What causes a decrease in vitamin D?

December 26, 2025 •

3 min read

According to data from the National Health and Nutrition Examination Survey, a significant portion of the U.S. population has insufficient vitamin D levels. Many people wonder what causes a decrease in vitamin D, a question with answers that range from lifestyle factors to complex medical conditions.

Quick Summary

Low vitamin D levels often result from inadequate sunlight, insufficient dietary intake, or impaired absorption due to conditions like Crohn's disease or celiac disease. Age, obesity, kidney or liver disease, and specific medications can also contribute to this deficiency.

Key Points

Sun Exposure is Key: The primary source of vitamin D is UVB radiation from sunlight, and insufficient exposure due to geography, season, skin color, or lifestyle is a major cause of deficiency.
Dietary Deficiencies Matter: Relying on food alone for vitamin D is often insufficient, especially for vegans, as few foods naturally contain it; consumption of fortified foods and supplements is necessary.
Medical Conditions are a Factor: Diseases affecting fat absorption, such as celiac or Crohn's disease, significantly impair the uptake of fat-soluble vitamin D.
Organ Health is Critical: The liver and kidneys must function correctly to activate vitamin D; diseases affecting these organs, like cirrhosis and chronic kidney disease, will lead to reduced active vitamin D.
Medications Can Interfere: Certain drugs, including anticonvulsants, steroids, and some cholesterol medications, can disrupt vitamin D absorption or accelerate its breakdown.
Age and Weight Play a Role: The body's ability to produce and utilize vitamin D decreases with age, and obesity can cause vitamin D to be sequestered in fat cells, making it less available.

Primary Factors Influencing Vitamin D Levels

The synthesis and maintenance of adequate vitamin D levels in the body are a complex process involving sun exposure, diet, and metabolic functions. When one or more of these elements are compromised, a deficiency can occur. Understanding these primary causes is the first step toward prevention and management.

Insufficient Sun Exposure

Sunlight is the most significant natural source of vitamin D, as the skin produces it in response to ultraviolet-B (UVB) radiation. A lack of adequate sun exposure is a leading cause of low vitamin D levels for a variety of reasons, including geographical location, increased melanin content in darker skin, and spending extended periods indoors. Protective measures like sunscreen use and clothing also block UVB rays necessary for production.

Inadequate Dietary Intake

Few foods naturally contain significant amounts of vitamin D. Therefore, diets lacking fortified foods or vitamin D-rich options can contribute to deficiency, particularly for those following vegan diets who avoid animal-based sources like fatty fish and egg yolks.

Medical Conditions and Health Factors

Beyond external factors, several internal health issues can impair the body's ability to absorb or activate vitamin D, leading to low levels.

Malabsorption Syndromes

Since vitamin D is fat-soluble, conditions affecting fat absorption, such as Celiac disease, Crohn's disease, Ulcerative Colitis, Cystic Fibrosis, and post-Gastric Bypass Surgery, can significantly reduce vitamin D uptake. Impaired bile production also hinders absorption.

Liver and Kidney Disease

Vitamin D requires two conversions to become active: one in the liver and one in the kidneys. Chronic liver disease, such as cirrhosis, decreases the initial conversion. Kidney disease impairs the final conversion to the active hormone, calcitriol.

Obesity

Higher body mass index (BMI) is linked to lower circulating vitamin D levels as the vitamin gets stored in fat cells, making it less available. Individuals with obesity may need higher doses of supplements.

Age

Aging increases the risk of vitamin D deficiency due to decreased skin synthesis, reduced kidney function for activation, and less time spent outdoors.

Medications and Other Factors

Certain medications and lifestyle choices can also play a role in lowering vitamin D levels.

Medications that Interfere

Some drugs increase the breakdown or interfere with vitamin D absorption. These include certain anticonvulsants (phenobarbital, phenytoin, carbamazepine), corticosteroids (prednisone, dexamethasone), weight-loss drugs (Orlistat), and cholesterol-lowering bile acid sequestrants (cholestyramine).

Lifestyle and Other Considerations

Exclusively breastfed infants are at risk of deficiency and need supplementation as breast milk has low vitamin D content. Wearing extensive clothing for cultural or personal reasons also limits essential sun exposure.


Cause Category	Effect on Vitamin D Levels	Key Examples	Prevention/Management
Sunlight	Decreased synthesis in the skin	Living in high latitudes, sunscreen use, spending time indoors, darker skin tone	Safe, limited sun exposure, consider supplementation
Diet	Insufficient intake from food sources	Vegan diets, limited consumption of fatty fish or fortified dairy	Consume fortified foods, supplements
Absorption Issues	Decreased intestinal absorption of fat-soluble vitamins	Celiac disease, Crohn's, gastric bypass surgery, cystic fibrosis	Medical treatment of underlying condition, potentially higher dose supplements
Metabolism Issues	Impaired conversion to active form in liver and kidneys	Chronic liver disease, chronic kidney disease	Treatment of underlying disease, medical monitoring
Medications	Accelerated breakdown or reduced absorption	Anticonvulsants, corticosteroids, bile acid sequestrants	Discuss medication regimen and potential supplementation with a doctor

Conclusion

A decrease in vitamin D is influenced by a range of factors, from sunlight exposure and dietary intake to underlying medical conditions and certain medications. Addressing a deficiency often requires identifying the specific cause through consultation with a healthcare professional, who can recommend appropriate strategies such as supplementation or dietary adjustments. Understanding these diverse factors is vital for maintaining optimal vitamin D levels and overall health. More information on vitamin D and other nutrients can be found through authoritative sources like the Harvard T.H. Chan School of Public Health Nutrition Source.

Frequently Asked Questions

Yes, living in northern or southern latitudes with less intense sunlight during winter months can significantly decrease the skin's vitamin D production, contributing to deficiency.

Melanin, the pigment that makes skin darker, acts as a natural sunblock, which reduces the skin's ability to synthesize vitamin D from sunlight. People with darker skin tones need longer sun exposure to produce the same amount of vitamin D as those with lighter skin.

Conditions that impair fat absorption, such as celiac disease, cystic fibrosis, Crohn's disease, and post-gastric bypass surgery, can cause a decrease in vitamin D absorption because it is a fat-soluble vitamin.

Obesity is associated with lower circulating vitamin D because the fat-soluble vitamin gets trapped within fat cells, making it less bioavailable for the body's use.

Yes, some medications, including certain anti-seizure drugs (phenobarbital), steroids (prednisone), and cholesterol-lowering drugs (cholestyramine), can interfere with vitamin D absorption or metabolism.

The liver and kidneys are crucial for activating vitamin D. Chronic diseases of these organs, such as cirrhosis or kidney failure, hinder the two conversion steps required to make vitamin D hormonally active.

No, window glass blocks the specific UVB rays needed for the skin to produce vitamin D. Exposure through a window is not an effective way to synthesize the vitamin.