Vitamin D is often described as a vitamin, yet in biological terms it behaves more like a hormone. It plays a central role in calcium absorption, bone metabolism, muscle function and immune regulation. In the United Kingdom, maintaining adequate levels is a recognised public health issue, particularly for women, older adults and people with darker skin.
According to the NHS, vitamin D deficiency and insufficiency are common, especially during autumn and winter when ultraviolet B sunlight is too weak at UK latitude to stimulate meaningful skin production.
How the Body Makes and Processes Vitamin D
Vitamin D is synthesised in the skin from 7 dehydrocholesterol when exposed to ultraviolet B light. It can also be obtained from diet and supplements.
There are two principal forms:
Vitamin D2 Ergocalciferol
Derived from plant sources and fortified foods. It is effective but generally less potent than vitamin D3 in raising and maintaining blood levels.
Vitamin D3 Cholecalciferol
Produced in human skin and found in oily fish, liver and eggs. Evidence suggests it is more effective for long term maintenance of adequate vitamin D status.
Both forms are converted in the liver to 25 hydroxyvitamin D, known as 25 OH D, which is the main circulating form measured in blood tests. This is then converted in the kidneys to the active hormone 1,25 dihydroxyvitamin D, which exerts biological effects.
Insufficiency Versus Deficiency
Clinically, vitamin D status is defined by serum 25 OH D levels.
- Insufficiency is often defined as 25 to 50 nanomoles per litre. Levels are below optimal but not critically low.
- Deficiency is typically defined as below 25 nanomoles per litre and is associated with increased risk of rickets in children and osteomalacia in adults.
Guidance from National Institute for Health and Care Excellence and the NHS reflects these thresholds in routine clinical practice.
Why It Occurs
Several factors contribute:
- Limited ultraviolet B exposure in the UK
- Increased skin pigmentation reducing vitamin D synthesis
- Age related decline in cutaneous production
- Obesity, due to sequestration in adipose tissue
- Malabsorption conditions such as coeliac disease or Crohn’s disease
- Inadequate dietary intake without supplementation
Women, particularly postmenopausal women, are at increased risk because of bone density changes associated with hormonal shifts.
Bone Health and Osteoporosis
The strongest evidence for vitamin D relates to bone metabolism.
Vitamin D supports calcium and phosphate homeostasis, enabling proper bone mineralisation. Without sufficient vitamin D, calcium absorption falls, leading to secondary hyperparathyroidism, bone demineralisation and increased fracture risk.
Clinical guidance from NICE and the NHS supports supplementation in at risk groups to reduce the likelihood of osteomalacia and to support osteoporosis management. In older women, adequate vitamin D combined with calcium has been shown to reduce fall and fracture risk in defined populations.
Muscle Function and Falls
Vitamin D receptors are present in muscle tissue. Low levels are associated with muscle weakness and increased fall risk, particularly in older adults. This has important implications for women over the age of 65, where fracture risk is already elevated.
Immune Function
Vitamin D influences both innate and adaptive immunity. It enhances macrophage and dendritic cell function and modulates inflammatory responses.
Observational studies link low vitamin D levels with increased susceptibility to infection and certain autoimmune conditions. However, while associations are strong, intervention trials show mixed results. Regulator approved guidance therefore supports correction of deficiency but does not endorse high dose supplementation for broad immune enhancement in the absence of deficiency.
Skin Health
Vitamin D regulates keratinocyte proliferation and differentiation, contributing to skin barrier integrity and wound repair. Topical vitamin D analogues are used in dermatology for inflammatory conditions such as psoriasis, reflecting established biological mechanisms. However, oral supplementation beyond correcting deficiency is not routinely recommended for cosmetic skin outcomes.
Renal Impairment and Alfacalcidol
In chronic kidney disease, the kidney’s ability to convert 25 OH D into active 1,25 dihydroxyvitamin D is impaired.
In these circumstances, active analogues such as alfacalcidol may be prescribed. Alfacalcidol is converted in the liver to calcitriol, bypassing the need for renal activation.
According to the British National Formulary and NICE guidance, indications include secondary hyperparathyroidism and renal bone disease. Dosing requires careful titration and regular monitoring of calcium and phosphate to prevent hypercalcaemia.
This is a specialist treatment and should only be used under medical supervision.
Dietary Sources
Few foods naturally contain significant vitamin D. Useful sources include:
- Oily fish such as salmon, mackerel and sardines
- Egg yolks
- Liver in moderation
- Fortified cereals, plant based milks and spreads
Given dietary limitations, supplementation is often necessary.
Supplementation in the United Kingdom
The NHS recommends:
- 10 micrograms or 400 international units daily for adults, particularly during autumn and winter
- Year round supplementation for individuals at higher risk, including those with limited sun exposure or darker skin
Higher doses to correct deficiency should be prescribed and monitored by a clinician using serum 25 OH D measurements.
Excessive intake can cause hypercalcaemia and associated complications, so more is not necessarily better.
The Evidence Based Conclusion
Vitamin D is not a wellness trend. It is a biologically essential hormone precursor with regulator recognised roles in bone integrity, muscle strength and metabolic stability.
The evidence robustly supports:
- Prevention and treatment of deficiency
- Maintenance of bone health
- Reduction of fall risk in defined older populations
- Essential management in renal related bone disease
The evidence does not support:
- High dose supplementation in people with normal levels
- Broad claims of immune enhancement without documented deficiency
For women in the United Kingdom, particularly postmenopausal women, maintaining adequate vitamin D through sensible sunlight exposure, diet and appropriate supplementation is a clinically sound and regulator aligned approach to long term health.
- National Institute for Health and Care Excellence
Vitamin D deficiency in adults Treatment and prevention. NICE Clinical Knowledge Summary.
Provides UK thresholds for deficiency and insufficiency, investigation guidance and treatment dosing strategies. - NHS
Vitamin D.
National guidance on recommended intake of 10 micrograms daily, at risk populations and safe supplementation advice. - British Dietetic Association
Food Fact Sheet Vitamin D.
Evidence based overview of dietary sources, forms D2 and D3 and population risk groups. - British National Formulary
Alfacalcidol Monograph.
Dosing, monitoring requirements and interaction profile in renal impairment and secondary hyperparathyroidism. - The Lancet Diabetes and Endocrinology
Bolland MJ et al. Vitamin D supplementation and musculoskeletal health. 2018.
Large meta analysis evaluating fracture prevention and musculoskeletal outcomes. - BMJ
Reid IR et al. Effects of vitamin D supplements on bone mineral density. 2014.
Systematic review examining impact of supplementation on bone density in adults. - American Journal of Clinical Nutrition
Tripkovic L et al. Comparison of vitamin D2 and D3 supplementation in raising serum 25 hydroxyvitamin D. 2012.
Demonstrates greater potency of vitamin D3 in maintaining serum levels. - Nature Reviews Endocrinology
Bouillon R et al. Vitamin D and human health. 2019.
Comprehensive review of mechanisms including immune modulation and extra skeletal effects.
