Why do some people spend time in the sun, eat relatively well, and still end up with low vitamin D levels?
Why can two people with similar lifestyles have completely different results on their blood tests?
And how much does sunlight actually help?
The answer is more complex than it seems.
Vitamin D is one of the few nutrients that the human body can produce on its own, yet it remains one of the most common deficiencies worldwide. While it is often associated simply with sun exposure, the reality is that vitamin D levels depend on a combination of biological processes and individual factors.
In the body, vitamin D functions more like a hormone than a typical vitamin. It plays a key role in calcium and phosphorus absorption, supports bone health, contributes to normal muscle function, and is involved in immune system regulation.
How the Body Produces Vitamin D
When the skin is exposed to ultraviolet B (UVB) radiation from sunlight, a compound derived from cholesterol is converted into vitamin D3 (cholecalciferol). However, this form is not yet active.
To become usable, vitamin D must go through two important steps. First, it is processed in the liver into 25-hydroxyvitamin D, which is the form measured in blood tests. Then, it is converted in the kidneys into its active form, known as calcitriol.
This means that sunlight exposure alone is not always enough, as the body must also be able to properly process and activate vitamin D.
Does Sun Exposure Help?
Yes, but not under all conditions.
Vitamin D production depends on UVB rays, which are most effective between approximately 10:00 a.m. and 4:00 p.m., especially during the warmer months. In winter, particularly in regions farther from the equator, the angle of the sun reduces UVB exposure to the point where the body produces little to no vitamin D.
Several factors can also limit this process:
- sunscreen significantly reduces vitamin D production
- UVB rays do not pass through glass, so indoor sunlight does not help
- air pollution can decrease the intensity of effective sunlight
For many people, around 10 to 30 minutes of sun exposure on uncovered skin (such as the face and arms) may be sufficient, but this varies widely.
Why Some People Produce or Absorb Less Vitamin D
Not everyone produces or uses vitamin D in the same way, and several factors influence this.
Skin pigmentation plays an important role. Individuals with darker skin have higher levels of melanin, which reduces the skin’s ability to produce vitamin D, meaning they may require longer sun exposure to generate the same amount.
Age is another factor, as the skin becomes less efficient at producing vitamin D over time, and the body’s ability to activate it may also decline.
Body composition matters as well. Because vitamin D is fat-soluble, it can be stored in body fat, making it less available for use in individuals with higher body fat levels.
Digestive conditions, such as celiac disease or inflammatory bowel diseases, can impair the absorption of vitamin D from food and supplements.
Liver and kidney function are essential for converting vitamin D into its active form, so any impairment in these organs can affect vitamin D status.
In addition, genetic differences can influence how vitamin D is transported and utilized in the body, which helps explain why some individuals have low levels despite similar lifestyles.
Dietary Sources of Vitamin D
Diet can contribute to vitamin D intake, but it is usually not enough on its own to meet daily requirements.
Natural sources include:
- fatty fish such as salmon, mackerel, and sardines
- cod liver oil
- egg yolks
- fortified dairy products
- certain UV-exposed mushrooms
Even with these foods, dietary intake alone is often insufficient compared to what the body can produce with adequate sun exposure.
How Much Vitamin D Do We Need?
Recommended daily intake varies depending on age, but general guidelines suggest:
- about 600–800 IU per day for adults
- around 800–1000 IU per day for older adults
In practice, individual needs may differ, especially in cases of deficiency. The most reliable way to assess vitamin D status is through a blood test measuring 25-hydroxyvitamin D.
Why Vitamin D Matters
Vitamin D deficiency is not limited to bone health.
It has been associated with:
- increased risk of osteoporosis
- muscle weakness
- persistent fatigue
- higher susceptibility to infections
Ongoing research continues to explore additional connections, although not all findings are fully established.
Conclusion
Vitamin D levels are influenced by more than just time spent in the sun. They reflect a balance between sun exposure, the body’s ability to produce and activate the vitamin, dietary intake, and individual factors.
This is why two people can have similar habits and very different results.
Sunlight helps, but it is not always enough.
And sometimes, even when we feel we are doing everything right, the body may still need additional support.