Vitamin D deficiency is very common in the UK and affects people of all ages and backgrounds. Despite vitamin D being produced naturally in the skin from sunlight, the UK’s climate, modern indoor lifestyles, and certain medical conditions make deficiency widespread.

This guide explains all major UK-validated causes of vitamin D deficiency -including lifestyle, medical, biological, and environmental factors – based on NHS and SACN guidance.

Why Vitamin D Deficiency Happens

Vitamin D deficiency occurs when the body cannot produce, absorb, or activate enough vitamin D. Because vitamin D supports bones, muscles, immunity, and general health, identifying the root cause is essential for proper treatment.

Sunlight and Vitamin D Production (UK Context)

In the UK, sunlight is the main natural source of vitamin D, but UVB radiation is only strong enough between:

Late March to early September (UKHSA)
During autumn and winter, the skin cannot make vitamin D at all.

Vitamin D synthesis steps:

1. UVB rays strike the skin

2. Skin converts 7-dehydrocholesterol → vitamin D3

3. Liver converts vitamin D3 → 25(OH)D

4. Kidneys convert 25(OH)D → active 1,25(OH)₂D

Any disruption at any stage can cause deficiency.

Lifestyle-Related Causes of Vitamin D Deficiency in the UK

This is the most common reason for deficiency among UK adults.

1. Lack of Sun Exposure

Common reasons:

• Working indoors (offices, factories, schools, hospitals)

• Spending daylight hours inside

• Minimal outdoor activity

• Staying indoors during peak UVB hours (11am-3pm)

• Poor mobility or housebound status

2. Clothing That Covers Most of the Skin

NHS lists this as a high-risk factor.
Examples:

• Cultural or religious garments

• Clothing covering arms, legs, and face

These significantly reduce UV exposure.

3. Sunscreen Use

Sunscreen is essential for skin cancer protection, but:

• SPF 30+ can reduce vitamin D synthesis

• People using sunscreen consistently may produce less vitamin D

The NHS emphasises that people should not avoid sunscreen to increase vitamin D.

4. Living in Northern Latitudes

The UK’s position far from the equator means:

• Weak UVB for much of the year

• No vitamin D production during autumn/winter

• Increased risk for people in Scotland and northern England

This is why NHS recommends supplements for everyone during autumn & winter.

5. Indoor or Screen-Based Lifestyles

Long working hours indoors limit natural light exposure.
A major factor in adults aged 20-60.

6. Air Pollution

Air pollution blocks or scatters UVB rays.
People in polluted urban areas (London, Birmingham, Manchester) produce less vitamin D even outdoors.

Biological & Genetic Causes

Some individuals naturally produce less vitamin D due to skin type, age, or genetics.

1. Darker Skin (African, Caribbean, South Asian Heritage)

NHS lists this group at high risk.
Melanin reduces the skin’s ability to produce vitamin D.
People with darker skin require more sunlight to produce equivalent levels.

2. Ageing

After age 60:

• The skin makes less vitamin D

• Mobility decreases

• Elderly adults often spend less time outdoors

This is why older adults are at higher risk.

3. Genetic Variations

Genetic differences can reduce:

• Vitamin D receptor efficiency

• Liver activation

• Kidney activation

• Vitamin D transport proteins

These individuals may require higher supplementation under medical guidance.

Medical Conditions That Cause Vitamin D Deficiency

The NHS identifies several medical causes:

1. Kidney Disorders (Impaired Activation)

Kidneys convert inactive vitamin D into its active form.
Conditions include:

• Chronic kidney disease (CKD)

• Renal failure

• Nephrotic syndrome

These conditions often require medically supervised supplementation.

2. Liver Disorders (Reduced Conversion)

The liver converts vitamin D into 25(OH)D.
Deficiency may occur with:

• Hepatitis

• Cirrhosis

• Alcohol-related liver disease

• Fatty liver

3. Obesity

Vitamin D becomes stored in fat tissue.
People with obesity often have:

• Lower circulating vitamin D

• Higher requirements

This is recognised by UK clinical guidelines.

4. Malabsorption Disorders

Vitamin D is fat-soluble and requires normal digestion to absorb.

Conditions that reduce absorption:

• Coeliac disease

• Crohn’s disease

• Ulcerative colitis

• Short bowel syndrome

• Pancreatic insufficiency

• Cystic fibrosis

5. Bariatric / Gastric Bypass Surgery

These procedures reduce:

• Surface area of intestine

• Fat absorption

• Vitamin D uptake

Patients usually require lifelong supplementation.

6. Bile Acid or Pancreatic Disorders

Fat malabsorption leads directly to vitamin D deficiency.

Medication-Induced Vitamin D Deficiency (NHS Acknowledged)

Several drugs interfere with vitamin D levels.

1. Anticonvulsants

e.g. phenytoin, carbamazepine
Increase vitamin D breakdown in the liver.

2. Corticosteroids

Long-term use:

• Reduces vitamin D activation

• Reduces calcium absorption

• Weakens bones

3. Cholesterol-binding medications

e.g. cholestyramine
Bind fats and impair vitamin D absorption.

4. Weight-loss drug Orlistat

Reduces fat absorption → reduces vitamin D absorption.

Dietary Causes (UK Diet Patterns)

Diet alone usually cannot provide enough vitamin D, but deficiency occurs more easily when intake is low.

Foods Naturally High in Vitamin D (limited in UK diets)

• Oily fish

• Egg yolks

• Liver

• Red meat

• Fortified foods

Many people in the UK eat very little oily fish, which contributes to low intake.

Vegan & Vegetarian Diets

Plant-based diets contain vitamin D2, which is:

• Less potent

• Shorter acting

• Less stable

Vegans heavily rely on fortified foods or supplements.

Low-fat diets

Vitamin D is fat-soluble; very low fat intake reduces absorption.

High-Risk Groups for Vitamin D Deficiency (NHS)

The NHS identifies the following groups at increased risk:

• People with darker skin

• People who cover their skin

• People who spend little time outdoors

• Older adults (65+)

• Pregnant and breastfeeding women

• Infants and young children

• People with malabsorption conditions

• People who are obese

• People taking specific medications

• People living in polluted or urban areas

Summary: Main Causes of Vitamin D Deficiency (UK-Specific)

Vitamin D deficiency occurs due to:

• Insufficient sunlight (major cause in the UK)

• Living at northern latitudes

• Indoor lifestyle

• Darker skin

• Ageing skin

• Obesity

• Malabsorption

• Kidney or liver disease

• Medications

• Pollution

• Low dietary intake

• Cultural/clothing practices

• Pregnancy or increased physiological needs

Understanding the cause helps ensure proper diagnosis and treatment.