Why is my vitamin D low? Understanding the Common Causes

January 7, 2026 •

4 min read

According to the National Health and Nutrition Examination Survey (NHANES), over 40% of adults in the U.S. have insufficient vitamin D levels. If you've been asking, "why is my vitamin D low?" you are not alone; a deficiency can stem from many factors beyond just a lack of sun exposure.

Quick Summary

Limited sun exposure, dietary issues, medical conditions, and lifestyle factors can cause low vitamin D levels. Genetic predispositions and medication use can also interfere with your body's ability to produce or absorb this vital nutrient.

Key Points

Sunlight Exposure: Not getting enough time in the sun is a primary reason for low vitamin D, especially for people in northern climates or those who stay indoors.
Skin Pigmentation: The pigment melanin in darker skin tones reduces the skin's ability to produce vitamin D from sunlight, requiring more sun exposure for the same effect.
Malabsorption: Conditions like celiac disease and Crohn's disease can prevent the intestines from properly absorbing fat-soluble vitamins, including vitamin D.
Weight and Body Fat: Higher body fat levels can sequester vitamin D, preventing it from circulating in the blood and leading to lower measurable levels.
Underlying Health Conditions: Liver and kidney diseases can impair the body's ability to convert vitamin D into its active form, disrupting its function.
Aging: With age, the skin's ability to produce vitamin D from sun exposure decreases, and the kidneys become less efficient at activating it.
Medications: Certain prescription drugs, such as steroids and some anti-seizure medications, can interfere with vitamin D metabolism.

The Primary Reasons for Low Vitamin D Levels

Vitamin D is a crucial fat-soluble vitamin that plays an essential role in calcium absorption, bone health, and immune system function. While sunshine is a major source, a variety of interconnected factors can contribute to a deficiency. Below are the most common reasons your vitamin D might be low, even if you think you're getting enough sun.

Inadequate Sun Exposure

As the "sunshine vitamin," the most direct cause of a deficiency is simply not getting enough exposure to ultraviolet-B (UVB) rays.

Limited Outdoor Time: Modern lifestyles often involve spending long hours indoors at home, school, or work. This significantly limits the opportunity for your skin to synthesize vitamin D naturally.
Geographic Location: Living in a northern or southern latitude where the sun's angle is low during the winter months means the UVB rays are less effective at reaching the Earth's surface.
Consistent Sunscreen Use: While crucial for preventing skin cancer, sunscreen with an SPF of 8 or higher blocks the UVB rays necessary for vitamin D production.
Covering Clothing: For cultural or personal reasons, some people consistently wear clothing that covers most of their body, preventing sun exposure.

Darker Skin Pigmentation

The amount of melanin, the pigment responsible for skin color, is a significant factor in vitamin D production. People with darker skin have more melanin, which acts as a natural sunscreen, reducing the skin's ability to produce vitamin D from sunlight. Studies show that people with dark skin need significantly more sun exposure to produce the same amount of vitamin D as those with lighter skin. This is why certain ethnic groups, like African Americans and Hispanics, are at a higher risk of deficiency.

Dietary Factors and Malabsorption

Though sunlight is the primary source, diet plays a role, especially for those with limited sun access. However, many dietary sources of vitamin D are naturally limited.

Low Vitamin D Diet: Few foods are naturally rich in vitamin D. Primary sources include fatty fish (salmon, tuna), fish liver oils, egg yolks, and fortified foods like milk, cereals, and orange juice. Vegetarian or vegan diets can be particularly low in vitamin D.
Fat Malabsorption: Since vitamin D is fat-soluble, conditions that affect the gut's ability to absorb fat can lead to low vitamin D levels. These can include celiac disease, Crohn's disease, cystic fibrosis, and certain types of liver disease.
Weight-Loss Surgery: Some bariatric procedures, such as gastric bypass, can bypass sections of the small intestine where vitamin D is absorbed, leading to deficiency.

Health Conditions and Medications

Several health issues and medications can interfere with the body's vitamin D metabolism.

