Vitamin D plays a vital role in maintaining strong, healthy bones throughout life by regulating calcium absorption and bone mineralisation. Whilst sunlight exposure enables the body to produce vitamin D naturally, many people in the UK require supplementation, particularly during autumn and winter months when UVB radiation is insufficient. Certain groups—including older adults, individuals with darker skin, and those with limited sun exposure—face heightened deficiency risk and may need year-round supplementation. This article examines how vitamin D supports skeletal health, identifies who needs supplements, and provides evidence-based guidance on appropriate dosing and product selection to optimise bone strength and reduce fracture risk.

How Vitamin D Supports Bone Health

Vitamin D plays a fundamental role in maintaining skeletal integrity throughout life. This fat-soluble vitamin functions as a hormone precursor, with its active form—calcitriol (1,25-dihydroxyvitamin D)—regulating calcium and phosphate homeostasis, two minerals essential for bone mineralisation. Without adequate vitamin D, the body cannot effectively absorb calcium from the diet, regardless of calcium intake levels.

The mechanism involves vitamin D binding to receptors in the small intestine, where it enhances the absorption of dietary calcium and phosphorus. In bone tissue itself, vitamin D works synergistically with parathyroid hormone (PTH) to maintain serum calcium concentrations within the narrow physiological range required for optimal bone formation. When vitamin D levels are insufficient, PTH secretion increases, leading to secondary hyperparathyroidism. This compensatory mechanism mobilises calcium from bone, potentially weakening the skeletal structure over time.

In children, severe vitamin D deficiency causes rickets—a condition characterised by soft, deformed bones due to inadequate mineralisation of the growing skeleton. In adults, deficiency leads to osteomalacia, presenting with bone pain, muscle weakness, and increased fracture risk. Low vitamin D status may contribute to bone loss and osteoporosis risk, particularly in older adults. Adequate vitamin D status is therefore important for bone health across the lifespan.

Research shows that maintaining sufficient vitamin D levels, particularly when combined with adequate calcium intake, may help reduce the risk of falls and fractures in high-risk populations such as older, institutionalised adults. The relationship between vitamin D and bone health represents one of the most well-established nutrient-tissue associations in clinical medicine.

If you experience persistent bone pain, proximal muscle weakness, or recurrent falls, you should consult your GP, as these could be signs of vitamin D deficiency or other bone health concerns.

Who Needs Vitamin D Supplements for Bones

UK health authorities recommend that certain population groups require vitamin D supplementation to protect bone health, as dietary sources and sunlight exposure alone may prove insufficient. All adults and children over one year should consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D, particularly during autumn and winter months when UVB radiation in the UK is inadequate for cutaneous vitamin D synthesis.

Specific groups face elevated risk of deficiency and require year-round supplementation:

• Children aged 1-4 years: Should take 10 micrograms (400 IU) of vitamin D daily throughout the year.

• Older adults (aged 65+): Age-related decline in skin synthesis capacity, reduced outdoor activity, and decreased renal conversion of vitamin D to its active form increase deficiency risk. This population experiences the highest rates of osteoporotic fractures.

• Individuals with limited sun exposure: Those who are housebound, institutionalised, or who cover their skin for cultural or religious reasons cannot rely on sunlight for vitamin D production.

• People with darker skin tones: Higher melanin concentrations reduce UVB penetration, requiring longer sun exposure times that are impractical in the UK climate. British residents of African, African-Caribbean, and South Asian heritage show particularly high deficiency rates.

• Individuals with malabsorption conditions: Coeliac disease, Crohn's disease, ulcerative colitis, and chronic pancreatitis impair fat-soluble vitamin absorption, including vitamin D.

• People with obesity (BMI >30): Vitamin D can be sequestered in adipose tissue, reducing bioavailability.

• Those taking certain medications: Anticonvulsants (phenytoin, carbamazepine), glucocorticoids, some antiretroviral drugs, orlistat, and cholestyramine can affect vitamin D metabolism or absorption, increasing requirements.

Pregnant and breastfeeding women require supplementation both for their own bone health and to ensure adequate foetal skeletal development and infant vitamin D stores. Women should take 10 micrograms daily throughout pregnancy and breastfeeding. Some may be eligible for free Healthy Start vitamins.

Routine testing for vitamin D deficiency is not recommended for asymptomatic individuals. However, testing may be appropriate for those with symptoms or multiple risk factors where results would influence treatment decisions. Healthcare professionals should proactively identify at-risk individuals and recommend appropriate supplementation strategies tailored to individual circumstances.

