What Would Cause Vitamin D to be Low?

January 1, 2026 •

3 min read

According to the National Institutes of Health, nearly one out of four people in the United States have vitamin D levels that are too low for optimal health. A vitamin D deficiency can stem from a variety of factors beyond simple lack of sun, affecting everything from bone density to mood.

Quick Summary

Several factors can lead to low vitamin D, including insufficient sun exposure, dietary deficiencies, malabsorption issues, and liver or kidney dysfunction. Age, skin pigmentation, certain medical conditions, and medications also play a role in the body's ability to produce, absorb, or activate this essential nutrient.

Key Points

Sun Exposure is Key: The most common reason for low vitamin D is inadequate exposure to sunlight, particularly during winter months or for those with limited outdoor activity.
Dietary Intake is Often Insufficient: Few foods naturally contain high levels of vitamin D, so those with restricted diets, like vegans, are at risk.
Fat Malabsorption is a Major Cause: Medical conditions such as celiac disease, Crohn's disease, and cystic fibrosis can prevent the intestines from properly absorbing fat-soluble vitamins, including vitamin D.
Chronic Diseases Impair Activation: Liver and kidney diseases interfere with the body's ability to convert inactive vitamin D into its usable, active form.
Age and Skin Pigment Matter: Older adults and individuals with darker skin tones naturally produce less vitamin D from sun exposure, increasing their risk of deficiency.
Certain Medications Affect Levels: Some medications, including steroids and anti-seizure drugs, can accelerate the breakdown of vitamin D in the body.
Obesity Can Lower Levels: Higher body fat can sequester vitamin D, preventing it from circulating in the bloodstream and leading to lower blood levels.
Diagnosis is via Blood Test: A blood test to measure 25-hydroxyvitamin D levels is the most reliable way to confirm a deficiency.
Supplementation is Often Needed: For many people, especially those in high-risk groups, dietary intake and sun exposure are not enough, and supplements are necessary.

Understanding Vitamin D: The "Sunshine Vitamin"

Vitamin D is a fat-soluble nutrient essential for absorbing calcium and phosphorus, critical for bone health and proper immune function. While often called the "sunshine vitamin," relying solely on sun exposure for adequate levels is challenging due to modern lifestyles and health risks. Low vitamin D levels can have a significant impact on your overall health, leading to weakened bones, muscle pain, and fatigue.

Primary Causes of Low Vitamin D

Multiple factors can contribute to low vitamin D levels, ranging from lifestyle choices to underlying medical conditions.

1. Inadequate Sun Exposure

Sunlight is the most efficient natural source of vitamin D. When ultraviolet B (UVB) rays hit the skin, a cholesterol compound is converted into vitamin D3. Factors limiting this process include geographic location, season, time of day, lifestyle (like spending more time indoors or using sunscreen), and skin pigmentation. Darker skin, for instance, produces less vitamin D from sun exposure due to more melanin.

2. Dietary Deficiencies

Few foods naturally contain high amounts of vitamin D. Natural sources include fatty fish, fish liver oils, beef liver, and egg yolks. Many foods are fortified with vitamin D, but those with dietary restrictions like vegans may find it difficult to obtain enough.

3. Malabsorption Disorders

Vitamin D is fat-soluble, and its absorption is tied to dietary fat absorption. Conditions that impair fat absorption, such as celiac disease, Crohn's disease, ulcerative colitis, cystic fibrosis, and certain weight-loss surgeries, can lead to deficiency.

Other Factors Influencing Vitamin D Levels

4. Medical Conditions Affecting Metabolism

Diseases affecting the liver and kidneys can hinder the conversion of vitamin D into its active form.

5. Age

As individuals age, their skin's ability to produce vitamin D from sunlight decreases, and kidney function in converting it to the active form becomes less efficient.

6. Medications

Certain medications, including steroids, anti-seizure drugs, cholesterol-lowering drugs, and weight-loss medications, can interfere with vitamin D metabolism.

Comparing Causes of Low Vitamin D


Factor	How it Leads to Low Vitamin D	Who is at Risk?
Limited Sun Exposure	Reduced conversion of cholesterol to vitamin D3 in the skin.	People in northern latitudes, indoor workers, those with high sun protection habits.
Dietary Restrictions	Insufficient intake from food, especially for vegans and those with low consumption of fortified products.	Vegans, individuals with limited access to fortified foods.
Malabsorption Issues	Poor absorption of fat-soluble vitamin D in the intestines due to disease or surgery.	People with celiac disease, Crohn's, cystic fibrosis, or after gastric bypass.
Chronic Diseases	Liver and kidney diseases hinder the body's ability to convert vitamin D into its active form.	Individuals with liver cirrhosis or chronic kidney failure.
Aging	Decreased production by the skin and impaired kidney function to activate the vitamin.	Adults over 65 years old.
Medications	Certain drugs accelerate the metabolic breakdown of vitamin D.	Individuals on steroids, anti-seizure, or some cholesterol-lowering medications.

Conclusion: Taking Action to Address Low Vitamin D

Low vitamin D levels can result from a combination of factors. Identifying the cause is key to effective action. This may involve increasing safe sun exposure, dietary changes, or supplements. A blood test is the best way to diagnose a deficiency. Supplementation can help restore levels and reduce health risks.

This article provides general information and is not medical advice. Consult a healthcare professional for diagnosis and treatment. For more information on health and wellness, visit Healthline.

Frequently Asked Questions

The fastest way to increase vitamin D levels is through targeted supplementation, as advised by a doctor. A healthcare provider can determine the appropriate dosage and form (D2 or D3), as dietary changes and sun exposure take longer to have a significant effect.

While stress isn't a direct cause of low vitamin D, chronic stress can lead to behavioral changes, such as less time spent outdoors, poor dietary choices, and inflammation, which can indirectly contribute to lower levels.

No, you cannot get vitamin D from sunlight coming through a window. The glass blocks the UVB rays necessary for your skin to synthesize vitamin D, although it allows UVA rays to pass through.

Yes, individuals with obesity often have lower circulating vitamin D levels. This is because vitamin D is stored in fat tissue, and with higher amounts of body fat, more of the vitamin is sequestered and not released into the bloodstream.

Yes, older adults are at a higher risk of low vitamin D. As we age, our skin's ability to produce vitamin D from sunlight decreases, and the kidneys are less efficient at converting it into its active form.

Since many natural sources of vitamin D are animal-based, strict vegans and vegetarians may struggle to get enough from diet alone. They often need to rely on fortified foods or supplements to meet their needs.

Certain medications can interfere with vitamin D metabolism. This includes some steroids, anti-seizure drugs, weight-loss drugs, and cholesterol-lowering drugs.

Skin conditions are not listed as primary factors causing low vitamin D, but some issues like eczema or autoimmune skin diseases may have a correlation. However, the main causes relate to synthesis, absorption, and metabolism.