Understanding normal vitamin D levels by age is essential for maintaining bone health, immune function, and overall wellbeing throughout life. In the UK, vitamin D status is measured using serum 25-hydroxyvitamin D [25(OH)D], with sufficiency defined as levels above 50 nmol/L. Whilst this threshold applies across all age groups, requirements vary significantly from infancy through to older adulthood. Factors including sunlight exposure, skin pigmentation, and underlying health conditions influence individual vitamin D status. This article explains the reference ranges for different age groups, identifies risk factors for deficiency, and provides guidance on when testing and supplementation are appropriate.

What Are Normal Vitamin D Levels?

Vitamin D is a fat-soluble hormone essential for calcium absorption, bone health, immune function, and numerous physiological processes. In the UK, vitamin D status is assessed by measuring serum 25-hydroxyvitamin D [25(OH)D], the most stable and reliable biomarker of vitamin D stores in the body.

The UK classification system defines vitamin D status as follows:

• Deficiency: serum 25(OH)D below 25 nmol/L (10 ng/mL)

• Insufficiency: serum 25(OH)D between 25–50 nmol/L (10–20 ng/mL)

• Sufficiency: serum 25(OH)D above 50 nmol/L (20 ng/mL)

The Scientific Advisory Committee on Nutrition (SACN) recommends that everyone in the UK should maintain serum 25(OH)D concentrations of at least 25 nmol/L year-round to protect musculoskeletal health. This underpins the UK Reference Nutrient Intake (RNI) of 10 micrograms (400 IU) of vitamin D daily for the general population. UK clinical practice generally considers levels above 50 nmol/L as adequate for most people.

It is important to note that whilst these reference ranges provide general guidance, individual requirements may vary based on age, health status, skin pigmentation, and geographical location. The interpretation of vitamin D levels should always be undertaken in clinical context, considering symptoms, risk factors, and concurrent medical conditions. Healthcare professionals may recommend different target levels for specific patient groups, such as those with osteoporosis, malabsorption disorders, or chronic kidney disease.

Vitamin D Reference Ranges by Age Group

Whilst the biochemical definition of vitamin D sufficiency remains consistent across age groups (serum 25(OH)D ≥50 nmol/L), different life stages have varying requirements and vulnerabilities to deficiency.

Infants and young children (0–4 years): Newborns and young children are particularly vulnerable to vitamin D deficiency due to rapid skeletal growth and limited dietary sources. The Department of Health and Social Care recommends that all breastfed infants from birth to one year receive a daily supplement of 8.5–10 micrograms (340–400 IU) of vitamin D. Formula-fed infants who consume more than 500 mL of infant formula daily do not need additional vitamin D as formula is already fortified. Children aged 1–4 years should receive 10 micrograms (400 IU) daily.

Children and adolescents (5–18 years): During periods of rapid growth, particularly in adolescence, adequate vitamin D is crucial for bone mineralisation and peak bone mass development. The recommended daily intake is 10 micrograms (400 IU), particularly during autumn and winter months. Children with darker skin, limited outdoor time, or who wear concealing clothing should take supplements year-round.

Adults (19–64 years): Healthy adults should maintain serum 25(OH)D levels above 50 nmol/L. The recommended daily intake is 10 micrograms (400 IU), particularly during autumn and winter months (October to March in the UK) when UVB radiation is insufficient for cutaneous vitamin D synthesis. Adults with darker skin, limited sun exposure, or who cover their skin should consider year-round supplementation.

Older adults (65+ years): Older adults are at heightened risk of deficiency due to reduced skin synthesis capacity, decreased outdoor activity, and potential malabsorption. NICE guidance recommends that all adults over 65 should take a daily supplement of 10 micrograms (400 IU) year-round. Some frail or housebound elderly individuals may require treatment for confirmed deficiency under medical supervision.

The UK safe upper limit for vitamin D is 100 micrograms (4000 IU) daily for adults, including pregnant and breastfeeding women, and children aged 11-17 years.

Factors That Affect Vitamin D Levels

Multiple physiological, environmental, and lifestyle factors influence an individual's vitamin D status, making deficiency common in certain populations despite adequate sunlight exposure.

Sunlight exposure and geographical location: Most vitamin D is synthesised in the skin following exposure to UVB radiation (wavelength 290–315 nm). In the UK, UVB intensity is insufficient for adequate vitamin D synthesis between October and March. Latitude, season, time of day, weather conditions, and air pollution all affect UVB availability. Individuals who spend limited time outdoors, work indoors, or consistently use high-factor sunscreen may have reduced synthesis.

Skin pigmentation: Melanin absorbs UVB radiation, reducing vitamin D synthesis in the skin. People with darker skin (Fitzpatrick skin types IV–VI) require longer sun exposure to produce equivalent amounts of vitamin D compared to those with lighter skin. This places individuals of African, African-Caribbean, and South Asian heritage at substantially higher risk of deficiency in the UK.

Age-related changes: The capacity for cutaneous vitamin D synthesis declines with age. Older adults produce less vitamin D than younger individuals following equivalent sun exposure. Additionally, age-related reductions in kidney function may impair conversion of 25(OH)D to its active form, 1,25-dihydroxyvitamin D.

