What is the reason behind low vitamin D? A Comprehensive Guide

December 31, 2025 •

3 min read

According to a 2011-2012 NHANES study, a significant portion of the U.S. population showed insufficient or deficient vitamin D levels. Many people wonder what is the reason behind low vitamin D, as the causes are multifaceted, ranging from lifestyle choices to serious medical conditions.

Quick Summary

Several factors contribute to low vitamin D, including inadequate sun exposure, limited dietary intake, certain medical conditions affecting absorption and metabolism, and specific medications. Age, skin tone, and obesity are also key risk factors. Treatment involves addressing the root cause through diet, safe sun exposure, and supplementation.

Key Points

Limited Sun Exposure: The most common cause is a lack of direct sunlight, due to indoor lifestyles, clothing, or living in northern latitudes during winter.
Poor Diet: Few natural foods contain high levels of vitamin D, making dietary intake insufficient for many, especially those who avoid animal products.
Medical Conditions: Diseases like celiac disease, Crohn's, cystic fibrosis, and liver or kidney failure impair the body's ability to absorb or activate vitamin D.
Obesity: Higher body fat levels can sequester vitamin D, making less of it available for use by the body and increasing the risk of deficiency.
Aging Skin: The skin's efficiency at synthesizing vitamin D from sunlight diminishes with age, making older adults more susceptible to deficiency.
Certain Medications: Drugs such as specific anti-seizure medications and steroids can increase the metabolism of vitamin D, lowering its levels in the body.
Darker Skin Tone: Higher melanin content reduces the skin's ability to produce vitamin D from the sun, placing individuals with darker skin at higher risk.

The Primary Source of Vitamin D: Sunlight Exposure

Sunlight is the most efficient and natural source of vitamin D, as the skin produces the vitamin when exposed to ultraviolet B (UVB) rays. However, many modern lifestyle and environmental factors limit our ability to get enough sun. These include spending most of our time indoors, wearing clothing that covers most of the body, and living in higher latitudes where UVB rays are less effective, especially during winter months. For example, studies have shown significant seasonal fluctuations in vitamin D levels, with the lowest concentrations occurring at the end of winter. People who use sunscreen diligently also reduce their vitamin D synthesis, although regular, brief sun exposure without sunscreen on certain body parts is often recommended.

Dietary Insufficiency and Malabsorption

While sunlight is a key source, diet also plays a crucial role. Many people do not consume enough vitamin D-rich foods to maintain adequate levels, especially those following vegan or vegetarian diets. Natural food sources containing significant amounts of vitamin D are limited, making fortification and supplements important. Beyond intake, the body's ability to absorb and utilize the vitamin can be compromised by a range of medical issues.

Conditions affecting vitamin D absorption:

Celiac disease: An autoimmune disorder that damages the small intestine, leading to poor absorption of fat-soluble vitamins like D.
Crohn's disease and ulcerative colitis: Inflammatory bowel diseases that interfere with nutrient absorption.
Cystic fibrosis: A genetic disorder that can cause chronic pancreatic insufficiency, reducing the absorption of fats and, therefore, fat-soluble vitamins.
Gastric bypass surgery: Alterations to the small intestine make it harder to absorb nutrients, requiring lifelong supplementation.

The Role of Metabolism and Underlying Health Issues

The body must metabolize inactive vitamin D into its active form, and any disruption in this process can cause low levels. The kidneys and liver play a vital role in this conversion, so chronic diseases affecting these organs can lead to deficiency.

Other critical health factors:

Obesity: Individuals with a BMI over 30 tend to have lower serum vitamin D levels. Body fat sequesters vitamin D, preventing it from circulating freely in the blood.
Kidney and liver disease: Conditions like cirrhosis and renal failure inhibit the enzymes needed to convert vitamin D into its active form.
Aging: As people age, their skin's ability to synthesize vitamin D from sunlight decreases. Older adults also tend to spend less time outdoors and may have a reduced dietary intake.
Medications: Certain drugs can accelerate the breakdown of vitamin D in the liver. Examples include specific anti-seizure medications (e.g., phenytoin, phenobarbital), steroids (e.g., prednisone), and some cholesterol-lowering drugs.

Risk Factors and Demographic Variations

Certain populations are at a significantly higher risk for developing vitamin D deficiency, primarily due to genetic, lifestyle, and environmental factors.

Comparison of Risk Factors for Low Vitamin D


Risk Factor	Reason	Affected Populations
Skin Pigmentation	Melanin acts as a natural sunscreen, reducing the skin's ability to produce vitamin D from sunlight.	Individuals with darker skin tones, including African American and Hispanic populations.
Age	Decreased skin synthesis and lower outdoor mobility, often accompanied by reduced dietary intake.	Older adults over 65, and infants exclusively breastfed (human milk is low in vitamin D).
Geographic Location	Less access to adequate UVB radiation, especially during winter months.	People living at higher latitudes (far from the equator).
Malabsorption	Underlying medical conditions prevent the proper absorption of nutrients from the gut.	People with celiac disease, Crohn's disease, or post-bariatric surgery.
Lifestyle	Spending most time indoors for work, health, or personal preference reduces sun exposure.	Office workers, housebound individuals, and people in institutions like nursing homes.

Conclusion

The reason behind low vitamin D is not a single factor but a combination of complex dietary, lifestyle, and physiological issues. Addressing deficiency often requires a multi-pronged approach, involving a balanced diet with vitamin D-rich foods, appropriate and safe sun exposure, and targeted supplementation, especially for high-risk individuals. Consulting a healthcare provider for a proper diagnosis and treatment plan is crucial. Correcting low vitamin D is vital for bone health and overall well-being. For more in-depth information, you can visit the National Institutes of Health Office of Dietary Supplements website.

Frequently Asked Questions

The most common reason for low vitamin D is inadequate exposure to sunlight, as the body primarily produces the vitamin through skin exposure to UVB rays.

Yes, people with darker skin have more melanin, which acts as a natural sunscreen and reduces the skin’s ability to produce vitamin D from sunlight. They may need more sun exposure or supplementation.

Yes, studies show a strong link between obesity and low vitamin D levels. Body fat stores vitamin D, effectively trapping it and making it less available for use in the bloodstream.

Medical conditions that affect fat absorption in the intestines, such as Crohn’s disease, celiac disease, and cystic fibrosis, can lead to low vitamin D levels.

Older adults are at higher risk because their skin's ability to produce vitamin D from sunlight decreases with age. They also tend to spend less time outdoors and may have reduced dietary intake.

Yes, some medications, including certain anticonvulsants and steroids, can interfere with vitamin D metabolism by speeding up its breakdown in the liver.

You can increase your vitamin D levels through a combination of safe, moderate sun exposure, consuming fortified foods like milk and cereals, eating vitamin D-rich foods such as fatty fish, and taking supplements as advised by a doctor.