What Is Vitamin D?
Although classified among the fat-soluble vitamins, vitamin D behaves more like a hormone than a classic vitamin. The fact that the body doesn't have to obtain it from outside sources — it can produce it in the skin under the right conditions, with production rising and falling according to blood levels — is one of the traits that marks vitamin D as a steroid-based molecule that acts hormonally. Its main job is to tightly regulate calcium and phosphorus balance in the circulation; its principal target tissues are the intestine, kidneys, and bone.
For this reason, the earliest and best-documented consequences of vitamin D insufficiency relate to the skeletal system: osteomalacia, the impaired bone mineralization seen in adults, is classically attributed to vitamin D deficiency. Low vitamin D levels have also been linked to numerous conditions outside the skeleton, though how many of these associations are truly causal remains an active area of scientific debate.
How does the body produce vitamin D?
Most vitamin D synthesis begins when the sun's ultraviolet B (UVB) rays reach the skin. This process, which starts in the skin, is completed through further steps in the liver and kidneys, converting the vitamin into its active form. The value measured in blood tests is generally 25-hydroxyvitamin D, or 25(OH)D, which reflects the body's stores.
How Widespread Is the Problem?
Vitamin D insufficiency is recognized as a global public health issue. A large pooled analysis combining population-based studies published between 2000 and 2022, covering more than 7.9 million participants, concretely illustrates the scale of the problem. The analysis reported that roughly 15.7% of participants had serum 25(OH)D levels below 30 nmol/L, 47.9% below 50 nmol/L, and 76.6% below 75 nmol/L. In other words, while "severe deficiency" affects a relatively smaller share, falling below what is considered the "optimal" threshold is the rule for most of the world's population.
Türkiye is no exception to this picture. Factors such as geographic location, season, age, skin tone, sunscreen use, clothing choices, spending long periods indoors, and air pollution are cited as contributing to the widespread prevalence of vitamin D deficiency in the country as well.
How are blood values interpreted?
In the classification commonly used in practice, a 25(OH)D level below 20 ng/mL is defined as "deficiency," 20–29 ng/mL as "insufficiency," and above 30 ng/mL as "sufficient." However, it is emphasized that there is no full consensus on this deficiency/insufficiency terminology; the Endocrine Society's 2024 guideline states that specific 25(OH)D thresholds defining sufficiency, insufficiency, and deficiency are no longer endorsed. For this reason, a single lab result should not be interpreted in isolation but evaluated together with a person's age, risk group, bone health, and clinical findings.
Symptoms: Subtle and Easily Mistaken for Something Else
Vitamin D deficiency usually progresses silently; symptoms build up gradually over time and can be difficult to notice because they are easily confused with other health problems. The most frequently reported findings in clinical sources include:
- Persistent fatigue and low energy: People may describe exhaustion severe enough to interfere with daily tasks.
- Muscle weakness and cramps: Cramps, especially in the legs, are commonly reported.
- Bone and joint pain: Widespread bone pain is one of the most typical complaints of deficiency.
- Frequent infections: A weakened immune system can lead to recurrent infections.
- Reduced bone mineral density: Long-term deficiency can pave the way for serious bone disorders.
Low vitamin D levels are also said to potentially have negative effects on the immune system, nervous system, and mood, in addition to bones and muscles. However, none of these symptoms are specific to vitamin D deficiency; similar complaints also occur in anemia, thyroid disorders, sleep problems, and depression, among many other conditions. This is why the diagnosis should never be made on one's own: anyone experiencing persistent fatigue or reduced immunity is advised to have their vitamin D level checked under a doctor's supervision.
Who Is at Risk?
The risk of deficiency is not the same for everyone. Insufficiency is more common among age groups with limited sun exposure and a reduced capacity to produce vitamin D. Skin's synthesis capacity declines with age; for the same amount of time in the sun, older adults produce less vitamin D than younger people. Skin tone also plays a determining role: the melanin in darker skin filters out more UVB rays, slowing synthesis.
Living in regions with limited sunlight and the winter months are also environmental factors that increase risk. This is more pronounced for people living in northern countries or in cities where tall buildings cast constant shade. Certain digestive conditions that cause malabsorption, as well as liver and kidney diseases, also raise the risk of deficiency.
Sources: Sun, Food, and Supplements
Sun
The amount of sun exposure needed for vitamin D synthesis cannot be reduced to a single number; skin type, age, clothing choices, season, and geography all change this duration. The amount of body surface exposed to the sun also affects the outcome. Experts note that, in terms of synthesis, the most efficient exposure is short, frequently repeated sessions around midday, when UVB levels peak. On the other hand, exposing the arms and legs to the sun a few times a week during peak sun hours is said to be sufficient. Balance matters here: excessive, unprotected exposure is not recommended because of skin cancer risk — the goal is not to get a tan, but brief and regular exposure.
Food
Vitamin D obtained through diet alone is generally not enough to meet daily needs, but good sources are well known. Fatty fish such as salmon, herring, mackerel, tuna, trout, and sardines stand out as prominent sources; cod liver oil is cited as one of the richest sources. Among shellfish, oysters and shrimp are relatively good sources. Egg yolks and fortified dairy products also contribute.
Supplements and safety
Confusion around units is common: 1 microgram (mcg) of vitamin D equals 40 international units (IU). The commonly cited recommended range for adults is 400–800 IU/day; based on assessments by the US Institute of Medicine, this range is said to be sufficient for the vast majority of people. When insufficiency or deficiency is detected, the dose can be increased under a doctor's supervision.
However, because vitamin D is fat-soluble and stored in the body, the "more is better" approach does not apply. Long-term, unsupervised use of high doses can raise blood calcium levels and cause organ damage, particularly to the kidneys. For this reason, the decision to supplement — along with the dose and duration — should be determined by a physician based on blood values and clinical status.
Conclusion
Vitamin D is one of the rare nutrients that directly conflicts with the indoor-centered rhythm of modern life: it comes largely from the sun, yet is easily depleted by a lifestyle that keeps us away from it. Global data show that low levels are the rule, not the exception. Still, the solution is not unsupervised high-dose supplements or excessive sun exposure, but balanced, brief sun exposure, regular consumption of vitamin D–rich foods, and doctor-supervised testing and supplementation for at-risk groups.
Sources
- Global and regional prevalence of vitamin D deficiency in population-based studies from 2000 to 2022: A pooled analysis of 7.9 million participants – Frontiers in Nutrition
- Vitamin D Synthesis, Metabolism and Functions (Review) – DergiPark
- Prevalence and Progression of Vitamin D Deficiency in Greater Beirut and Mount Lebanon From 2013 to 2022 – PMC
- What Are the Symptoms of Vitamin D Deficiency? What Foods Contain Vitamin D? – Acıbadem
- Vitamin D Deficiency: Symptoms, Causes and Treatment Methods – Medipol Health Group
- Causes, Symptoms and Treatment of Vitamin D Deficiency – Liv Hospital
- How Can Vitamin D Levels Be Raised? Which Foods Contain Vitamin D? – Medicana
- What Is the Vitamin D Reference Range? – Tüketici Sağlığı
- How Much Sunlight Do We Need? – Cumhuriyet
- Vitamin D and Sun Exposure – Etkin Eczacılık