Vitamin D deficiency primarily arises from insufficient exposure to sunlight and inadequate intake from dietary sources. It can also result from malabsorption in the intestines, chronic illnesses that interfere with synthesis processes in the body, and increased requirements due to certain medications. Since vitamin D is essential for bone tissue, its deficiency leads to rickets—a bone disease in children—and osteomalacia, a disorder in bone formation and mineralization in adults. Symptoms in children include delayed walking, frequent sitting, falling tendencies, and bowing of the legs. In osteomalacia, chronic muscle and bone pain are common. Other signs of vitamin D deficiency include muscle weakness, chronic fatigue, a tendency toward depression, and increased sweating.
Vitamin D exists in two forms: D2 and D3. D3 is synthesized in the skin through exposure to sunlight and can also be obtained through the diet, especially from fatty fish. D2 is produced when ergocalciferol found in yeast and mushrooms is exposed to sunlight and is consumed in food by humans. The vitamin D produced in the skin through sunlight is more active in the body.
Approximately 90% of vitamin D is synthesized in the skin with exposure to sunlight, while 10% comes from dietary sources. Foods such as fatty fish, dairy products, and eggs are beneficial for dietary vitamin D intake. For the synthesis of vitamin D in the skin, exposing 25% of the body to sunlight for 15 minutes three days a week is sufficient. Less vitamin D is synthesized in darker skin compared to lighter skin. UVB rays do not pass through glass, and excessive exposure to UVA and UVB rays poses a risk of skin cancer. Sunscreen creams block UVB penetration, thereby inhibiting vitamin D synthesis.
Vitamin D deficiency is observed not only in cases of insufficient sunlight exposure but also in malabsorption conditions such as celiac disease and short bowel syndrome. Vegans (those who do not consume meat or any other animal-derived foods) are also prone to vitamin D deficiency due to inadequate dietary intake. It is known that long-term use of certain medications (such as epilepsy drugs and some tuberculosis and antifungal medications) accelerates the breakdown of vitamin D in the body, leading to deficiency. Liver and kidney failures cause deficiency due to impaired synthesis in the later stages of vitamin D production.
Although it is a vitamin, vitamin D also functions in the body like a hormone. Its primary known role is to facilitate the absorption of calcium and phosphorus—essential minerals for bone tissue—from the intestines. In the absence of vitamin D, the absorption of dietary calcium from the intestines decreases significantly. Another effect of vitamin D on bone metabolism is the reabsorption of calcium from bones, maintaining a steady blood calcium level, and promoting the development and maturation of bone cells responsible for bone renewal.
Vitamin D has effects beyond bones and acts on various tissues throughout the body. While directly and indirectly regulating bone and mineral metabolism, it also plays a crucial role in immune function, cell growth, maturation, cell cycle, and the secretion of hormones such as insulin and renin. Its deficiency, which affects cardiovascular, hematological, immunological, hormonal, and metabolic systems, needs to be managed carefully to prevent a broad spectrum of health issues, ranging from cardiovascular diseases to cancers. Recurrent infections and prolonged recovery times should prompt consideration of vitamin D deficiency. In patients with low CD8-type T lymphocytes (critical immune system cells), vitamin D deficiency is associated with increased susceptibility to various cancers, multiple sclerosis (an autoimmune inflammatory disease of the brain and spinal cord), rheumatoid arthritis, and other autoimmune diseases. Autoimmune diseases occur when the body develops an immune response against its own tissues.
There is a close relationship between low levels of vitamin D and high blood pressure as well as cardiovascular diseases. Calcium levels maintained by vitamin D in the blood and tissues are vital for the blood vessel walls, heart muscles, and nerve transmission system.
Studies show that vitamin D deficiency is more commonly associated with increased visceral fat. The impact of vitamin D deficiency on impaired blood sugar regulation, insulin secretion, and insulin resistance, as well as a predisposition to obesity, has also been demonstrated. Exposure to vitamin D deficiency in the early stages of life increases the risk of developing type 1 diabetes in children and young people due to its effects on the immune system and pancreatic cells.
Vitamin D deficiency is one of the physiological and external causes of psychiatric disorders and has been observed to facilitate the development of schizophrenia and Alzheimer's disease and play a role in the onset and recurrence of depression. Unfortunately, vitamin D levels are generally low. Compared to previous years, when natural food sources were more available, and more time was spent outdoors in contact with sunlight, our need for vitamin D today has increased.
Considering all this information, vitamin D deficiency facilitates the onset of many diseases that can threaten human life and may require lifelong treatment. Therefore, educational and nutritional programs should be organized to prevent vitamin D deficiency, and additional daily supplements should be taken as needed based on age and circumstances that may lead to deficiency. Daily vitamin D needs vary according to age and personal conditions. To ensure sufficient blood levels of vitamin D for bone health and protection from the diseases, levels above 30 ng/ml are recommended, which can only be achieved with a daily intake of over 1,000 IU of vitamin D. There are also situations where the need for vitamin D increases. Pregnant women, breastfeeding mothers and their babies, postmenopausal women (especially to prevent bone fractures), elderly individuals (due to weakened cardiovascular and immune systems), those with heavy work conditions who cannot get enough sunlight, individuals with limited access to sufficient food sources (such as fatty fish and dairy products), those with chronic illnesses, obese individuals, patients with impaired vitamin D absorption and synthesis, and those experiencing increased vitamin D elimination due to medication use should be supported with vitamin D.
