What causes vitamin D levels to deplete?

November 17, 2025 •

3 min read

Globally, up to 50% of the population may experience vitamin D insufficiency. Understanding what causes vitamin D levels to deplete is the first step toward prevention and managing deficiencies that can impact bone health, immune function, and overall wellness.

Quick Summary

This article explores the multiple factors that can lead to low vitamin D, such as inadequate sunlight, dietary shortfalls, malabsorption issues, and specific health conditions.

Key Points

Limited Sun Exposure: The most common cause of low vitamin D is inadequate exposure to sunlight, especially in northern latitudes or during winter months.
Skin Pigmentation: Individuals with darker skin require significantly more sun exposure to produce the same amount of vitamin D as those with lighter skin due to melanin levels.
Dietary Deficits: Few foods naturally contain vitamin D, making adequate intake challenging without fortified products or supplements.
Obesity and Malabsorption: Excess body fat can sequester vitamin D, while conditions like Crohn's and Celiac disease impair nutrient absorption in the gut.
Age and Metabolism: Older adults have reduced skin synthesis, and liver or kidney diseases can hinder the body's conversion of vitamin D into its active form.
Certain Medications: Specific drugs, such as some anticonvulsants, can accelerate the metabolic breakdown of vitamin D in the body.

The Sun and Skin: The Primary Source Challenge

For most people, the skin's exposure to sunlight is the most significant source of vitamin D. Ultraviolet B (UVB) rays from the sun interact with a cholesterol precursor in the skin to produce vitamin D3. However, several factors can prevent this process from being efficient.

Geographical Location and Season

People living at higher latitudes, further from the equator, receive less intense UVB radiation, especially during the winter months. This can cause a significant drop in vitamin D levels, as the winter sun may not be strong enough to trigger synthesis, even with unprotected sun exposure.

Skin Pigmentation and Age

The amount of melanin, the pigment that determines skin color, affects how much vitamin D the skin can produce. People with darker skin have more melanin, which acts as a natural sunscreen and reduces vitamin D synthesis. Consequently, they may need more sun exposure to produce the same amount of vitamin D as a person with lighter skin. Additionally, as people age, their skin's ability to produce vitamin D from sunlight significantly decreases.

Sunscreen and Indoor Lifestyle

While sunscreen is crucial for preventing skin cancer, it blocks the UVB rays necessary for vitamin D production. Similarly, spending most of your time indoors or covering up with clothing can severely limit sun exposure, even during sunny periods.

Dietary Insufficiency and Malabsorption

Diet is another critical component, though few foods naturally contain significant vitamin D.

Limited Dietary Sources

Few foods are naturally rich in vitamin D. The best sources include fatty fish like salmon, tuna, and mackerel, as well as fish liver oils and egg yolks. For those with limited access to these foods or with dietary restrictions, such as vegans, relying on supplements or fortified foods becomes essential.

Malabsorption Syndromes

Since vitamin D is a fat-soluble vitamin, conditions that impair fat absorption can lead to a deficiency. These include:

Celiac disease
Crohn's disease
Cystic fibrosis
Chronic pancreatic insufficiency
Gastric bypass surgery

Medical Conditions and Medications

Beyond absorption, certain health issues and medications interfere with the body's ability to utilize vitamin D.

Liver and Kidney Disease

The body must convert vitamin D into its active form through a two-step process, first in the liver and then in the kidneys. Chronic liver diseases, like cirrhosis, and kidney diseases can impair this crucial conversion, leading to lower active vitamin D levels.

Obesity

Individuals with a body mass index (BMI) of 30 or higher often have lower blood vitamin D levels. This is not because they produce less but because excess body fat sequesters the vitamin, keeping it locked away and preventing it from circulating in the bloodstream.

Medications

Certain medications can increase the breakdown of vitamin D in the liver. These include:

Anticonvulsants (e.g., phenobarbital, carbamazepine)
Glucocorticoids
Antiretroviral drugs

Comparison of Key Depletion Factors


Factor	Mechanism of Depletion	Impact on Vitamin D Production	Impact on Absorption or Metabolism
Sun Exposure	Limited sun time or use of sunscreen blocks UVB rays.	Primary production significantly reduced.	Minimal impact on absorption.
Poor Diet	Insufficient intake of naturally rich or fortified foods.	Production unaffected.	Leads to insufficient overall intake.
Malabsorption	Medical conditions prevent intestinal absorption of fats.	Production unaffected.	Significantly reduced absorption of fat-soluble vitamin D.
Obesity	Excess body fat sequesters vitamin D from the bloodstream.	Production unaffected.	Bioavailability and circulation are reduced.
Aging	Skin's reduced efficiency and potential decreased mobility.	Skin production is less efficient.	Decreased intake or exposure often occurs.
Medications	Certain drugs accelerate the breakdown of vitamin D.	Production unaffected.	Increased metabolic catabolism.

Conclusion

Understanding what causes vitamin D levels to deplete is key to maintaining adequate health. From limited sun exposure and geographical location to dietary habits, obesity, and underlying medical conditions, many factors contribute to lower levels. While sun exposure is a primary driver, it is not always a reliable or safe option. Therefore, a multifaceted approach involving dietary changes and, when necessary, targeted supplementation is often required. Consult a healthcare provider to assess your individual risk and needs.

Frequently Asked Questions

The most common cause of vitamin D depletion is inadequate exposure to sunlight. The body synthesizes most of its vitamin D through sun exposure, and insufficient time outdoors significantly lowers levels.

For most people, it is very difficult to get enough vitamin D from diet alone because few foods naturally contain significant amounts. While some foods like fatty fish are good sources, many people rely on fortified foods or supplements to meet their needs.

Living at higher latitudes means receiving less intense UVB radiation, especially during winter. This can cause a substantial seasonal drop in vitamin D levels, as the body's ability to produce it from sunlight is diminished.

Yes, obesity is associated with lower vitamin D levels. Excess body fat can sequester vitamin D, meaning it is stored in fat tissue and not released into the bloodstream for the body to use efficiently.

Several medical conditions can cause depletion, including malabsorption syndromes like Celiac and Crohn's disease, chronic liver disease, and chronic kidney disease, which affect absorption or conversion.

Yes, some medications can interfere with the body's vitamin D metabolism. Certain anticonvulsants and steroids are known to increase the breakdown of vitamin D, leading to lower levels.

Older adults are more susceptible because the skin's capacity to synthesize vitamin D from sunlight declines with age. Additionally, they tend to spend less time outdoors, limiting their exposure.