What causes vitamin D to go so low? Underlying factors explained

December 22, 2025 •

4 min read

According to the National Institutes of Health (NIH), a significant portion of the global population suffers from vitamin D deficiency, a condition that can have widespread health implications. Understanding what causes vitamin D to go so low is the first step toward effective prevention and treatment, which can help maintain bone health, support the immune system, and regulate mood.

Quick Summary

This guide explains the root causes of low vitamin D, detailing the roles of limited sun exposure, inadequate diet, fat malabsorption, and specific medical conditions. It provides an overview of the key factors involved in vitamin D deficiency.

Key Points

Inadequate Sun Exposure: Limited sunlight, particularly in northern latitudes or with darker skin, is a primary driver of low vitamin D levels.
Malabsorption Disorders: Conditions like celiac disease and Crohn's prevent the small intestine from properly absorbing dietary vitamin D.
Obesity and Body Fat: Body fat can trap vitamin D, isolating it from the bloodstream and leading to lower circulating levels.
Declining Kidney and Liver Function: These organs are essential for converting vitamin D to its active form; poor function can cause deficiency.
Certain Medications: Drugs such as steroids and some cholesterol-lowering medicines can interfere with vitamin D metabolism and cause a deficiency.
Age-Related Factors: The elderly produce vitamin D less efficiently in their skin and kidneys, making them a high-risk group.
Limited Dietary Intake: Relying solely on diet is often insufficient, as few foods naturally contain enough vitamin D to meet daily needs.

The Primary Role of Sun Exposure

Sunlight is the body's most effective natural source of vitamin D. When ultraviolet-B (UVB) rays from the sun hit the skin, a compound called 7-dehydrocholesterol is converted into previtamin D3, which then becomes vitamin D3. Numerous factors can prevent this crucial process, leading to low vitamin D levels.

Melanin and Skin Pigmentation

Individuals with darker skin have more melanin, a pigment that protects the skin from sun damage. However, this increased melanin acts as a natural sunscreen, significantly reducing the skin's ability to produce vitamin D from sunlight. Research has shown that individuals with darker skin may require significantly more sun exposure to produce the same amount of vitamin D as those with lighter skin.

Geographical Location and Seasonality

For those living in northern latitudes, the angle of the sun is insufficient to trigger vitamin D synthesis in the skin for much of the year, particularly during winter. Even in sunnier climates, seasonal fluctuations can cause levels to drop. Those who spend most of their time indoors, such as the elderly in nursing homes or office workers, also receive very little sun exposure.

Use of Sunscreen

Consistent and proper use of sunscreen, while vital for preventing skin cancer, can block UVB rays and inhibit vitamin D production. This is a trade-off that many people make, highlighting the need for alternative sources like diet or supplementation to maintain adequate levels.

Dietary Insufficiency and Absorption Problems

While sunlight is a major source, dietary intake also plays a role, especially for those with limited sun exposure. Unfortunately, very few foods naturally contain significant amounts of vitamin D.

Limited Dietary Sources

Key dietary sources include fatty fish (salmon, tuna, mackerel), fish liver oils, and smaller amounts in egg yolks, beef liver, and cheese. However, relying solely on diet is often not enough to meet daily requirements, and many people do not regularly consume these foods.

Malabsorption Syndromes

Certain medical conditions can prevent the small intestine from absorbing nutrients properly, including vitamin D. These malabsorption syndromes are a significant cause of deficiency and include:

Celiac Disease: An autoimmune disorder where consuming gluten damages the small intestine lining.
Crohn's Disease and Cystic Fibrosis: Inflammatory and genetic disorders, respectively, that affect digestive function.
Gastric Bypass Surgery: This weight-loss surgery alters the digestive tract, limiting nutrient absorption from the small intestine.

Medical Conditions Affecting Vitamin D Metabolism

The body must convert inactive vitamin D into its active form to be used efficiently. Problems with the liver or kidneys can disrupt this conversion process.

Liver and Kidney Disease

Liver disease, such as cirrhosis, can impair the liver's ability to perform the initial conversion of vitamin D. Chronic kidney disease similarly reduces the kidney's capacity to perform the second conversion step, leading to inadequate active vitamin D.

