Vitamin D – Functions, Sources

Vitamin D, often known as the “sunshine vitamin,” is one of the most vital nutrients for maintaining overall health and well-being. Unlike many other vitamins that must be obtained primarily from food, Vitamin D has a unique feature, it can be synthesized naturally in the skin when exposed to sunlight.

Over the years, Research has highlighted the importance of Vitamin D far beyond its traditional link to strong bones and teeth. It is essential for the absorption of calcium and phosphorus, two minerals that are crucial for bone development and strength. At the same time, it supports the immune system, muscle function, and even mental health. A growing body of scientific evidence suggests that adequate Vitamin D levels may reduce the risk of certain chronic diseases, infections, and age-related conditions.

Vitamin D deficiency is a global health concern affecting people across different age groups and regions. Factors such as limited sun exposure, indoor lifestyles, air pollution, darker skin pigmentation, and poor dietary intake have made deficiency increasingly common. This has brought Vitamin D into the spotlight of medical discussions, emphasizing its role not only in preventing rickets and osteoporosis but also in supporting long-term health.

What is Vitamin D?

Vitamin D is a fat-soluble vitamin that plays a crucial role in maintaining health. Unlike many other vitamins that must be obtained entirely from food, Vitamin D has a distinctive feature—it can be produced by the body itself when the skin is exposed to ultraviolet B (UVB) rays from sunlight. Because of this, it is often called the “sunshine vitamin.”

Scientifically, Vitamin D is not a single substance but a group of compounds with similar biological functions. The two major forms are Vitamin D2 (ergocalciferol) and Vitamin D3 (cholecalciferol). Vitamin D2 is mainly derived from plant sources and fortified foods, while Vitamin D3 is produced in human and animal skin upon exposure to sunlight and is also found in certain animal-based foods like fish, eggs, and dairy. Both forms are beneficial, but Vitamin D3 is considered more effective in raising and maintaining Vitamin D levels in the blood.

Once Vitamin D enters the body—either through sunlight, diet, or supplements—it is inactive and must undergo two conversion steps to become functional. The first step occurs in the liver, where Vitamin D is converted into calcidiol [25-hydroxyvitamin D]. The second step takes place in the kidneys, where calcidiol is further converted into calcitriol [1,25-dihydroxyvitamin D], the active form of the vitamin. Calcitriol functions as a hormone, regulating calcium and phosphorus balance in the body, which is essential for bone formation, growth, and repair.

Functions of Vitamin D

The most important function of Vitamin D is regulating calcium and phosphorus metabolism. It enhances the absorption of calcium and phosphorus from the intestine and ensures these minerals are deposited into bones and teeth, keeping them strong and healthy. Without adequate Vitamin D, calcium absorption becomes inefficient, leading to weak bones, increased risk of fractures, and conditions like rickets in children or osteoporosis in adults.

Vitamin D also acts as a key player in bone growth and remodeling. In growing children, it helps in the proper formation of the skeletal system. In adults, it supports the continuous renewal of bone tissue, which is essential for maintaining bone density and preventing bone loss with age.

Another crucial function of Vitamin D is its role in the immune system. It stimulates the production of antimicrobial peptides, which are natural substances that help the body fight against bacteria, viruses, and other pathogens. Studies have shown that adequate Vitamin D levels are linked with reduced susceptibility to respiratory infections, autoimmune disorders, and inflammatory diseases.

Vitamin D also supports the muscular system. It enhances muscle contraction by maintaining the proper balance of calcium within muscle cells. This improves strength, coordination, and overall physical performance. Deficiency in Vitamin D has been associated with muscle weakness, cramps, and an increased risk of falls, especially in the elderly.

In addition, Vitamin D contributes to cardiovascular health. Research suggests that it helps regulate blood pressure, improves vascular function, and may reduce the risk of heart disease. While this area is still being explored, it shows the wider impact of Vitamin D beyond bones and immunity.