Liver and Kidney Disease: The liver and kidneys are essential for converting vitamin D to its active form. Diseases affecting these organs can disrupt this process and cause low levels.
Obesity: A higher body fat percentage is linked to lower blood vitamin D levels because fat cells sequester the vitamin, preventing its release into the blood. Obese individuals may require larger doses of supplements to reach adequate levels.
Medications: Certain drugs can increase the metabolism or breakdown of vitamin D. Examples include laxatives, some anti-seizure medications, and steroid drugs like prednisone.

Aging

As people age, the skin's efficiency in producing vitamin D from sunlight decreases. Older adults also tend to spend less time outdoors and may have a reduced dietary intake, all of which contribute to a higher risk of deficiency. The kidneys also become less efficient at converting vitamin D to its active form with age.

Low Vitamin D: Sun Exposure vs. Internal Processing


Factor	Impact on Vitamin D Levels	Who is Affected?
Sun Exposure	Reduced UVB exposure limits skin's production of vitamin D.	People living at higher latitudes, those with indoor lifestyles, and consistent sunscreen users.
Skin Pigmentation	Higher melanin content requires more sun exposure for the same amount of vitamin D synthesis.	Individuals with naturally darker skin tones.
Dietary Intake	Insufficient intake of vitamin D-rich or fortified foods.	Vegans, those with limited access to fortified dairy or fatty fish.
Absorption Issues	Conditions like Celiac or Crohn's disease interfere with nutrient absorption in the gut.	Individuals with gastrointestinal or malabsorption disorders.
Organ Function	Impaired liver or kidney function prevents conversion to the active form.	Those with chronic liver or kidney disease.
Weight	Excess body fat sequesters vitamin D, making it less bioavailable.	Individuals with a higher body mass index (BMI).
Medications	Certain drugs accelerate the breakdown of vitamin D in the body.	Patients taking medications like steroids or some anti-seizure drugs.

Conclusion

Addressing the question, "why is my vitamin D low?" requires a holistic look at various factors, not just sunlight. While spending more time outdoors and eating fortified foods can help, for many, the solution lies in understanding more complex issues like dark skin pigmentation, underlying medical conditions, or medication side effects. A blood test from a healthcare provider can pinpoint the extent of the deficiency and help determine the most effective course of action, which may include targeted supplementation. Understanding the root cause is the first step toward restoring healthy vitamin D levels and mitigating the associated health risks.

For more information on the vitamin D synthesis process and health recommendations, refer to the National Institutes of Health (NIH) fact sheet on Vitamin D.

Frequently Asked Questions

Yes, it is possible to have low vitamin D despite regular sun exposure. Factors such as darker skin pigmentation, advanced age, living at high latitudes, consistently using sunscreen, or having an underlying medical condition can all interfere with your body's ability to produce or absorb the vitamin effectively.

Medical conditions that can cause low vitamin D levels include fat malabsorption disorders like celiac disease, Crohn's disease, and cystic fibrosis. Chronic liver and kidney disease also impair the conversion of vitamin D into its active form, and obesity can cause the vitamin to be stored in fat cells.

Yes, skin color is a significant factor. People with darker skin have higher levels of melanin, which protects the skin from UV rays but also reduces its ability to produce vitamin D. This means individuals with darker skin need more sun exposure than those with lighter skin to produce the same amount of the vitamin.

The best dietary sources of vitamin D include fatty fish (such as salmon, tuna, and mackerel), fish liver oils, egg yolks, and fortified foods. Common fortified items include milk, certain cereals, and orange juice.

To increase vitamin D levels quickly, a healthcare provider will likely recommend supplements, particularly vitamin D3, which is more easily absorbed by the body. They may start you on a higher dose to correct a deficiency, combined with dietary changes and safe, short periods of sun exposure.

Yes, some medications can accelerate the breakdown of vitamin D in your liver. Examples include certain anti-seizure drugs (like phenobarbital), steroids (like prednisone), and some cholesterol-lowering medicines.

Symptoms of low vitamin D can be subtle but may include fatigue, bone pain, muscle aches or weakness, and mood changes such as depression. Severe deficiency can lead to complications like osteomalacia (soft bones) in adults and rickets in children.