in different Arab countries, Archives of osteoporosis, 2023