Why Is Vitamin D Important?

Dr. Desislava Ivanova answers the most important questions about vitamin D intake and deficiency — a topic that often becomes the subject of speculation during the winter months.

Why is it important to monitor vitamin D levels, especially in winter?

Vitamin D is produced in the skin under the influence of UVB rays. The penetration of UVB radiation decreases in poor weather conditions, when using SPF sunscreens, or in people with darker skin. Effective absorption depends on the angle of sunlight — the higher the sun, the more UVB rays are absorbed. During winter, the sun is lower, which means less UVB absorption and therefore reduced vitamin D synthesis.
Regardless of the season, certain groups are at higher risk of deficiency — infants and young children, elderly people (whose ability to synthesize vitamin D declines with age), individuals with obesity, pregnant women, and those who deliberately avoid sun exposure.

How can we tell if we suffer from vitamin D deficiency?

The first signs are often muscle and joint pain, as vitamin D plays a crucial role in calcium absorption. Its deficiency can lead to pain and stiffness, disrupting daily life.

Other possible symptoms include:
- Loss of bone density – leading to thinning and fragility of bones, and increased fracture risk.
- Muscle weakness – due to limited calcium absorption, often accompanied by fatigue and dizziness.
- Chronic tiredness – a feeling of exhaustion even after sleep and rest.
- Depression – negative thoughts, anxiety and apathy, especially common during the darker months.
- Hair loss – low vitamin D levels affect hair growth and strength, causing thinning and shedding.
- Weakened immunity – frequent infections and susceptibility to illness may indicate a serious deficiency that, if untreated, can lead to chronic inflammation or more severe diseases.

What tests are used to diagnose deficiency?

Vitamin D deficiency is diagnosed through a blood test measuring 25-hydroxyvitamin D (25(OH)D) — the most accurate indicator of vitamin D status. Blood is drawn in the morning on an empty stomach, and ideally after at least 10 days without taking vitamin D supplements.

What happens if the deficiency goes undetected for a long time?

The active form of vitamin D3 (calcitriol) plays a vital role in calcium-phosphorus balance, crucial for bone formation and maintenance. Deficiency may lead to bone diseases such as osteoporosis or rickets.

Vitamin D3 also supports various physiological functions, regulating mineral metabolism and influencing many tissues.

- Bone health:
Vitamin D3 helps form the bone matrix and maintain mineralization.
- Immune system:
It modulates immune responses and reduces inflammation. Deficiency is associated with increased susceptibility to infections.
- How can we get enough vitamin D?
There are two main sources — food and sun exposure.

Mushrooms are a major plant-based source, producing ergocalciferol (D2) when exposed to sunlight. Just 50 grams of maitake mushrooms can provide nearly the full daily requirement. Shiitake, button, porcini, and chanterelle mushrooms are also rich in D2.

Vitamin D3 occurs in certain algae and lichens, as well as in eggs, liver, salmon, fatty fish, and cheese.

Many plant-based milks, tofu, cereals, orange juice, and dried fruits are fortified with vitamin D — including them in your diet can help improve your levels. Spending 15–30 minutes in sunlight daily is another natural and pleasant way to increase vitamin D.

However, caution is needed — avoid midday sun to prevent skin burns. Sunscreens significantly reduce vitamin D synthesis, so short, controlled exposure is best.

- How should vitamin D be taken, and for how long?

If not otherwise prescribed by a doctor:
- Infants and children: 400–600 IU daily (up to 1000–3000 IU in therapeutic cases).
- Adults: 600–800 IU for maintenance, up to 4000 IU per day if treating deficiency.
Active supplementation is recommended in winter, after testing your vitamin D levels.

Can vitamin D deficiency cause weight gain?

Not directly, but people with obesity tend to have lower circulating vitamin D levels because the vitamin is fat-soluble and gets stored in body fat, reducing its availability.

In all cases, anyone suspecting deficiency should consult a healthcare professional for proper diagnosis and dosage.