Vitamin D Deficiency: Blood Test, Normal Levels & Supplementation

Why Vitamin D Matters

Vitamin D is essential for healthy bones, teeth, and muscles. It helps the body absorb calcium and phosphate from food and plays a role in immune function. Unlike most vitamins, the majority of our vitamin D does not come from diet — it is produced in the skin when exposed to ultraviolet B (UVB) sunlight.

In the UK, we are at a geographical disadvantage. Between October and March, the sun is too low in the sky for the skin to produce meaningful amounts of vitamin D. This means that without supplementation or adequate dietary intake, many people become deficient during the darker months.

Severe deficiency can cause rickets in children and osteomalacia (bone softening) in adults. Milder deficiency is associated with fatigue, muscle weakness, bone pain, low mood, and impaired immune function. There is also growing research linking low vitamin D to increased risk of cardiovascular disease, autoimmune conditions, and certain cancers, though these associations are not yet fully established.

The 25-Hydroxyvitamin D Test

The standard blood test for vitamin D measures 25-hydroxyvitamin D (also written as 25(OH)D). This is the main circulating form of vitamin D and the best indicator of your overall vitamin D status. It reflects both dietary intake and skin production.

In the UK, results are reported in nanomoles per litre (nmol/L). The commonly used thresholds are:

• Below 25 nmol/L — Deficient. At this level, there is a risk of bone disease (rickets in children, osteomalacia in adults). Treatment doses of vitamin D are recommended.
• 25 to 50 nmol/L — Insufficient. May not have overt symptoms but is considered suboptimal. Supplementation is advised.
• 50 to 75 nmol/L — Adequate for most people.
• 75 to 150 nmol/L — Optimal according to many expert bodies, though there is no universal consensus above 50 nmol/L.
• Above 220 nmol/L — Toxicity risk. Can cause hypercalcaemia. This is extremely rare and almost always caused by excessive supplementation, not sunlight or diet.

Some laboratories and international guidelines use different units (ng/mL). To convert: 1 nmol/L = 0.4 ng/mL, so 50 nmol/L = 20 ng/mL.

UK Prevalence of Vitamin D Deficiency

Vitamin D deficiency is widespread in the UK. Data from the National Diet and Nutrition Survey shows:

• Approximately 20% of adults have levels below 25 nmol/L year-round.
• During winter and spring (January to March), this figure rises to around 30 to 40%.
• Deficiency is more common in Scotland and northern England due to lower sunlight exposure.
• Certain population groups are at significantly higher risk (see below).

Who Is at Risk?

While anyone in the UK can become vitamin D deficient, certain groups are at particular risk:

• People with darker skin — Melanin reduces the skin's ability to produce vitamin D from sunlight. People of African, Caribbean, and South Asian heritage are at significantly higher risk.
• People who cover their skin — For religious, cultural, or personal reasons.
• Housebound individuals and care home residents — Limited sun exposure.
• Older adults (65+) — The skin becomes less efficient at producing vitamin D with age.
• Pregnant and breastfeeding women — Higher requirements and often already insufficient.
• People with obesity (BMI above 30) — Vitamin D is sequestered in fat tissue, reducing circulating levels.
• People with malabsorption conditions — Including coeliac disease, Crohn's disease, and cystic fibrosis.

PHE and NICE Supplementation Guidance

Public Health England (now the UK Health Security Agency) recommends:

• All adults and children over 4 — Should consider taking a daily supplement of 10 micrograms (400 IU) of vitamin D during autumn and winter (October to March).
• At-risk groups — Should take a 10 microgram supplement throughout the year.
• Babies from birth to 1 year — 8.5 to 10 micrograms daily (unless having more than 500ml of infant formula per day, which is already fortified).
• Children aged 1 to 4 — 10 micrograms daily year-round.

If a blood test shows deficiency (below 25 nmol/L), NICE Clinical Knowledge Summaries recommend a higher loading dose, typically 1,000 to 4,000 IU daily for 6 to 12 weeks, followed by a maintenance dose. The exact protocol depends on the severity of deficiency and your GP's assessment.

Dietary Sources of Vitamin D

It is difficult to get enough vitamin D from diet alone, but the following foods contribute:

• Oily fish (salmon, mackerel, sardines, herring) — the richest natural source.
• Egg yolks.
• Red meat and liver.
• Fortified foods (some cereals, margarine, milk alternatives).
• Mushrooms exposed to UV light.

When Should You Get Tested?

The NHS does not routinely test vitamin D in everyone, but your GP may request it if you have symptoms of deficiency (fatigue, bone pain, muscle weakness, frequent infections) or if you are in a high-risk group. Many people choose to have a private vitamin D test for peace of mind, particularly at the end of winter when levels are lowest.

The test does not require fasting and can be taken at any time of day.

Getting Tested at Home

Vitamin D is one of the most commonly requested tests through home phlebotomy services. A qualified phlebotomist from Lola Dispatch can visit your home, take a venous blood sample (which is more accurate than finger-prick tests), and send it to an accredited laboratory.

Whether you want a standalone vitamin D check or a comprehensive wellness panel, a home blood test saves time and gives you reliable, laboratory-grade results. Book online and a phlebotomist will be in touch to arrange a convenient appointment.