Recommended Vitamin D Doses for Bone Strength

The recommended vitamin D dosage for bone health varies according to age, baseline vitamin D status, and individual risk factors. For general population bone health maintenance, UK health authorities recommend 10 micrograms (400 IU) daily for all individuals aged one year and above. This dose aims to ensure that the majority of the population maintains serum 25-hydroxyvitamin D [25(OH)D] concentrations above 25 nmol/L year-round, the threshold below which bone health becomes compromised.

For individuals with confirmed vitamin D deficiency (serum 25(OH)D below 25 nmol/L), higher therapeutic doses are required. NICE guidance recommends treatment regimens typically involving:

• Loading doses: A total of approximately 300,000 IU over 6-10 weeks. This can be achieved through regimens such as 50,000 IU weekly for 6 weeks, 20,000 IU twice weekly for 7 weeks, or 4,000 IU daily for 10 weeks to rapidly replenish depleted stores.

• Maintenance therapy: Following repletion, maintenance doses of 800–2,000 IU (20–50 micrograms) daily are often recommended, particularly for those with ongoing risk factors.

For osteoporosis prevention and treatment, clinical trials supporting fracture reduction have typically used 800–1,000 IU (20–25 micrograms) daily, combined with calcium supplementation where dietary intake is insufficient. The UK recommendation for calcium intake is 700-1,200 mg daily, primarily from dietary sources. This combination therapy often forms part of standard osteoporosis management alongside bone-protective medications.

Higher-risk groups—including care home residents, those with malabsorption, or individuals on medications affecting vitamin D metabolism—may require doses of 800–1,000 IU daily as standard maintenance, with periodic monitoring of serum 25(OH)D levels to ensure adequacy.

It is important to note that vitamin D is fat-soluble and can accumulate in tissues. The UK upper limit for vitamin D supplementation is 100 micrograms (4,000 IU) per day unless under medical supervision. Excessive intake can cause hypercalcaemia, leading to nausea, weakness, kidney stones, and cardiac arrhythmias. Very high single bolus doses should be avoided as they may be associated with adverse outcomes. After loading treatment, checking adjusted serum calcium is advisable, with selective rechecking of vitamin D levels at 3-6 months in some cases. Supplementation should follow evidence-based guidelines, and individuals with kidney disease or conditions affecting calcium metabolism should consult their GP before starting supplements.

Choosing the Right Vitamin D Supplement

Selecting an appropriate vitamin D supplement requires understanding the different formulations available and matching product characteristics to individual needs. Vitamin D supplements come in two main forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Research indicates that vitamin D3 is more effective at raising and maintaining serum 25(OH)D concentrations, making it the preferred choice for bone health supplementation.

When selecting a supplement, consider the following factors:

Formulation and strength: Products are available as tablets, capsules, oral sprays, and liquid drops. Liquid formulations may benefit individuals with swallowing difficulties or malabsorption conditions, as vitamin D is fat-soluble and liquid preparations often contain oil-based carriers that enhance absorption. Check that the stated dose matches your requirements—products range from 400 IU (10 micrograms) to 4,000 IU (100 micrograms) per dose.

Quality assurance: In the UK, vitamin D supplements are regulated as food supplements rather than medicines (unless specifically licensed). Choose reputable brands from established manufacturers to ensure product quality and accurate labelling. For treatment of diagnosed deficiency, licensed vitamin D medicines (with a PL number) may be prescribed by clinicians.

Combination products: Some supplements combine vitamin D with calcium, magnesium, or vitamin K2. Calcium co-supplementation benefits those with inadequate dietary calcium intake, but assess your dietary calcium intake first and aim for a total intake (diet plus supplements) of 700-1,200 mg daily. While vitamin K2 is sometimes marketed alongside vitamin D, UK guidance does not routinely recommend this combination for bone health.

Dietary considerations: Vegetarians and vegans should note that most vitamin D3 is derived from lanolin (sheep's wool), though lichen-derived vegan D3 supplements are increasingly available. Vitamin D2 is plant-derived but less effective.

Timing and absorption: Take vitamin D supplements with a meal containing some fat to optimise absorption. There is no significant advantage to taking vitamin D at a particular time of day—consistency matters more than timing.

If you are taking other medications, consult your GP or pharmacist before starting supplementation to avoid potential interactions. Calcium supplements (not vitamin D itself) should be taken separately from bisphosphonates. Adequate vitamin D and calcium are important before starting denosumab to reduce hypocalcaemia risk. Orlistat and cholestyramine can reduce vitamin D absorption. Individuals with kidney disease, sarcoidosis, or hyperparathyroidism require specialist advice before supplementation.

If you experience any suspected side effects from vitamin D or calcium supplements, report them to the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or via the Yellow Card app).

Frequently Asked Questions