Vitamin D is often called the “sunshine vitamin” because the body can produce it when skin is exposed to ultraviolet (UV-B) rays from sunlight. Yet, vitamin D deficiency remains one of the most common nutritional deficiencies globally. It is largely a consequence of modern lifestyles, diet, skin biology, health conditions, ageing and public health advice around sun protection.
Vitamin D is not just a vitamin in the ordinary sense. It acts more like a hormone once it is converted into its active form in the body. It helps regulate calcium and phosphate, supports healthy bones and teeth, assists muscle function, influences immune activity and plays a role in normal cell growth. Low vitamin D levels are most strongly linked to bone and muscle problems, but research has also explored its relationship with immunity, pregnancy, diabetes risk, autoimmune disease, mood, cardiovascular health and cancer. The strength of evidence varies widely, so it is important to separate proven benefits from promising but less certain ones.
Functions of Vitamin D
Vitamin D has several important functions in the body. Although it is best known for supporting bone health, its role is much wider. Once vitamin D is produced in the skin or absorbed from food and supplements, it is converted by the liver and kidneys into its active form. This active form helps regulate several biological processes.
1. Calcium Absorption
One of vitamin D’s most important functions is helping the intestines absorb calcium from food. Calcium is essential for strong bones, healthy teeth, muscle contraction, nerve signalling and normal heartbeat function. Without enough vitamin D, the body cannot absorb calcium efficiently, even if a person eats calcium-rich foods.
2. Bone and Teeth Strength
Vitamin D works with calcium and phosphate to build and maintain healthy bones and teeth. In children, too little vitamin D can lead to rickets, a condition where bones become soft and may bend or deform. In adults, deficiency can contribute to osteomalacia, bone pain, muscle weakness and increased fracture risk. Over time, poor vitamin D status may also worsen osteoporosis risk, especially in older adults.
3. Muscle Function
Muscles need vitamin D for normal performance. Low vitamin D levels may contribute to muscle weakness, cramps, aches and poor balance. This is especially important in older adults, where muscle weakness and poor balance can increase the risk of falls and fractures.
4. Immune System Regulation
Vitamin D helps regulate the immune system. It supports the body’s ability to respond to infections while also helping prevent excessive inflammation. This is one reason vitamin D has been studied in relation to respiratory infections, autoimmune conditions and general immune health.
5. Phosphate Regulation
Along with calcium, phosphate is important for bone structure, cell repair, energy production and normal tissue function. Vitamin D helps the body absorb phosphate from the gut and maintain the correct balance of calcium and phosphate in the bloodstream.
6. Cell Growth and Repair
Vitamin D plays a role in cell growth, differentiation and repair. This means it helps cells mature and function properly. Because of this, researchers have studied vitamin D in relation to cancer, inflammation and chronic disease, although supplementation should not be viewed as a cancer-prevention treatment.
7. Inflammation
Vitamin D has anti-inflammatory effects in the body. It influences immune cells and inflammatory signalling pathways. Adequate vitamin D may help the body maintain a healthier inflammatory balance, although the degree of benefit depends on the person’s health status and whether they are deficient.
8. Nervous System Function
Vitamin D receptors are found in parts of the nervous system. Vitamin D may play a role in nerve development, nerve signalling and brain health. Research has explored possible links between low vitamin D and mood disorders, cognitive decline and neurological conditions, but evidence is still developing.
9. Pregnancy and Infant Development
During pregnancy, vitamin D plays an important role in maintaining the the expectant mother’s bone health and contributes to the skeletal development of the foetus. Adequate vitamin D is also important for infants because bones grow rapidly during early life. This is why vitamin D supplementation is often recommended for breastfed infants.
10. Mineral Balance
Vitamin D works closely with parathyroid hormone, calcium and phosphate to maintain mineral balance in the body. When vitamin D is low, the body may increase parathyroid hormone levels to keep blood calcium stable. Over time, this can draw calcium from bones and weaken bone structure.
Why Vitamin D Deficiency Is So Common?
1. Limited sunlight exposure
The most important natural source of vitamin D is sunlight. When UVB rays reach the skin, they trigger vitamin D production. However, modern life has reduced regular sun exposure. Many people work indoors, commute by car, exercise inside gyms and spend leisure time in shaded or indoor environments.
