6 winter vitamin D deficiency facts you need to know

6 things to know about vitamin D deficiency in winter

In the colder months, seeing the sun does not always mean enough vitamin D can be made; test results, diet and medical advice guide the decision on supplements.

HABER.NET · Newsroom • 29 August 2026 14:31 • 2 min read AI

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Why does vitamin D come up more often in winter?

Vitamin D deficiency is talked about more often during periods when sunlight exposure decreases. Vitamin D status is assessed with a blood test for 25-hydroxy vitamin D (25(OH)D), but diagnosis, follow-up and the decision to use supplements should be made by a doctor on an individual basis.

Because Türkiye lies roughly between 36 and 42 degrees north latitude, the angle of the sun from November to February significantly reduces UVB-driven vitamin D synthesis in the skin. For that reason, winter sunlight alone may not be enough for everyone.

6 things you should know about vitamin D deficiency

Winter sunlight is not always enough. Being outdoors is beneficial for overall health, but vitamin D production in winter may not be as effective as it is in the summer. For that reason, relying only on sun exposure is not a sound approach.
How should a 25 OH vitamin D level be interpreted? According to the Endocrine Society thresholds commonly used in clinical practice, levels below 20 ng/mL are considered deficient, 20-30 ng/mL insufficient, and above 30 ng/mL sufficient. Even so, results should be evaluated together with age, symptoms and any accompanying diseases.
Glass and clothing can limit synthesis. Getting daylight and exposing the skin directly to UVB are not the same thing. Thick clothing, spending time behind glass and using sunscreen are among the factors that can reduce synthesis in the skin.
Foods help, but may not be enough on their own. Oily fish, eggs and some vitamin D-fortified products can provide support. However, meeting the full daily requirement through food alone is not always easy.
The Ministry of Health has been providing free 400 IU daily vitamin D drops to all babies under one year old since May 2005. The Preventing Vitamin D Deficiency in Infants and Protecting Bone Health Programme aims to prevent rickets and support bone health; the ministry says it has provided support to nearly 15 million babies to date.
High-dose supplements should not be taken without medical advice. Vitamin D is a fat-soluble vitamin and can accumulate in the body. That is why using high-dose vitamin D supplements without a doctor’s recommendation can create a risk of poisoning.

Why is the decision to take a supplement individual?

The decision to use a vitamin D supplement can vary depending on age, pregnancy, infancy, chronic illnesses and the medicines being used. Rather than interpreting a value seen online on its own or using a dose recommended for someone else, it is safer to consult a family doctor or the relevant specialist.

Regular monitoring is important for bone health across childhood, adulthood and older age, especially when it comes to vitamin D for babies. For people who experience fatigue, muscle or bone complaints in winter, the best approach is to consider the laboratory result together with the clinical assessment.

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