Vitamin D Deficiency

What is Vitamin D?

Vitamin D, also known as the 'sunshine vitamin', is an essential nutrient for the maintenance of healthy bones and teeth, and for good immune system functioning. Vitamin D is produced naturally by plants and the skin on exposure to sunlight, specifically to ultraviolet B (UVB) radiation. Vitamin D has two main forms, D2 and D3. Vitamin D2 is produced by plants exposed to sunlight and is found in vegetables in the diet, vitamin-fortified foods, and supplements. Vitamin D3 is primarily from skin exposure to sunlight UVB radiation, as well as from a standard diet, vitamin-fortified foods, and supplements.

Neither Vitamins D2 nor D3 are biologically-active forms. The sun's rays trigger the process that converts cholesterol (7-dihydrocholesterol) into Vitamin D3. Once absorbed from the intestine, both vitamins D2 and D3 are broken down in the liver to 25-hydroxyvitamin D (25-OH D, known as calcidiol). In turn, 25-OH D is converted in the kidney to the active compound 1,25-dihydroxyvitamin D (1,25-(OH)2 D, known as calcidiol).

Why is Vitamin D important?

Vitamin D is required for the regulation of the body's calcium level, which is necessary to maintain bone density and promote bone health. A deficiency can lead to an increased risk of bone fractures and bone diseases, such as rickets in children, and osteomalacia and osteoporosis in adults.

Vitamin D is critical to immune system function. Having many effects on the different cells within the immune system, Vitamin D modulates immune responses. Vitamin D deficiency is associated with an increased susceptibility to infection, and increased development and progression of autoimmune diseases, such as multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease and diabetes. Additionally, there is evidence that low vitamin D levels may be associated with other chronic diseases, including heart disease, asthma (children), and bowel and other cancer, and may contribute to all-cause mortality.

Who should be tested for Vitamin D deficiency?

Vitamin D levels are checked in the screening and monitoring of bone disorders. Bone related symptoms of a Vitamin D deficiency include weak or soft bones, bone malformations, and fractures. People with osteoporosis or other bone diseases are at high risk of having low Vitamin D levels.

Vitamin D tests are also used to monitor Vitamin D levels in people with chronic diseases such as autoimmune disease, or who are at high risk of having a deficiency. Risk factors for a vitamin D deficiency include older age, darker skin, obesity, strict vegan diet, and pregnancy; people with gastric bypass surgery or diseases of the intestines (such as inflammatory bowel disease) which affect the absorption of nutrients; liver or kidney disease; and those with limited exposure to sunlight or avoid the sun (e.g., people who are housebound, wear whole-body clothing, burn easily, or with skin cancer).

How to test for Vitamin D deficiency?

Vitamin D status is most accurately (and usually) determined by measuring the level of 25-OH D in the blood. Not everyone is tested, but it may be of benefit to you if you are at risk of deficiency (i.e., decreased intake, and intestinal, liver or kidney problems) or have clinical findings associated with deficiency (e.g., fragility fractures, low bone mineral density, and low serum calcium or phosphorus levels). In these cases, 25-OH D level may help to determine your need for vitamin D therapy and an effective dose.

A blood test for Vitamin D can be done using a blood sample taken at home by patients. Your healthcare professional will provide you with a self-collection sample pack. Within the pack there are instructions on how to collect a blood sample, which must be read before starting the procedure. In summary, using clean hands, the 600 ml blood sample is collected from a finger. The skin is pierced using a lancet, that activates when firmly pressed against the skin. Blood drops are collected in the collection tube, which is filled to the top line. Once you have sealed the tube with the cap, carefully invert the tube from 5 to 10 times. It is important that you stick the completed name label to the tube before packing. Your healthcare professional will tell you where to send the blood sample for laboratory analysis. Results of your test will be sent to your healthcare professional who will then explain the findings to you.

Results take 4 hours after the sample arrives at the lab.

What do Vitamin D test results indicate?

Vitamin D deficiency is relatively common, particularly in people with inadequate sun exposure, limited dietary intake, and poor intestinal absorption. Thus, a low blood level of 25-OH D may suggest that you are not getting sufficient exposure to sunlight or enough Vitamin D in your diet to meet the needs of your body. Low levels may also indicate problems with 25-OH D absorption from the intestines or production by the liver (some anti-seizure drugs can cause a reduction). On the other hand, high blood levels of 25-OH D are most often caused by excess Vitamin D supplementation.

For people in whom 1,25-(OH)2 D is measured, low levels can indicate low levels of 25-OH D and kidney disease, or be caused by some HIV medicines. 1,25 (OH)2 D levels in the blood may be measured if you have high calcium levels or a condition that might cause increased amounts of 1,25 (OH)2 D from its production outside of the kidney, including sarcoidosis, tuberculosis or some lymphomas.

What is a normal Vitamin D test result?

There are many factors affecting Vitamin D production, which means that what is 'normal' for you may not be for another person. Vitamin D reference ranges, as measured by 25-OH D, vary widely depending on the population (age, gender, ethnicity, geographic location, and season). Your optimum level of vitamin D might also depend on your health and disease status. Your healthcare professional will discuss your results in the context of your situation. There is also some variation in the 25-OH D reference ranges used by different medical establishments, such as the following guidance from two highly respected establishments:

The UK National Osteoporosis Society reference ranges (plasma 25-OH D):

• Deficiency: less than 25 nmol/l
• Inadequate in some people: 25-50 nmol/l
• Sufficient for almost the whole population: greater than 50 nmol/l

The US Mayo Medical Laboratories reference ranges (serum 25-OH D):

• Severe deficiency: less than 10 ng/l
• Mild to moderate deficiency: 10-24 ng/l
• Optimal range: 25-80 ng/l
• Possible toxicity: greater than 80 ng/l

Vitamin D reference ranges for healthy persons, as measured by 1,25(OH)2 D, are 18-64 pg/ml for men and 18-78 pg/mL for women.

How to maintain or achieve good Vitamin D levels

The best way to maintain or achieve optimal Vitamin D levels is through exposure to UVB radiation in sunlight, a Vitamin D-rich diet, and daily oral supplementation. The risk of low Vitamin D levels is greater during the winter months but it is often due to an inadequate dietary intake. Excluding oily fish, egg yolk, mushrooms, and fortified food products (e.g., milk products, margarines, cereals, and soy), many foods are poor sources of vitamin D, and vitamin D supplementation is required. Although both Vitamin D2 and D3 are required for good health, Vitamin D3 supplementation is more effective at improving and maintaining the body's levels of Vitamin D. Most Vitamin D supplements are higher in D3 than D2, or are D3 only. Vegans should be aware that Vitamin D3 supplementation is often derived from an animal source, but there are vegan D3 supplements available.

Vitamin D supplementation without testing is justified for people who have no apparent risk factors or deficiency but have inadequate sun exposure or dietary intake. Current advice is that all people over the age of one year should take a Vitamin D supplement of 10 μg (400 IU) per day, especially in autumn and winter. In particular, people who are at increased risk of vitamin D deficiency should take the supplement throughout the year. In cases of Vitamin D deficiency, an oral Vitamin D3 dose equivalent to 20-50 μg (800-2000 IU) daily is recommended as maintenance therapy (with higher doses taken initially, if necessary).

Reviewed by: Dr Loraine Haslam MBBS, DRCOG, DFSRH, LoC SDI, LoC IUT, MRCGP
GMC registration number: 4524038
Date: 16 August 2022
Next review: 15 August 2024
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