Urban lifestyles have changed how people interact with the sun. Even in sunny countries, vitamin D deficiency can be common because people may avoid midday sun, wear covering clothing, use sunscreen, or spend most of the day indoors.
2. Sunscreen and sun-safety behaviour
Sun protection is important because excessive ultraviolet exposure increases the risk of sunburn, premature skin ageing and skin cancer. However, the same UVB rays that can damage skin are also needed for vitamin D production.
The key is balancing sufficient sun exposure to support vitamin D production, but not excessive which may increase the risk of skin damage.
In practice, many people avoid direct sunlight almost completely or use high-SPF sunscreen whenever outdoors. This is sensible for skin protection, but it can contribute to lower vitamin D production if dietary intake or supplementation does not make up the difference.
3. Darker skin produces vitamin D more slowly
Melanin, the pigment that gives skin its colour, acts as a natural sunscreen. People with darker skin have more melanin, which helps protect against UV damage but also reduces the skin’s ability to produce vitamin D from the same amount of sunlight.
As a result, people with darker skin may need longer sun exposure than those with lighter skin to produce similar amounts of vitamin D. This is one reason vitamin D deficiency can occur even in countries with abundant sunshine. A person’s skin tone, clothing habits, time outdoors and sun angle all matter.
4. Limited food source of vitamin D
Unlike many nutrients, vitamin D is not widely available in common everyday foods. Good natural sources include oily fish such as salmon, sardines, mackerel and tuna, as well as egg yolks, liver and some mushrooms exposed to ultraviolet light. Many people do not eat these foods regularly.
Some countries fortify milk, cereals, margarine or plant-based alternatives with vitamin D, but fortification policies vary. People who are vegan, avoid dairy, have milk allergy, are lactose intolerant, or eat little fish may have lower dietary vitamin D intake.
5. Ageing reduces vitamin D production
Older adults are at greater risk of deficiency for several reasons. Ageing skin is less efficient at producing vitamin D. Older people may also spend less time outdoors, have lower dietary intake, take multiple medicines, or have kidney changes that affect vitamin D activation.
This is one reason newer guidance places special emphasis on older adults. The Endocrine Society’s 2024 guideline suggests vitamin D supplementation for adults older than 75, while noting that healthy adults under 75 are unlikely to benefit from taking more than the standard recommended daily intake.
6. Body weight and fat storage can affect vitamin D levels
Vitamin D is fat-soluble, meaning it can be stored in body fat. People with obesity often have lower measured blood levels of vitamin D, partly because more vitamin D may be distributed into fat tissue rather than remaining available in circulation. This does not mean body fat “causes” deficiency by itself, but it is one reason people with obesity may require medical assessment and sometimes different supplementation strategies.
7. Gut, liver and kidney conditions can interfere with vitamin D
Vitamin D must be absorbed through the gut and then processed by the liver and kidneys. Conditions that affect fat absorption, such as coeliac disease, Crohn’s disease, inflammatory bowel disease, pancreatic disease, bariatric surgery or chronic liver disease, can reduce vitamin D absorption or metabolism.
Kidney disease can also impair conversion of vitamin D into its active form. In such cases, ordinary over-the-counter vitamin D may not be enough. It is important to consult with a healthcare professional when considering vitamin D supplementation in the backdrop of a kidney condition.
8. Certain medicines can lower vitamin D levels
Some medications affect vitamin D metabolism. These may include certain anti-seizure medicines, glucocorticoids, some HIV medicines, antifungal drugs and weight-loss medicines that reduce fat absorption. People taking long-term medication should not automatically supplement at high doses, but they may need testing or advice from a healthcare provider.
9. Breastfed infants can be at risk
Breast milk is the ideal food for infants, but it usually contains limited vitamin D unless the mother has high vitamin D intake or supplementation. This is why many health authorities recommend vitamin D supplementation for breastfed infants. The NIH consumer guidance lists recommended daily intake for infants from birth to 12 months as 400 IU.
Common Benefits of Vitamin D Supplementation
1. Stronger bones and prevention of rickets and osteomalacia
The most established benefit of vitamin D is bone health. Vitamin D helps the body absorb calcium, which is essential for building and maintaining strong bones. Without enough vitamin D, calcium absorption falls, and the body may pull calcium from bones to maintain normal blood levels.
