What is the Root Cause of Low Vitamin D?

October 1, 2025 •

4 min read

According to numerous studies, vitamin D deficiency is a widespread issue, affecting a significant portion of the global population. Pinpointing what is the root cause of low vitamin D? can be complex, as it often stems from a combination of lifestyle, environmental, and physiological factors.

Quick Summary

Low vitamin D levels result from multiple factors, including limited sun exposure, inadequate dietary intake, and poor absorption due to medical conditions. Obesity, chronic kidney and liver diseases, and certain medications can also disrupt vitamin D metabolism, contributing to deficiency.

Key Points

Sunlight is a Primary Source: Inadequate exposure to UVB rays is a major cause of deficiency, affected by geography, season, and time spent indoors.
Dark Skin Reduces Synthesis: Higher melanin content in darker skin acts as a natural sunblock, requiring more sun exposure to produce the same amount of vitamin D as lighter skin.
Malabsorption is a Medical Cause: Conditions like celiac disease, Crohn's, and cystic fibrosis can prevent the gut from absorbing dietary vitamin D.
Organ Function is Critical: The liver and kidneys are essential for converting inactive vitamin D into its usable form; therefore, chronic diseases in these organs impair this process.
Obesity Can Lower Levels: Excess body fat can sequester and store vitamin D, making it less available for use by the body.
Certain Drugs Interfere: Some medications, including anti-seizure drugs, steroids, and cholesterol-lowering agents, can negatively impact vitamin D metabolism.
Multiple Factors Often Involved: For most people, deficiency is caused by a mix of environmental, dietary, and medical issues, rather than a single root cause.

The Primary Driver: Inadequate Sun Exposure

For most people, the most significant source of vitamin D is sunlight. When ultraviolet B (UVB) rays from the sun hit the skin, it triggers the synthesis of vitamin D3. A lack of consistent sun exposure is arguably the most fundamental cause of vitamin D deficiency worldwide.

Geographical Location and Season

Those living in higher latitudes, especially during winter, receive minimal UVB radiation due to the sun's angle and spend more time indoors. This seasonal dip in sun exposure is a direct cause of lower vitamin D levels.

Lifestyle and Demographics

Modern lifestyles often involve spending most of the day indoors, limiting natural sun exposure. Certain demographics are also at higher risk:

Older adults: As we age, the skin's ability to synthesize vitamin D from sunlight decreases.
Individuals with darker skin: Melanin, the pigment responsible for darker skin tones, acts as a natural sunscreen and inhibits vitamin D production.
People who cover their skin: For cultural, religious, or health reasons, extensive clothing coverage can prevent adequate sun exposure for vitamin D synthesis.

Dietary Intake and Absorption Issues

While sunlight is primary, diet is another crucial factor, especially for those with limited sun exposure. Few foods naturally contain high levels of vitamin D, making intake a common issue.

Inadequate Dietary Sources

Unless a diet is rich in fatty fish or includes fortified foods, it may not provide enough vitamin D. Strict vegan diets, in particular, may lack sufficient vitamin D sources.

Good dietary sources of vitamin D include:

Fatty fish (salmon, mackerel, trout)
Cod liver oil
Mushrooms exposed to UV light
Egg yolks
Fortified dairy products, cereals, and orange juice

Malabsorption Syndromes

Even with sufficient dietary intake, certain gastrointestinal disorders can prevent the body from properly absorbing the fat-soluble vitamin D.

Common malabsorption causes include:

Celiac disease: This autoimmune condition damages the small intestine lining, impairing nutrient absorption.
Crohn's disease and cystic fibrosis: These and other inflammatory bowel diseases can interfere with absorption in the small intestine.
Gastric bypass surgery: Bypassing a portion of the small intestine reduces the surface area available for vitamin D absorption.

Medical Conditions and Metabolism

Several chronic health issues can interfere with the body's ability to metabolize and utilize vitamin D, even if sun exposure and dietary intake are sufficient.

Liver and Kidney Disease

Vitamin D is a prohormone that requires activation by the liver and kidneys. Impaired function in these organs directly impacts the conversion of vitamin D into its active form.