The nervous system also benefits from Vitamin D. It plays a role in brain development and function, influences neurotransmitters, and is believed to have protective effects against neurodegenerative disorders. Low levels of Vitamin D have been linked with mood disorders, including depression and anxiety.

Another emerging function of Vitamin D is its role in controlling cell growth and reducing inflammation. It influences genes that regulate cell division and repair, helping to prevent abnormal growth, which may contribute to lowering cancer risk. It also modulates inflammation, providing protection against chronic inflammatory conditions.

Sources of Vitamin D

Vitamin D can be obtained from three major sources: sunlight, food, and supplements. Since it is not found in abundance in natural foods, and modern lifestyles often limit sun exposure, understanding these sources is essential to maintain adequate levels of this vital nutrient.

The most natural and efficient source of Vitamin D is sunlight. When ultraviolet B (UVB) rays from the sun fall on the skin, they trigger a chemical reaction that converts a cholesterol-based compound present in the skin into Vitamin D3 (cholecalciferol). This is then processed by the liver and kidneys into its active form. Just 10 to 30 minutes of sun exposure on the face, arms, and legs a few times a week can help many people meet their Vitamin D needs. However, factors such as skin pigmentation, use of sunscreen, clothing, geographic location, season, and pollution levels greatly influence how much Vitamin D the body can produce from sunlight.

Dietary sources of Vitamin D are relatively limited, but they play an important role, especially in individuals who do not get enough sunlight. Fatty fish like salmon, mackerel, tuna, and sardines are among the richest natural food sources of Vitamin D. Cod liver oil is another concentrated source that has been traditionally used to prevent rickets. Egg yolks, liver, and certain dairy products also provide smaller amounts of Vitamin D. In addition, some mushrooms, especially those exposed to ultraviolet light, can produce Vitamin D2, which adds to dietary intake.

Because natural food sources are not sufficient for most people, fortified foods have become an important way of meeting Vitamin D requirements. Many countries fortify milk, breakfast cereals, orange juice, and plant-based milk alternatives such as soy or almond milk with Vitamin D. This has been a successful public health strategy to reduce widespread deficiency, particularly in populations at higher risk.

Supplements are another significant source, especially for individuals with very low sun exposure, dietary restrictions, or specific medical conditions. Vitamin D supplements are available in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). Among the two, Vitamin D3 is considered more effective in raising and maintaining blood Vitamin D levels. Supplements are often prescribed to people with confirmed deficiency or those at risk, such as the elderly, infants, and individuals living in northern latitudes where sunlight is limited during winter.

While sunlight remains the most reliable natural source of Vitamin D, food and supplements play a crucial role in ensuring sufficient levels in the body.

Recommended Daily Intake of Vitamin D

The amount of Vitamin D a person needs each day depends on age, health status, lifestyle, and in some cases, geographical location. Since Vitamin D functions like a hormone and influences several body systems, maintaining an adequate daily intake is crucial for both children and adults. Health authorities worldwide, such as the Institute of Medicine (IOM) in the United States and the World Health Organization (WHO), have provided guidelines for recommended daily intake.

For infants up to 12 months of age, the requirement is around 400 International Units (IU) or 10 micrograms (mcg) per day. This is essential for bone growth and to prevent rickets, a condition characterized by weak or deformed bones. Since breast milk contains very little Vitamin D, infants who are exclusively breastfed often need supplements to meet this requirement.

Children and adolescents between 1 to 18 years require 600 IU (15 mcg) daily. During these years, bones are developing rapidly, and sufficient Vitamin D ensures proper skeletal strength, reduces fracture risk, and supports overall growth.

For healthy adults up to 70 years of age, the recommended intake is also 600 IU (15 mcg) daily. This level is considered sufficient to maintain bone density, support immune function, and regulate calcium balance. However, in individuals with limited sunlight exposure, higher intake may be necessary.