Obesity

Individuals with a body mass index (BMI) over 30 are at a higher risk of vitamin D deficiency. This is because body fat cells sequester vitamin D, preventing it from circulating in the blood. As a result, obese individuals often require larger doses of supplements to reach healthy levels.

Age

As people age, their skin's efficiency at producing vitamin D from sunlight decreases. Additionally, older adults may spend more time indoors and have lower dietary intake, compounding the risk of deficiency. The kidneys' ability to convert vitamin D also declines with age.

Medications

Certain medications can accelerate the breakdown of vitamin D in the body, leading to lower levels. These include:

Steroids, such as prednisone.
Anti-seizure medications like phenobarbital and phenytoin.
Weight-loss drugs such as orlistat.

Factors Contributing to Low Vitamin D Levels

Here is a list of the key risk factors that can lead to a deficiency:

Limited sun exposure due to lifestyle or location.
Having darker skin pigmentation.
Older age, as metabolic processes slow down.
Malabsorption issues from certain intestinal diseases.
Obesity, which can trap vitamin D in fat tissue.
Kidney or liver disease affecting conversion.
Taking certain medications that interfere with metabolism.
Strict vegan diets lacking fortified foods.

Comparison of Vitamin D Sources

This table outlines the pros and cons of the main ways people obtain vitamin D.


Source	Pros	Cons
Sunlight	Free, natural, and highly effective for most people.	Melanin and latitude can limit effectiveness; risk of skin cancer from excessive exposure.
Dietary Intake	Can supplement sun exposure; provides other nutrients.	Few foods contain significant amounts; difficult to get enough solely from food.
Fortified Foods	Accessible and easy way to boost intake.	Content can vary; may not be sufficient for severe deficiencies.
Supplements (D3)	Convenient, reliable, and easily dosed. Recommended for high-risk groups.	Requires consistency; risk of toxicity with very high doses over time.

Conclusion

Low vitamin D levels are a complex issue, often caused by a combination of factors rather than a single reason. From reduced sun exposure due to geography, darker skin, or lifestyle, to underlying health conditions and medications, the causes are multifaceted. For many people, a holistic approach that includes modest sun exposure, a balanced diet with vitamin D-rich or fortified foods, and supplementation as recommended by a healthcare professional is the most effective strategy. Given the wide-ranging health impacts, addressing the root causes of vitamin D deficiency is crucial for maintaining overall wellness. Read more on the topic from MedlinePlus here.

Ultimately, understanding the specific reasons behind an individual's low vitamin D is the most important step toward correction. Regular testing, especially for those in high-risk categories, can help identify and manage this common but often overlooked health problem. By proactively addressing the contributing factors, it is possible to prevent the associated health complications and improve quality of life.

Frequently Asked Questions

For most people, it is very difficult to get enough vitamin D from diet alone. While some foods like fatty fish and fortified milk contain it, dietary intake is often insufficient, especially without adequate sun exposure or supplements.

Yes, skin color is a significant factor. Individuals with darker skin have higher levels of melanin, which reduces the skin's ability to produce vitamin D from sunlight. This means they need more sun exposure to produce the same amount.

For people with a higher body mass index (BMI), vitamin D is sequestered by fat cells and not released into the bloodstream. This means more vitamin D is needed to maintain sufficient circulating levels.

The liver and kidneys are crucial for converting vitamin D into its active form. Liver disease can impair the first conversion step, and chronic kidney disease affects the final conversion, both leading to low levels.

Yes, consistent and proper use of sunscreen blocks the UVB rays necessary for vitamin D production in the skin. While sunscreen protects against skin cancer, it can inhibit this synthesis, requiring other sources to compensate.

Yes, older adults are at increased risk due to several factors. Their skin produces vitamin D less efficiently, they may get less sun exposure, and their kidneys are less able to convert it to its active form.

Some medications, including certain anti-seizure drugs, steroids, and weight-loss medicines, can accelerate the metabolism of vitamin D, leading to lower levels in the body.