Vitamin D (also known as calciferol) is a fat-soluble vitamin naturally present in some foods, added to others, and available as a dietary supplement. It is an essential nutrient for the body, playing key roles such as maintaining bone health, supporting immune system function, and regulating cell growth. Often referred to as the 'sunshine vitamin,' vitamin D is produced by the skin when exposed to sunlight. It is also found in foods such as fish oil, dairy products, and egg yolks.

Vitamin D exists in two main forms, D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D2 is obtained from the diet, mostly from fungi but also from plants, and is often used in supplements. Fish such as trout, salmon, tuna, and mackerel can also provide some of the vitamin D needed. Vitamin D3 can be obtained from the diet (from animal products) or synthesized in the skin by converting the cholesterol precursor 7-dehydrocholesterol after exposure to sufficient sunlight, i.e. ultraviolet B (UVB) radiation.

Sun exposure for vitamin D synthesis can only be effective when the angle of the sun's rays is greater than 45°. As a result, those living in the northern hemisphere do not get enough vitamin D through skin synthesis during the winter months, and in some northern regions, low sun exposure can last up to 6 months of the year. To increase vitamin D intake, some countries have implemented a policy of fortifying dairy products and margarine with vitamin D. At the same time, using light bulbs for artificial UVB exposure is another way to increase vitamin D synthesis.

The main source of vitamin D for humans is from 7-dehydrocholesterol, the precursor of vitamin D synthesis in the skin from sunlight (UVB rays), while natural food sources of vitamin D (e.g. fish or mushrooms) play only a minor role in overall vitamin D intake.

3.1. Bone health

Vitamin D improves the absorption of calcium and phosphorus from the intestines, promoting the development, strengthening and protection of bones and teeth.

In combination with calcium, it helps protect older adults from osteoporosis.

Maintains adequate serum calcium and phosphate concentrations to prevent hypocalcemic tetany (involuntary contraction of muscles, leading to cramps and spasms) by increasing intestinal calcium absorption. It is also required for bone growth and bone remodeling by osteoblasts and osteoclasts.

3.2. Muscle function

Vitamin D has an important role in muscle health. Recent studies show that this vitamin helps maintain and regenerate muscle tissue. Vitamin D contributes to the muscle repair process through receptors found in muscle cells. It also has positive effects on mitochondria's health, which support muscle tissue regeneration after muscle damage. 

Some studies on middle-aged and elderly individuals show that vitamin D supplementation can increase muscle strength. Significant improvements in muscle strength were seen, especially in the lower limbs. However, the effects on muscle strength may vary depending on individual factors, including physical activity level, vitamin D status, and type of exercise.

3.3. Immune system

Vitamin D is an important immune system regulator that influences the immune system in both innate and adaptive immune processes

Vitamin D specifically regulates T and B cells. It suppresses T cell proliferation and reduces the production of pro-inflammatory cytokines. This has an overall anti-inflammatory effect.

In B cells, it suppresses immunoglobulin production and B cell proliferation, which is important in reducing the risk of autoimmune diseases. It also acts on macrophages and regulates their cytokine production and cell renewal processes.

3.4. Cardiovascular health

Vitamin D deficiency is closely associated with cardiovascular diseases, especially coronary artery disease, heart failure, and atrial fibrillation. 

Low levels of vitamin D are linked to cardiovascular problems such as arterial hardening, hypertension, and atherosclerosis. It has also been observed that patients with vitamin D deficiency have worse prognoses, with an increased frequency of heart attacks and other cardiovascular events.

However, the effect of vitamin D supplements on heart health is still controversial. Although some studies suggest that vitamin D supplementation reduces cardiovascular risks, several clinical trials do not support this association. This is because the complex interactions between vitamin D metabolism and cardiovascular effects are not fully understood. In summary, vitamin D deficiency may adversely affect cardiovascular health, but more clinical research is needed.

3.5. Its effects on cancer

According to previous research, vitamin D may have both protective and therapeutic effects on cancer. Vitamin D deficiency has been linked to several types of cancer, particularly breast, colon, prostate, and lung cancer. Adequate vitamin D levels have been linked to lower mortality rates and improved survival rates in some types of cancer. Vitamin D supplementation in particular has been observed to reduce cancer recurrence and mortality in some patients, but there are inconsistent results. Therefore, more clinical research is needed.

3.6. Mental health

Vitamin D has an important role in mental health. Studies show that vitamin D deficiency is associated with depression and that adequate vitamin D levels can support mental health. Over the last 5 years, studies have found that vitamin D supplementation can help alleviate symptoms of depression, with particularly positive effects in older adults and individuals prone to depression.  For example, one study showed that individuals taking vitamin D supplements experienced a significant reduction in symptoms of depression. These effects were specifically associated with a reduction in pro-inflammatory biomarkers (such as IL-6, and IL-1β), as these inflammation molecules have been linked to depression.

