Many people wonder whether sitting by a sunny window can boost their vitamin D levels, particularly those who work indoors or have limited outdoor mobility. Unfortunately, the answer is no – standard window glass blocks the ultraviolet B (UVB) radiation essential for vitamin D synthesis in the skin. Whilst sunlight through windows feels warm and appears bright, it cannot trigger the photochemical reaction needed to produce this vital nutrient. This limitation has important implications for maintaining adequate vitamin D status in the UK, especially during winter months when UVB availability is already severely restricted by our northern latitude.

Can You Get Vitamin D Through a Window?

The short answer is no – you cannot produce vitamin D through standard window glass. Whilst sunlight streaming through your window may feel warm and bright, the glass blocks the specific ultraviolet B (UVB) radiation required for vitamin D synthesis in the skin. This is a common misconception, particularly relevant for office workers, those who spend considerable time indoors, or individuals with limited outdoor mobility.

Vitamin D is often called the 'sunshine vitamin' because our bodies produce it when skin is directly exposed to UVB rays from sunlight. This process occurs in the epidermis, where a cholesterol derivative (7-dehydrocholesterol) is converted to previtamin D3, which then transforms into vitamin D3 (cholecalciferol). However, this photochemical reaction requires UVB radiation with wavelengths between 290–315 nanometres.

Standard window glass, whether in homes, offices, or vehicles, effectively filters out the majority of UVB radiation whilst allowing visible light and UVA rays to pass through. This protective property of glass prevents sunburn indoors but simultaneously prevents vitamin D production. Even sitting beside a sunny window for extended periods will not contribute meaningfully to your vitamin D status, despite the warmth and brightness you experience.

This limitation has important implications for vulnerable populations in the UK, including housebound elderly individuals, care home residents, and those with mobility restrictions who may spend most of their time indoors. Understanding this fundamental barrier to vitamin D production is essential for implementing appropriate supplementation strategies and preventing deficiency.

How UVB Rays and Glass Affect Vitamin D Production

The interaction between ultraviolet radiation and glass is governed by the physical properties of both. UVB radiation (290–315 nm wavelength) is the specific component of sunlight responsible for vitamin D synthesis, but it is also the primary cause of sunburn and contributes to skin cancer risk. Standard window glass, composed primarily of silica (silicon dioxide), has optical properties that absorb and reflect UVB wavelengths almost entirely, typically blocking the vast majority of UVB rays.

In contrast, UVA radiation (315–400 nm) penetrates glass much more readily, with a substantial proportion passing through standard windows. Whilst UVA contributes to skin ageing and some skin damage, it does not trigger vitamin D production. This explains why you can still experience sun-related skin changes (such as tanning or photosensitivity reactions to certain medications) through windows, but cannot synthesise vitamin D.

The glass composition matters: laminated windscreens in vehicles often block even more UV radiation than standard window glass, whilst some specialised glass types (such as quartz glass used in laboratories) may allow limited UVB transmission. However, architectural and automotive glass used in the UK is specifically designed to filter UV radiation for occupant protection.

Environmental factors further complicate indoor vitamin D production. Even if minimal UVB were to penetrate glass, the angle of incidence, time of day, season, and latitude all affect UVB availability. In the UK (latitudes 50–60°N), UVB intensity is insufficient for vitamin D synthesis from approximately October through March, regardless of outdoor exposure. This 'vitamin D winter' means that even direct sunlight exposure during these months produces negligible vitamin D, making the window barrier even more significant during the limited months when UVB is available.

Alternative Ways to Maintain Vitamin D Levels in the UK

Given the limitations of obtaining vitamin D through windows and the UK's northern latitude, maintaining adequate vitamin D status requires a multifaceted approach combining dietary sources, safe sun exposure, and supplementation.

Dietary sources of vitamin D are limited but important. Naturally rich foods include:

• Oily fish (salmon, mackerel, sardines, herring) – providing approximately 400–1,000 IU per 100g serving

• Egg yolks – approximately 50 IU per egg

• Red meat and liver – modest amounts (20–50 IU per serving)

• Fortified foods – including breakfast cereals, fat spreads, and some plant-based milk alternatives

However, it is challenging to obtain sufficient vitamin D from diet alone, as achieving the recommended 400 IU (10 micrograms) daily would require consuming substantial quantities of these foods regularly.

Safe sun exposure during UK summer months (late March to September) can contribute to vitamin D production. The NHS recommends short periods of direct sun exposure on forearms, hands, or lower legs without sunscreen several times weekly during midday hours when UVB is strongest. The exact time needed varies widely based on skin tone, with darker skin requiring longer exposure than fair skin. This should be balanced against skin cancer risk; you should take care not to burn, and prolonged or excessive UV exposure should always be avoided.

