Vitamin D and B12 are essential micronutrients that support bone health, neurological function, and overall wellbeing. Whilst vitamin D can be synthesised through sunlight exposure and regulates calcium metabolism, vitamin B12 must be obtained from dietary sources and is crucial for DNA synthesis and red blood cell formation. Deficiencies in either vitamin are common in the UK, particularly during winter months and amongst certain population groups. Understanding the distinct roles, sources, and signs of deficiency can help you maintain optimal levels and prevent potentially serious health complications.

What Are Vitamin D and B12?

Vitamin D and vitamin B12 are two essential micronutrients that play distinct but equally vital roles in maintaining human health. Despite often being discussed together due to their prevalence in deficiency states, they have different biochemical functions, sources, and mechanisms of action within the body.

Vitamin D is a fat-soluble vitamin that exists primarily in two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The body can synthesise vitamin D3 when skin is exposed to ultraviolet B (UVB) radiation from sunlight. Once produced or ingested, vitamin D undergoes hydroxylation in the liver to form 25-hydroxyvitamin D [25(OH)D], the main circulating form used to assess vitamin D status, and then in the kidneys to produce the active hormone 1,25-dihydroxyvitamin D. This active form regulates calcium and phosphate metabolism, supporting bone mineralisation. While vitamin D receptors are present in numerous tissues throughout the body, suggesting potential roles in immune function and cellular growth, the clinical benefits beyond bone health remain under investigation.

Vitamin B12 (cobalamin) is a water-soluble vitamin essential for DNA synthesis, red blood cell formation, and neurological function. It acts as a cofactor for two critical enzymes: methionine synthase, which is involved in DNA methylation and homocysteine metabolism, and methylmalonyl-CoA mutase, which participates in fatty acid metabolism. Unlike vitamin D, humans cannot synthesise vitamin B12 and must obtain it entirely from dietary sources, primarily animal products. Absorption of B12 is complex, requiring intrinsic factor—a glycoprotein secreted by gastric parietal cells—to facilitate uptake in the terminal ileum. This intricate absorption mechanism contributes to B12 deficiency risk, particularly in certain population groups. The body can store B12 for several years, meaning deficiency often develops gradually.

Signs You May Be Deficient in Vitamin D or B12

Deficiencies in vitamin D and B12 can present with overlapping symptoms, though each has distinctive clinical features that aid diagnosis.

Vitamin D deficiency is widespread in the UK, particularly during autumn and winter months when sunlight exposure is limited. Common manifestations include:

• Musculoskeletal symptoms: Bone pain, muscle weakness, and increased susceptibility to fractures. In severe cases, adults may develop osteomalacia (soft bones), whilst children may develop rickets

• Fatigue and low mood: Persistent tiredness and depressive symptoms, though there is no official link definitively established between vitamin D levels and mood disorders

• Increased infection susceptibility: Some evidence suggests a link with recurrent respiratory infections, though clinical trial evidence is mixed and benefits appear modest

• Hair loss: Some observational studies have associated diffuse hair thinning with low vitamin D levels, though this relationship requires further research

Vitamin B12 deficiency typically develops gradually and can cause significant neurological and haematological complications:

• Anaemia symptoms: Fatigue, pallor, shortness of breath, and palpitations due to megaloblastic anaemia

• Neurological manifestations: Paraesthesia (pins and needles), particularly in hands and feet; peripheral neuropathy; balance problems; and cognitive changes including memory impairment. In severe cases, subacute combined degeneration of the spinal cord may occur

• Glossitis and mouth ulcers: A sore, red tongue and recurrent oral ulceration

• Mood changes: Depression, irritability, and in rare cases, psychosis

It is important to note that symptoms can be subtle initially, and some individuals remain asymptomatic despite biochemical deficiency. If you experience persistent fatigue, neurological symptoms, or unexplained anaemia, contact your GP for assessment. Seek urgent medical attention for new or worsening neurological symptoms, severe breathlessness, chest pain, or fainting, as these may indicate severe deficiency requiring prompt treatment. Early detection and treatment can prevent irreversible complications, particularly the neurological sequelae of B12 deficiency.

How to Get Enough Vitamin D and B12

Maintaining adequate levels of vitamin D and B12 requires different strategies due to their distinct sources and absorption mechanisms.