For adults above 70 years, the requirement increases to 800 IU (20 mcg) per day. As people age, their skin becomes less efficient at synthesizing Vitamin D from sunlight, and the risk of bone loss, fractures, and muscle weakness increases. Therefore, a slightly higher intake helps protect against osteoporosis and mobility-related problems in older adults.

Pregnant and lactating women are advised to take 600 IU (15 mcg) per day. Adequate Vitamin D during pregnancy is vital for the healthy development of the baby’s bones and teeth and also supports the mother’s immunity. In certain cases, doctors may recommend higher doses if deficiency is detected.

How Vitamin D is Produced in the Body Through Sunlight

The process begins when sunlight containing UVB radiation penetrates the skin. Within the skin, a cholesterol-based compound known as 7-dehydrocholesterol is present in the outer layers (epidermis). When UVB rays strike this compound, they convert it into previtamin D3 through a photochemical reaction. This step is entirely dependent on sunlight exposure and does not occur without it.

Previtamin D3 is unstable and, with the help of body heat, it naturally transforms into Vitamin D3 (cholecalciferol). At this stage, the vitamin is still inactive and cannot perform its biological functions. To become active, it must undergo two additional steps inside the body.

The first conversion takes place in the liver, where Vitamin D3 is changed into 25-hydroxyvitamin D [calcidiol], the main circulating form measured in blood tests to assess Vitamin D status. The second conversion occurs in the kidneys, where calcidiol is further converted into 1,25-dihydroxyvitamin D [calcitriol], the active form of Vitamin D. Calcitriol functions like a hormone, regulating calcium and phosphorus absorption, bone health, muscle activity, and immune system function.

Vitamin D production in the body begins when UVB rays from sunlight convert 7-dehydrocholesterol in the skin into previtamin D3, which then becomes Vitamin D3. Through further processing in the liver and kidneys, it is transformed into its active form, calcitriol, which performs vital functions in the body.

Deficiency of Vitamin D

Vitamin D deficiency occurs when the body does not have enough of this essential nutrient to perform its normal functions. Since Vitamin D is crucial for calcium absorption, bone strength, immunity, and several metabolic processes, its deficiency can affect multiple systems of the body.

Symptoms and Health Effects of Vitamin D Deficiency

One of the earliest signs of Vitamin D deficiency is persistent fatigue and low energy levels. People often report feeling tired even after adequate rest. This happens because Vitamin D influences energy metabolism and muscle efficiency, and a lack of it can lead to a general feeling of weakness.

Bone pain and muscle weakness are also common. Since Vitamin D is essential for calcium absorption, its deficiency weakens the bones and muscles. Adults may experience dull, aching pain in the lower back, hips, legs, or ribs. Children may complain of leg pain, especially at night, which is sometimes mistaken for “growing pains.”

In children, severe deficiency leads to rickets, a condition where bones become soft and weak, resulting in bowed legs, delayed growth, and skeletal deformities. In adults, deficiency causes osteomalacia, which is characterized by softening of the bones, frequent fractures, and difficulty in performing daily activities due to bone and muscle pain. Long-term deficiency also increases the risk of osteoporosis, a condition marked by brittle bones and a higher likelihood of fractures.

Frequent infections are another major effect of Vitamin D deficiency. This nutrient plays a vital role in strengthening the immune system by stimulating the production of antimicrobial peptides that fight viruses and bacteria. Low levels of Vitamin D make individuals more prone to colds, flu, and other respiratory infections.

Muscle cramps, twitching, and weakness are seen in both children and adults. This occurs because Vitamin D deficiency disrupts calcium balance in muscle tissues, reducing muscle performance and coordination. In older adults, this weakness increases the risk of falls and fractures, which can severely impact mobility and independence.

In infants, deficiency can cause delayed tooth eruption and dental problems due to impaired calcium regulation. Pregnant women with low Vitamin D may experience complications such as pre-eclampsia, and their babies may have a higher risk of weak bones and poor growth.