In addition, another study in older adults found that individuals with vitamin D deficiency had a higher risk of depression. These studies suggest that vitamin D plays an important role in regulating brain function and mood.

As a result, vitamin D supplementation may have positive effects on reducing symptoms of depression and improving overall mental health, especially in deficient individuals.

3.7. Its effect on metabolism

Vitamin D is a vitamin that has important effects on metabolism. Recent studies have shown that vitamin D can reduce the risk of metabolic syndrome, insulin resistance, and type 2 diabetes. Especially in pre-diabetic patients, vitamin D supplementation has been observed to reduce progression to type 2 diabetes by 10%. 

In addition, vitamin D deficiency has been associated with hypertension, obesity, and dyslipidemia, components of the metabolic syndrome. Adequate vitamin D levels may provide positive effects on overall metabolic health and mitigate such risk factors.  In particular, vitamin D metabolites are thought to influence mechanisms such as blood glucose regulation and insulin sensitivity. However, more intervention studies are needed to fully prove these associations in a cause-and-effect manner. 

Vitamin D deficiency can occur when the body does not produce enough vitamin D, does not get enough sunlight, or does not get enough vitamin D from the diet. Vitamin D insufficiency or deficiency has been described as very common condition in the general population and some authors use the terms “vitamin D deficiency epidemic or pandemic”:

Symptoms:

Bone and muscle pain: Vitamin D deficiency affects the absorption of calcium, which can lead to muscle weakness and bone pain.

Fatigue and weakness: Low vitamin D levels are associated with a lack of energy and can cause fatigue and weakness.

Frequent infections: Vitamin D helps the immune system to function properly; a deficiency can cause you to get sick more often.

Depression and mood disorders: Several studies suggest a link between low vitamin D levels and depression.

Bone loss (osteoporosis) and fracture risk: Vitamin D deficiency can reduce bone density, especially in older people, which increases the risk of fractures.

Risks:

Obesity and some chronic diseases: Obesity causes vitamin D to become trapped in fat cells, which lowers its circulating levels. In addition, certain medical conditions such as Crohn's disease, celiac disease, and kidney failure also negatively affect the body's absorption of vitamin D.

Age and dark skin color: As we age, the skin loses its capacity to produce vitamin D from sunlight. In addition, dark-skinned individuals have more difficulty producing vitamin D due to the excess melanin pigment.

Prevention and treatment of vitamin D deficiency is usually achieved through adequate sunlight exposure and dietary supplements. Exposure to UV-B rays is especially important to promote vitamin D synthesis through the skin. In addition, vitamin D supplements are recommended, especially for high-risk groups.

Over the last 5 years, studies have shown that low vitamin D levels are associated with autoimmune diseases, infections, and increased susceptibility to respiratory diseases such as COVID-19. Vitamin D supplementation in particular has been found to help reduce inflammatory markers and balance immune system responses.

Prevention and Treatment Methods:

Exposure to sunlight: Getting 15-30 minutes of sunlight at least 2-3 times a week, especially in summer, is effective in increasing vitamin D levels. However, over-exposure should be avoided, taking into account the risks of sunlight.

Dietary supplements: Foods such as oily fish, egg yolks, and fortified dairy products can contribute to vitamin D intake, but the amount from food is usually limited.

Vitamin D supplements: Table 2 shows the daily amounts of vitamin D that the Food and Nutrition Board (FNB) of the American National Academies of Sciences, Engineering, and Medicine has determined to be adequate daily intakes to maintain bone health and normal calcium metabolism in healthy individuals.

Age group

Recommended daily amount of vitamin D

Babies under 1 years old

10 microgram (mcg) (equivalent to 400 international units (IU))

1-70 years old

15 mcg (600 IU)

70 years old and older

20 mcg (800 IU)

These methods can contribute to bone health as well as support the functioning of the immune system. People at high risk (the elderly, people who are not exposed to enough sunlight and pregnant women) should have their vitamin D levels checked regularly.

The biggest risk of excessive vitamin D intake is the abnormal increase in blood calcium levels (hypercalcemia) and the associated health problems. Vitamin D toxicity typically occurs with the consumption of very high doses of supplements, as it is not caused by sunlight exposure or dietary intake.

Symptoms include nausea, vomiting, loss of appetite, constipation, excessive thirst, frequent urination, muscle weakness, fatigue, and kidney stone formation. Prolonged high calcium levels can lead to more severe complications such as kidney damage and bone pain. Neurological effects like confusion, drowsiness, and even coma may occur. In the cardiovascular system, high blood pressure and arrhythmias may develop.

To prevent vitamin D toxicity, it is essential to take supplements under medical supervision and monitor vitamin D levels through blood tests.