Why Do We Not Have Vitamin D in Our Body? Understanding the Root Causes of Deficiency

January 4, 2026 •

4 min read

According to NHANES data, a significant portion of the population has low vitamin D levels, leading many to question, 'why do we not have vitamin D in our body?' The reality is that our bodies can produce this vitamin, but modern lifestyles and physiological factors often hinder adequate synthesis and absorption.

Quick Summary

Numerous modern and biological factors cause a vitamin D deficiency. Key culprits include limited sun exposure, dietary shortcomings, poor absorption, and underlying health issues affecting its metabolism.

Key Points

Sunlight is the primary source: Our bodies produce vitamin D through skin exposure to UVB rays, but factors like season and lifestyle limit this.
Melanin reduces synthesis: Darker skin tones have more melanin, acting as a natural sunscreen and reducing the efficiency of vitamin D production.
Age affects production: The ability of the skin to synthesize vitamin D declines significantly with age, increasing the risk for older adults.
Absorption issues are common: Malabsorption syndromes (like Crohn's and celiac disease) prevent proper absorption of fat-soluble vitamin D from food.
Organ function is critical: The liver and kidneys must metabolize inactive vitamin D into its usable form; dysfunction in these organs causes deficiency.
Dietary intake can be low: Natural dietary sources of vitamin D are limited, often necessitating reliance on fortified foods or supplements.

The 'Sunshine Vitamin' Misnomer: How We Actually Get Vitamin D

While nicknamed the "sunshine vitamin," the term can be misleading. We don't magically have vitamin D. Instead, our bodies have the remarkable capability to synthesize vitamin D3 when our skin is exposed to the sun's ultraviolet B (UVB) rays. This process is the primary natural source of the nutrient for most people. Upon UVB exposure, a precursor molecule in the skin, 7-dehydrocholesterol, is converted into previtamin D3, which then thermally converts to vitamin D3. This vitamin D3 then circulates in the blood, where it is further processed by the liver and kidneys into its active form. However, a myriad of factors can disrupt this process, making consistent, natural production challenging and leading to widespread deficiency.

The Critical Role of Sunlight and Its Obstacles

In an ideal world, regular, sensible sun exposure would provide most of our vitamin D needs. However, modern life often presents significant roadblocks:

Geographic location: People living in northern latitudes receive less UVB radiation, especially during winter months, making natural synthesis difficult.
Lifestyle: Many individuals spend the majority of their time indoors due to work or other commitments, minimizing their sun exposure.
Sunscreen use: Correctly applied sunscreen with an SPF of 8 or more can significantly reduce vitamin D production in the skin. While crucial for skin cancer prevention, it also limits UVB absorption.
Protective clothing: For cultural, religious, or personal reasons, some people wear clothing that covers most of their body, blocking sunlight from reaching the skin.

Biological Barriers to Vitamin D Production

Beyond environmental factors, several biological and physiological characteristics can impact how efficiently our bodies produce and process vitamin D:

Skin pigmentation: Melanin, the pigment responsible for darker skin tones, acts as a natural shade, reducing the skin's ability to produce vitamin D from sunlight. This is why people with darker skin require significantly more sun exposure to produce the same amount of vitamin D as those with lighter skin.
Age: As we age, the skin's ability to synthesize vitamin D from sun exposure decreases. Furthermore, older adults are often less mobile or confined indoors, compounding the issue.
Obesity: Vitamin D is a fat-soluble vitamin. In individuals with a high body mass index (BMI), excess body fat can sequester the vitamin, trapping it away from the bloodstream and making it less bioavailable.

Dietary and Medical Causes of Vitamin D Deficiency

Even with adequate sun exposure, a person can become deficient due to problems with absorption and metabolism. This highlights the importance of dietary intake and the body's internal processing mechanisms.