Vitamin D sources and recommendations:

In the UK, sunlight exposure between April and September can stimulate adequate vitamin D synthesis in most people. However, factors such as skin pigmentation, age, use of sunscreen, and limited outdoor activity reduce production. The NHS recommends that everyone consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter. Certain groups should take supplements year-round, including:

• People with dark skin (African, African-Caribbean, or South Asian backgrounds)

• Those who are housebound or have limited sun exposure

• Residents of care homes

• People who cover their skin for cultural or religious reasons

• Pregnant and breastfeeding women

• Children aged 1-4 years (10 micrograms daily)

• Exclusively breastfed infants (8.5-10 micrograms daily, unless consuming more than 500ml of infant formula daily)

Dietary sources of vitamin D include oily fish (salmon, mackerel, sardines), egg yolks, fortified foods (breakfast cereals, spreads), and red meat, though it is difficult to obtain sufficient amounts from diet alone.

Vitamin B12 sources and recommendations:

Vitamin B12 is naturally present in animal products, making adequate intake straightforward for most omnivores. Rich sources include:

• Meat (particularly liver and kidney)

• Fish and shellfish

• Dairy products (milk, cheese, yoghurt)

• Eggs

• Fortified foods (some plant-based milk alternatives, fortified nutritional yeast, some breakfast cereals)

Vegans and vegetarians are at higher risk of deficiency and should consume B12-fortified foods regularly or take a B12 supplement. The recommended daily intake is 1.5 micrograms for adults. People taking certain medications may need monitoring for B12 deficiency. The MHRA advises that patients on long-term metformin should be monitored for B12 deficiency, particularly if they develop anaemia or peripheral neuropathy. Long-term use of proton pump inhibitors or H2-receptor antagonists can also impair B12 absorption. Discuss with your GP if you have risk factors for deficiency.

Testing and Treatment for Vitamin D and B12 Deficiency

Testing for deficiency:

Both vitamin D and B12 deficiencies are diagnosed through blood tests, though testing strategies differ.

Vitamin D testing measures serum 25-hydroxyvitamin D [25(OH)D] concentration. NICE guidance suggests that routine population screening is not recommended; testing should be targeted at individuals with:

• Symptoms suggestive of deficiency

• Conditions affecting vitamin D metabolism (chronic kidney disease, malabsorption disorders)

• Increased fracture risk or metabolic bone disease

Deficiency is generally defined as 25(OH)D levels below 25 nmol/L, with insufficiency between 25–50 nmol/L. Optimal levels remain debated, though most guidelines suggest maintaining levels above 50 nmol/L.

Vitamin B12 testing typically begins with serum B12 measurement. Reference ranges vary between laboratories (many UK labs report in pmol/L rather than ng/L), and borderline results may require additional testing. In uncertain cases, additional tests may include:

• Methylmalonic acid (MMA) and homocysteine levels (elevated in B12 deficiency)

• Full blood count (may show macrocytic anaemia)

• Folate levels (deficiency can present similarly to B12 deficiency)

• Intrinsic factor and parietal cell antibodies (to diagnose pernicious anaemia)

• Coeliac disease screening if clinically indicated

Treatment approaches:

For vitamin D deficiency, treatment depends on severity. UK guidance typically recommends:

• Loading doses of colecalciferol (e.g., total of approximately 300,000 IU over 6-10 weeks) for confirmed deficiency

• Maintenance therapy with 800–2,000 IU daily thereafter

• Higher doses may be required for malabsorption conditions

Before starting high-dose vitamin D, check corrected calcium and renal function. Use with caution in conditions such as sarcoidosis, hyperparathyroidism, or history of kidney stones. Recheck vitamin D levels approximately 3 months after loading treatment.

For vitamin B12 deficiency, treatment varies according to cause and severity:

• Dietary deficiency: Oral cyanocobalamin supplementation (50–150 micrograms daily between meals) is usually effective

• Pernicious anaemia or malabsorption: Intramuscular hydroxocobalamin injections are required, typically 1 mg three times weekly for two weeks, then maintenance injections every two to three months lifelong

• Neurological involvement: More intensive initial treatment with 1 mg hydroxocobalamin on alternate days until no further improvement

Follow-up should include symptom review and full blood count monitoring after 1-2 months of treatment.

When to seek medical attention:

Contact your GP if you experience persistent symptoms suggestive of deficiency. Seek urgent medical attention for new or progressive neurological symptoms, severe breathlessness, chest pain, or fainting, as these may indicate severe deficiency requiring prompt treatment. Regular monitoring is essential for those on long-term treatment to ensure adequate replacement and prevent complications.

Frequently Asked Questions