Liver Disease: Affects the first step of metabolism, the 25-hydroxylation.
Chronic Kidney Disease: Reduces the kidneys' ability to perform the final conversion to the active form, a problem that worsens with disease progression.

Obesity

Obesity is strongly linked to lower vitamin D levels. The reason is not necessarily a lack of intake but rather a sequestration effect, where the fat-soluble vitamin D is trapped in excess adipose tissue. This reduces the amount of circulating vitamin D in the bloodstream.

Medication Interference

Certain medications can affect vitamin D levels by increasing its breakdown in the liver or by reducing absorption in the gut. Patients on these drugs often require support under medical guidance to maintain adequate levels.

Medications that can affect vitamin D levels include:

Antiepileptic drugs (e.g., phenobarbital, Dilantin)
Steroids (e.g., prednisone)
Cholesterol-lowering drugs (e.g., cholestyramine)
Weight-loss drugs (e.g., Orlistat)
Some HIV treatments (e.g., Tenofovir)

Comparison of Root Causes


Root Cause	Mechanism	Affected Populations
Inadequate Sun Exposure	Reduced skin synthesis of vitamin D3 via UVB rays.	High-latitude residents, housebound individuals, older adults, people with darker skin.
Malabsorption	Impaired intestinal absorption of fat-soluble vitamin D.	People with celiac, Crohn's, or cystic fibrosis; post-bariatric surgery patients.
Obesity	Vitamin D stored and sequestered in excess adipose tissue.	Individuals with a BMI of 30 or higher.
Chronic Organ Disease	Liver and kidneys cannot properly convert vitamin D to its active form.	People with chronic liver or kidney disease.
Medications	Certain drugs accelerate vitamin D breakdown or inhibit absorption.	Patients taking specific anti-seizure, steroid, or weight-loss medications.

The Multifaceted Nature of Vitamin D Deficiency

Identifying the single root cause of low vitamin D can be challenging because several factors often contribute simultaneously. For example, an elderly person (age-related decline) living in a northern climate (lack of sun) who also has a mild malabsorption issue could be at significant risk. Similarly, someone with chronic kidney disease (impaired activation) who is also obese (sequestration in fat) faces multiple barriers to maintaining healthy vitamin D levels. A comprehensive approach that considers all potential influences—including environment, lifestyle, and underlying health—is necessary for proper diagnosis and effective management.

Ultimately, a combination of safe sun exposure, dietary changes, and, if necessary, supplementation under medical guidance, is the most effective strategy for managing and preventing vitamin D deficiency.

For more detailed information on vitamin D and its role in health, visit the National Institutes of Health (NIH) website.

Frequently Asked Questions

Symptoms of low vitamin D can include fatigue, bone and joint pain, muscle weakness, cramps, and in more severe cases, bone loss or osteomalacia. However, many people with a deficiency experience no noticeable symptoms.

For most people with light skin, about 5 to 30 minutes of midday sun exposure to the face, arms, hands, and legs a few times per week is often sufficient. The required time can vary based on skin tone, age, and location.

The best dietary sources of vitamin D include fatty fish like salmon and mackerel, fish liver oils, and fortified foods such as milk, cereals, and some orange juice. Mushrooms treated with UV light are another source.

Not automatically, but obesity is a significant risk factor for low vitamin D levels. Higher levels of body fat can sequester vitamin D, meaning more intake may be required to maintain normal circulating levels compared to a person with an average weight.

It can be challenging to meet vitamin D needs from diet alone, as few foods naturally contain significant amounts. Supplements and fortified foods are often necessary, especially for those with limited sun exposure.

Management usually involves increasing vitamin D intake through oral supplements or other methods, often starting with a strategy to address the deficiency, followed by a maintenance approach. A doctor determines the appropriate approach based on blood tests and individual factors.

Vitamin D2 (ergocalciferol) comes from plants, while vitamin D3 (cholecalciferol) comes from animals and is produced in the skin from sun exposure. Vitamin D3 is generally considered more effective at raising and sustaining blood vitamin D levels.