Challenges with Dietary Intake

Few foods naturally contain high levels of vitamin D. While some foods are fortified, relying solely on diet is often insufficient for maintaining optimal levels. The primary natural dietary sources are fatty fish like salmon and mackerel, fish liver oils, and egg yolks. For those with dietary restrictions or specific preferences, getting enough can be particularly challenging.

Medical Conditions Affecting Metabolism

Several health conditions can impair the body's ability to absorb or convert vitamin D into its active form:

Fat malabsorption syndromes: Conditions such as Crohn's disease, celiac disease, and cystic fibrosis can compromise the small intestine's ability to absorb dietary fat, and thus, fat-soluble vitamins like D.
Liver and kidney disease: The liver and kidneys are essential for converting inactive vitamin D into its active form. Chronic kidney or liver disease can inhibit this process, leading to low active vitamin D levels.
Certain medications: Some drugs, including certain cholesterol-lowering medicines and anti-seizure medications, can interfere with vitamin D metabolism.
Post-bariatric surgery: Procedures that bypass parts of the small intestine can significantly reduce nutrient absorption, requiring lifelong supplementation.

Comparison: Sun vs. Supplement vs. Diet


Source	Pros	Cons
Sunlight	The most natural method; free and effective for most people under optimal conditions.	Affected by season, latitude, time of day, skin pigmentation, and sunscreen use. Increases risk of skin cancer from overexposure.
Supplements	Guarantees a specific, consistent dose; bypasses sun exposure and absorption issues in the gut.	Possibility of taking an incorrect dose; requires consistency; quality can vary; high doses can cause toxicity.
Diet	Provides vitamin D alongside other essential nutrients; can be part of a healthy diet.	Few natural food sources; quantity can be low; relying on fortified foods is not always consistent; affected by fat malabsorption issues.

Conclusion

The perception that our bodies naturally lack vitamin D is a misconception. Instead, vitamin D deficiency is a complex issue resulting from a combination of modern lifestyle factors and biological limitations that interfere with the body's natural production and utilization pathways. The shift from outdoor living to indoor occupations, combined with darker skin tones needing more sun, and medical conditions affecting absorption and metabolism, all contribute to this global health challenge. Thankfully, strategic use of supplements and being mindful of diet can help mitigate the risks. Regular conversations with a healthcare provider about potential deficiency and appropriate supplementation are key to maintaining optimal levels for bone health and overall well-being. For more information, you can refer to the National Institutes of Health's fact sheet on vitamin D.

Frequently Asked Questions

No, that's a common misconception. The human body does produce vitamin D3. It does so when the skin is exposed to sunlight's ultraviolet B (UVB) rays, triggering a synthesis process. The reason many people have low levels is that this process is often hindered by modern lifestyles and other factors.

Melanin, the pigment that determines skin color, acts as a natural sunscreen. The higher the melanin content in your skin, the more it blocks UVB radiation. Therefore, people with darker skin need significantly more sun exposure to produce the same amount of vitamin D as people with lighter skin.

For most people, it is challenging to get enough vitamin D from diet alone because few foods are naturally rich in it. Primary sources include fatty fish and fish liver oils. Many people rely on fortified foods or supplements to meet their needs.

Several medical conditions can cause or contribute to a deficiency. These include fat malabsorption disorders like Crohn's and celiac disease, obesity, and liver or kidney diseases that prevent the conversion of vitamin D into its active form.

No, you cannot get too much vitamin D from sunlight. Your skin has a built-in mechanism that regulates production, ensuring any excess is converted into biologically inactive compounds. Toxicity is almost always caused by excessive intake from supplements.

Even in sunny climates, you may need supplements if you have limited outdoor time, always wear sunscreen or protective clothing, have darker skin, or have a medical condition affecting absorption. It's best to check with a healthcare provider.

Many people with low vitamin D have no symptoms. However, some may experience vague symptoms like fatigue, bone pain, muscle weakness, and an increased risk of bone fractures. A blood test is the most accurate way to diagnose a deficiency.