Body composition and medical conditions: Obesity is associated with lower serum 25(OH)D levels, possibly due to sequestration of vitamin D in adipose tissue. Malabsorption disorders (coeliac disease, Crohn's disease, cystic fibrosis), chronic kidney disease, liver disease, and certain medications can significantly impair vitamin D metabolism and increase deficiency risk. Medications affecting vitamin D include anticonvulsants, glucocorticoids, antiretrovirals, rifampicin, isoniazid, cholestyramine, and orlistat.

Additional at-risk groups: People who are housebound or living in care homes, those who cover most of their skin when outdoors, and pregnant and breastfeeding women are also at increased risk of deficiency.

Dietary intake: Few foods naturally contain vitamin D. Oily fish (salmon, mackerel, sardines), egg yolks, and fortified foods (breakfast cereals, spreads) provide modest amounts, but dietary sources alone are generally insufficient to maintain adequate levels without supplementation or sun exposure.

Symptoms of Vitamin D Deficiency Across Ages

Vitamin D deficiency presents differently across the lifespan, ranging from subtle non-specific symptoms to severe skeletal complications.

Infants and children: Severe deficiency in infancy and childhood causes rickets, characterised by impaired bone mineralisation and skeletal deformities. Clinical features include:

• Delayed motor milestones and muscle weakness

• Bowed legs (genu varum) or knock knees (genu valgum)

• Widening of wrists and ankles

• Frontal bossing and delayed fontanelle closure

• Rachitic rosary (enlarged costochondral junctions)

• Increased susceptibility to respiratory infections

Subclinical deficiency may present with non-specific symptoms such as irritability, poor growth, and recurrent infections. Hypocalcaemic seizures can occur in severe cases and require emergency medical attention.

Adults: In adults, chronic vitamin D deficiency leads to osteomalacia (softening of bones) with symptoms including:

• Persistent bone pain, particularly in the ribs, pelvis, and lower back

• Muscle weakness, especially proximal muscles (difficulty rising from chairs, climbing stairs)

• Fatigue and generalised malaise

• Increased fracture risk, even with minimal trauma

Many adults with insufficiency (25–50 nmol/L) remain asymptomatic but may experience subtle muscle weakness. Severe deficiency can occasionally cause hypocalcaemia with symptoms such as muscle cramps, tetany, and confusion requiring urgent medical care.

Older adults: In the elderly, vitamin D deficiency contributes to:

• Increased falls risk due to muscle weakness and impaired balance

• Accelerated bone loss and osteoporosis

• Hip and vertebral fractures

• Slower recovery from illness

Non-specific symptoms across all ages: Some research suggests associations between low vitamin D and mood disturbances, cognitive impairment, and increased susceptibility to infections. However, causality remains uncertain, and these associations require further investigation. Many of these symptoms are common and may have multiple causes. Patients experiencing persistent fatigue, muscle aches, or mood changes should discuss these symptoms with their GP rather than self-diagnosing vitamin D deficiency.

When to Get Your Vitamin D Levels Tested

Routine screening of vitamin D levels in the general population is not recommended by NICE or the NHS. Testing should be reserved for individuals with clinical suspicion of deficiency or specific risk factors.

Testing is recommended in the following circumstances:

• Symptomatic individuals: those presenting with bone pain, muscle weakness, unexplained fractures, or symptoms suggestive of osteomalacia or rickets

• Biochemical abnormalities: unexplained hypocalcaemia, hypophosphataemia, or raised alkaline phosphatase

• Bone disease: confirmed or suspected osteoporosis, osteomalacia, or rickets

• Increased fracture risk: recurrent fragility fractures or falls in older adults

• Malabsorption disorders: coeliac disease, inflammatory bowel disease, chronic pancreatitis, or following bariatric surgery

• Chronic kidney disease: stages 4–5, or those on dialysis

• Medications affecting vitamin D metabolism: long-term anticonvulsants, glucocorticoids, antiretrovirals, or other medications affecting vitamin D

• High-risk ethnic groups: individuals with darker skin living in the UK, particularly if symptomatic

When to contact your GP: You should arrange an appointment with your GP if you experience:

• Persistent bone or muscle pain that does not improve

• Unexplained muscle weakness affecting daily activities

• Recurrent falls or balance problems

• Symptoms of rickets in children (skeletal deformities, delayed development)

• Known risk factors for deficiency combined with concerning symptoms

Seek urgent medical attention if you or your child experiences seizures, severe muscle cramps, carpopedal spasm (hand spasms), tetany, confusion, or breathing difficulties, as these may indicate severe hypocalcaemia requiring emergency treatment.

Testing methodology: Vitamin D status is assessed via a blood test measuring serum 25(OH)D (not 1,25-dihydroxyvitamin D, which is not appropriate for deficiency screening). Results should be interpreted alongside clinical presentation, and management decisions should be made in consultation with a healthcare professional.

Important cautions: People with certain conditions including sarcoidosis, tuberculosis, some lymphomas, and primary hyperparathyroidism should only take vitamin D under specialist supervision, as it may cause hypercalcaemia.

For most healthy individuals without risk factors, routine supplementation with 10 micrograms (400 IU) daily during autumn and winter is more cost-effective than testing. Those at higher risk should consider year-round supplementation and discuss testing with their GP if symptoms develop.

Frequently Asked Questions