Supplementation represents the most reliable method for maintaining vitamin D status in the UK, particularly during autumn and winter. Vitamin D3 (cholecalciferol) supplements are widely available over-the-counter in various formulations including tablets, capsules, oral sprays, and liquid drops. The standard recommended dose for adults is 400 IU (10 micrograms) daily, though higher doses may be prescribed for deficiency treatment under medical supervision. Vitamin D is fat-soluble, so taking supplements with meals containing some fat may enhance absorption.

It's important to note that sunbeds are not recommended as a way to boost vitamin D levels due to their associated skin cancer risks.

NHS Guidance on Vitamin D Supplementation

UK Government and NHS recommendations emphasise that everyone should consider taking a daily vitamin D supplement, particularly during autumn and winter months. This guidance, aligned with recommendations from the Scientific Advisory Committee on Nutrition (SACN), recognises that sunlight exposure in the UK is insufficient for adequate vitamin D synthesis during approximately half the year.

The standard recommendation for adults and children over one year is 10 micrograms (400 IU) of vitamin D daily during autumn and winter (October to early March). However, certain groups are advised to take this supplement year-round due to increased deficiency risk:

• Individuals with limited sun exposure – including those who are housebound, in care homes, or who cover their skin for cultural or religious reasons

• People with darker skin (African, African-Caribbean, or South Asian backgrounds) – melanin reduces UVB absorption, requiring longer sun exposure for equivalent vitamin D production

• Infants and young children – babies from birth to one year require 8.5–10 micrograms (340–400 IU) daily, unless they consume more than 500ml of infant formula per day (as formula is already fortified with vitamin D)

• Pregnant and breastfeeding women

UK clinical guidance on vitamin D supplementation recommends that healthcare professionals should identify at-risk groups and provide advice on supplementation. The guidance emphasises that whilst vitamin D deficiency is associated with various health conditions, supplementation should focus on maintaining bone health rather than preventing non-skeletal diseases, where evidence remains inconclusive.

Vitamin D supplements are available on prescription for certain medical conditions, but most people are advised to purchase them over-the-counter. They are generally inexpensive and widely available from pharmacies and supermarkets. Some people may be eligible for free vitamin D supplements through the Healthy Start scheme. The NHS advises against exceeding 100 micrograms (4,000 IU) daily for adults unless under medical supervision, with lower upper limits for children (25 micrograms/day for infants under 1 year; 50 micrograms/day for children 1-10 years). Excessive vitamin D can cause hypercalcaemia, leading to weakened bones and kidney or heart damage over time.

When to Seek Medical Advice About Vitamin D Deficiency

Vitamin D deficiency often develops insidiously, and many individuals remain asymptomatic until levels become severely depleted. However, certain symptoms and risk factors warrant medical evaluation and potential testing of serum 25-hydroxyvitamin D [25(OH)D] levels.

Contact your GP if you experience:

• Persistent bone or muscle pain – particularly in the lower back, pelvis, hips, or legs

• Muscle weakness – especially proximal muscle groups (thighs, upper arms), affecting mobility or causing difficulty rising from chairs

• Frequent fractures or bone tenderness suggesting osteomalacia (softening of bones)

• Fatigue and low mood – whilst there is no official link definitively established, some studies suggest associations between low vitamin D and mood disorders

• Recurrent infections – vitamin D plays a role in immune function, though evidence for supplementation preventing infections remains uncertain

Medical assessment is particularly important for individuals with:

• Malabsorption conditions – including coeliac disease, Crohn's disease, ulcerative colitis, or chronic pancreatitis, which impair vitamin D absorption

• Chronic kidney disease – affecting conversion of vitamin D to its active form (calcitriol)

• Liver disease – impairing vitamin D metabolism

• Medications affecting vitamin D metabolism – including some anticonvulsants, glucocorticoids, and antiretroviral drugs

• Obesity – vitamin D is sequestered in adipose tissue, reducing bioavailability

Diagnostic testing involves measuring serum 25(OH)D concentration. UK reference ranges typically define deficiency as <25 nmol/L, insufficiency as 25–50 nmol/L, and sufficiency as >50 nmol/L. However, testing is not routinely recommended for asymptomatic individuals; UK guidance suggests targeted testing only for those with symptoms or significant risk factors.

Treatment of confirmed deficiency usually involves higher-dose vitamin D supplementation (typically a total loading dose of approximately 300,000 IU over 6-10 weeks) followed by maintenance therapy. Your GP will determine the appropriate regimen based on deficiency severity, underlying causes, and individual factors. Baseline and follow-up monitoring of calcium and renal function is typically recommended when receiving high-dose therapy. Some conditions (such as granulomatous diseases or primary hyperparathyroidism) may require specialist management due to altered vitamin D metabolism.

If you experience any suspected side effects from vitamin D supplements, report them to the Medicines and Healthcare products Regulatory Agency (MHRA) through the Yellow Card Scheme.

Frequently Asked Questions