Why Is Vitamin D Important?

Vitamin D supports bone density, growth, dental health, and muscle strength by increasing the absorption of calcium and phosphorus in the body. It also regulates immune functions and may play a protective role against infections.
In childhood, inadequate vitamin D can lead to serious consequences such as rickets, muscle weakness, and growth retardation. Even in countries with abundant sunlight, like Türkiye, vitamin D deficiency is common in children—meaning the belief that “sun exposure alone is sufficient” may not always hold true.
During summer, people often assume that increased sun exposure naturally meets vitamin D needs. However, in children, factors such as time of day, skin tone, clothing, and sunscreen use can significantly affect vitamin D synthesis. Additionally, there is uncertainty about the body’s capacity to store vitamin D and how long these stores remain adequate.
In this article, we examine the question “Is sun exposure enough?” from a scientific perspective.

1. Vitamin D Production from Sunlight

When skin is exposed to UVB radiation, 7-dehydrocholesterol is converted to pre-vitamin D₃, which is then activated into vitamin D.
However, this conversion is significantly influenced by clothing, cloud cover, air pollution, season, geographic latitude, skin color, and sunscreen use. For example, dark-skinned children require 3–5 times longer sun exposure than light-skinned children to produce the same amount of vitamin D.
Sunscreens in spray or cream form with SPF above 30 can block up to 95% of UVB rays, greatly reducing vitamin D synthesis. In real-life use, incomplete application may allow for minimal synthesis, but a significant decrease still occurs in children using sunscreen.
For these reasons, the approach of “playing outdoors is enough” is neither reliable nor sufficient.

2. Is Summer Sun Enough? Findings in Türkiye and Children

Despite abundant sunlight in southern latitudes like Türkiye, vitamin D deficiency remains common in children.
In one study of children aged 6–9 years, at the end of summer 3.4% had deficiency and 27.7% had insufficiency. This indicates that even during the sun-rich summer season, reaching adequate levels can be challenging.
A study in Italy similarly reported that even children with at least 5 months of annual sun exposure could still have low vitamin D levels.
These findings suggest that, even in summer, sun exposure alone may not be sufficient for some children, and supplementation and monitoring may be required.

3. Daily Sun Exposure Duration and Effectiveness

As a general guideline, minimal erythema dose exposure—approximately 10–20 minutes, 2–3 times per week—with face, arms, and legs uncovered, can be sufficient for vitamin D synthesis.
However, achieving this depends on balancing factors such as the optimal time of day (10:00–16:00), skin type, and local UV index.
According to a 2021 model, in locations like Türkiye, 30–40 minutes of UVB exposure at specific times of day may ensure sufficient vitamin D production for children. Still, this duration is not universal—urban air pollution, for example, can reduce UVB intensity even on seemingly sunny days.
It should also be remembered that UVA rays in sunlight are carcinogenic.

4. Comparison: Sunlight vs Vitamin D Supplementation

A recent randomized controlled trial found that giving breastfed infants 400 IU of vitamin D₃ daily was significantly more effective than weekly sun exposure alone.
In the sun exposure group, only 13.9% of infants achieved adequate vitamin D levels, compared to 35.1% in the supplementation group.
This demonstrates that, particularly for infants and young children, ensuring adequate sun exposure can be challenging.
Factors such as sleep schedules, frequency of outdoor activities, and time spent indoors in daycare or hospitals mean oral supplementation is generally a more reliable method.

5. Practical Recommendations and Considerations

Who Should Receive Supplements?
Even in sunny weather, infants, dark-skinned children, those who wear concealing clothing, and those who spend extended time indoors should have their vitamin D levels tested.
Based on results, supplementation can be initiated under medical supervision. The risk of deficiency increases during seasonal transitions, especially towards the end of winter (October–May).

Dosage and Duration
For children, the generally recommended daily dose of vitamin D is 400–600 IU, regardless of season.
Under medical guidance, serum 25-hydroxyvitamin D₃ levels can be measured to determine whether continuous or seasonal supplementation is required.

Sun Protection and Skin Health
In summer, it is also important to protect against excessive sun exposure: prolonged sunbathing increases cancer risk.
Therefore, sun exposure should be balanced with protection—limiting exposure time, and using sunscreen, hats, shade, and protective clothing—while still benefiting from its vitamin D effects.

Conclusion: Is Sun Enough, and When Is Supplementation Necessary?

  • Sunlight is the most natural source of vitamin D, but many factors can make it insufficient on its own.
  • Even in sunny regions like Türkiye, nearly 30% of children have vitamin D insufficiency at the end of summer.
  • Oral vitamin D₃ supplementation, especially during infancy and early childhood, can be more effective than sun exposure alone.
  • Considering skin type, lifestyle, seasonal changes, and safe sun exposure practices, a combination of regular sun exposure and supplementation is often the most reliable approach.
  • In short, moderate sun exposure is beneficial, but often not enough. A scientifically guided plan, in consultation with a healthcare provider, is the safest and most effective way to ensure adequate vitamin D for children.

References
Erol, M. (2015). Vitamin D Deficiency in Children and Adolescents: Seasonal Levels in Türkiye. Journal of Pediatric Endocrinology & Metabolism. https://pubmed.ncbi.nlm.nih.gov/26316436/
Goyal, A., et al. (2022). Sun Exposure vs Oral Vitamin D Supplementation for Breastfed Infants: Randomized Controlled Trial. Indian Pediatrics, 59(11), 835–836. https://pubmed.ncbi.nlm.nih.gov/36148748/