In children, severe deficiency can cause rickets, a condition where bones become soft, weak or deformed. In adults, deficiency can cause osteomalacia, which leads to bone pain, muscle weakness and increased fracture risk. Vitamin D, often together with calcium when needed, helps prevent and treat these deficiency-related bone conditions.
2. Support for muscle function
Vitamin D is important for muscle contraction and strength. Deficiency can cause muscle aches, weakness, fatigue and difficulty climbing stairs or rising from a chair, especially in older adults. Correcting deficiency may improve muscle function in people whose symptoms are related to low vitamin D.
This does not mean vitamin D is a performance-enhancing supplement for everyone. In people with adequate levels, extra supplementation is unlikely to produce dramatic improvements in strength. Its value is strongest when a deficiency is present.
3. Reduced risk of falls in some deficient or older people
Vitamin D has long been studied for fall prevention because it affects both bones and muscles. Some earlier research suggested that supplementation may reduce falls, particularly in older adults with deficiency. However, evidence is mixed, and benefit appears most likely in people who are deficient, frail, institutionalised, or at higher risk.
4. Support for immune function
Vitamin D plays a role in immune regulation. Immune cells have vitamin D receptors, and vitamin D influences antimicrobial peptides and inflammatory responses. This has led to interest in vitamin D for respiratory infections, autoimmune disease and general immune resilience.
A major meta-analysis found that vitamin D supplementation reduced the risk of acute respiratory infections overall, with stronger effects in people who were very deficient. This does not mean vitamin D prevents all colds, flu or infections. It means maintaining adequate vitamin D may form one part of immune health.
5. Pregnancy and infant health
Vitamin D is important during pregnancy because it supports maternal bone health, fetal skeletal development and immune regulation. Pregnant women should not use high-dose vitamin D without medical guidance. Lower dose supplementation along with a regular intake of foods rich in vitamin D may be a better option.
6. Support in older adults
Older adults are more likely to have lower vitamin D levels and are at greater risk of fractures, falls, muscle weakness and chronic disease. The benefit is not that vitamin D reverses ageing, but that it helps reduce the consequences of deficiency in a group where deficiency is common and clinically important.
Less Commonly Known Benefits
Some less widely known benefits of vitamin D supplementation are still being studied. Research indicates that vitamin D may assist in the following ways but it is not always conclusive. Further research is necessary.
- Helps regulate immune responses and possibly assists with the management of autoimmune diseases.
- May assist in pre-diabetes with slowing the progression to diabetes, alongside diet, lifestyle and weight management.
- May be useful in the management of depression and anxiety alongside medication and counselling.
- Assists in the prevention of cardiovascular disease possibly due to its benefit in reducing inflammation.
- May be helpful for chronic inflammatory diseases alongside the recommended medical therapy.
Dosage for Vitamin D Supplementation
The general recommended daily intake for vitamin D varies by age and pregnancy status.
- Infants 0–12 months: 400 IU
- Children and teens: 600 IU
- Adults up to 70: 600 IU
- Adults over 70: 800 IU
- Pregnant and breastfeeding women: 600 IU
These are general recommendations. People with confirmed deficiency may need higher treatment doses for a limited period, but this should be guided by a healthcare professional.
Vitamin D Excess
It is possible to take too much vitamin D resulting in toxicity. Vitamin D is fat-soluble, so excessive intake can build up in the body. Excess vitamin D can cause high calcium levels, which may lead to nausea, vomiting, constipation, weakness, confusion, excessive thirst, frequent urination, kidney stones and even kidney damage.
Vitamin D toxicity is usually caused by excessive supplement use, not sunlight. Sun exposure does not typically cause vitamin D toxicity because the body regulates production in the skin. This is why “more” is not always better. High-dose vitamin D should be used only when medically indicated.
References
- National Institutes of Health, Office of Dietary Supplements. Vitamin D — Fact Sheet for Health Professionals.
- Mayo Clinic. Vitamin D.
- Cleveland Clinic. Vitamin D Deficiency: Causes, Symptoms & Treatment.
- Endocrine Society. Vitamin D for the Prevention of Disease Guideline Resources.
