- Health and wellness
- 1 views
- What vitamin D is used for
- Where to find vitamin D
- Why vitamin D matters at different ages
- Deficiency and the use of supplements
- Frequently asked questions
What vitamin D is used for
Vitamin D is a fat-soluble nutrient that is essential for several functions in the body. It is best known for its role in bone health, but it also contributes to the normal function of muscles and the immune system. Unlike many other vitamins, it can be produced directly by the skin when exposed to sunlight, while some of the body's intake comes from food.
Maintaining adequate vitamin D levels is important at every stage of life, from childhood to old age. Requirements and the risk of developing a deficiency depend on several factors, including age, sun exposure, diet, health conditions and the body's individual ability to absorb and use the nutrient.
When sun exposure and diet are not sufficient, supplementation may need to be considered. However, not everyone needs to take vitamin D in supplement form: the decision should take individual needs into account and, when necessary, be based on test results and medical advice.
Benefits for bones and muscles
One of the main functions of vitamin D is related to calcium and phosphorus metabolism, involving two minerals that are essential for the formation and maintenance of bones and joints. Vitamin D promotes the intestinal absorption of calcium and helps maintain adequate levels of this mineral in the blood, thereby supporting normal bone mineralisation.
A prolonged insufficient intake can compromise skeletal health. In children, a significant deficiency can cause rickets, while in adults it can contribute to osteomalacia, a condition characterised by inadequate mineralisation of bone tissue. Along with other factors, deficiency can also contribute to a reduction in bone mineral density.
Vitamin D also contributes to the maintenance of normal muscle function. Its receptors are found in various tissues, including muscle cells, and the nutrient is involved in the processes required for muscle contraction and function. An adequate intake is therefore important for the normal functioning of the musculoskeletal system.
These aspects are particularly relevant with advancing age, when bone health and the ability to maintain strength and balance become important factors in reducing the risk of falls and fractures. However, vitamin D alone is not enough to prevent these events: physical activity, diet, calcium intake and overall health must also be taken into account.
Support for the immune system
Vitamin D contributes to the normal functioning of the immune system, taking part in the regulation of several processes involving the body's immune cells. Vitamin D receptors have been identified in numerous immune cells, including monocytes, macrophages and lymphocytes.
Its role is not simply to make the immune system more active. Vitamin D is involved in modulating the immune response, helping the body's defence and inflammation-regulation mechanisms function properly.
Maintaining an adequate vitamin D intake is therefore important for supporting the immune system, without attributing unproven preventive or therapeutic properties to this nutrient. A varied diet, adequate rest, regular physical activity and recommended preventive measures remain integral to protecting health.
Where to find vitamin D
Vitamin D can reach the body through two main routes: skin synthesis following exposure to UVB rays and diet. Production through the skin is an important source, while relatively few foods contain significant amounts of this vitamin.
There are different forms of vitamin D. The best known are vitamin D2 (ergocalciferol), found mainly in certain plant-based foods and mushrooms, and vitamin D3 (cholecalciferol), which is produced by the skin and found in some animal-derived foods. Both contribute to increasing vitamin D levels in the body, although they differ in structure and metabolism.
Synthesis through sunlight
The skin can produce vitamin D3 when exposed to ultraviolet B (UVB) rays. These rays reach the outer layers of the skin and trigger the conversion of 7-dehydrocholesterol, a substance naturally present in the body, into previtamin D3. Further transformations produce cholecalciferol, which is then converted into active metabolites in the liver and kidneys.
The amount of vitamin D produced depends on several factors, including:
- Season and latitude: UVB intensity varies throughout the year and according to geographical location;
- Time of exposure: the angle of the sun's rays affects the amount of UVB reaching the Earth's surface;
- Skin characteristics: skin pigmentation can affect vitamin D synthesis;
- Age: the skin's ability to produce vitamin D gradually decreases with ageing;
- Amount of exposed skin: the area of skin exposed can affect overall production;
- Protection and environmental conditions: clothing, glass, shade and sunscreen alter exposure to UVB rays.
There is no exposure time that works for everyone and guarantees a precise amount of vitamin D. It is also important not to try to increase skin synthesis through prolonged or unprotected exposure: ultraviolet rays can damage the skin and increase the risk of skin cancer. Sun exposure should therefore be managed carefully, following sun-protection recommendations and not treating it as a therapy for correcting a deficiency.
When skin synthesis is limited, particularly during months with less sunlight, it may become more important to assess dietary intake and, if necessary, targeted supplementation.
Foods with the highest vitamin D content
Relatively few foods contain significant amounts of vitamin D. The most notable sources include certain oily fish, cod liver oil, egg yolks and specific mushrooms exposed to UV rays. The amount present can vary depending on the species, farming conditions, origin and preparation methods.
Foods worth considering include:
- Oily fish: salmon, mackerel, herring, sardines and trout can provide vitamin D, with amounts varying according to species and origin;
- Cod liver oil: it is a concentrated source of vitamin D, but it may also contain high amounts of vitamin A. It is therefore important to consider the overall intake of these nutrients before using it;
- Egg yolk: it contains vitamin D, generally in lower amounts than oily fish. Its content can also vary depending on the hens' diet;
- Mushrooms exposed to UV rays: some varieties can contain interesting amounts of vitamin D2, particularly when exposed to ultraviolet radiation during growth or after harvesting;
- Fortified foods: some plant-based drinks, dairy products and cereals may be enriched with vitamin D. It is important to check the label because fortification is not available in every brand or formulation.
It is useful to regularly include dietary sources of vitamin D in a balanced diet, without relying on a single food to meet daily requirements. If specific needs or a risk of deficiency are present, assessing overall intake can help determine whether a supplement is necessary.
Why vitamin D matters at different ages
Vitamin D is important throughout life, but its role takes on particular characteristics at certain stages. During growth, it contributes to normal skeletal mineralisation; in adulthood, it supports the maintenance of bones and muscles; in older age, it becomes particularly relevant to bone health and muscle function.
According to EFSA reference values, the adequate intake for healthy people over one year of age is 15 µg per day, equivalent to 600 IU, assuming minimal skin synthesis. This is a nutritional reference value for the population and does not automatically correspond to the supplement dose each person should take. Actual requirements may also depend on skin production and individual circumstances.
In adults
In adulthood, vitamin D contributes to maintaining normal bone structure and muscle function. An adequate intake supports calcium absorption and normal bone tissue metabolism, helping maintain musculoskeletal health over the long term.
For healthy adults, EFSA indicates an adequate intake of 15 µg of vitamin D per day (600 IU), calculated on the assumption of minimal skin synthesis. This is a reference value that considers total intake from food and supplements, not a prescription requiring everyone to take a daily supplement.
Individual needs may change in the presence of limited sun exposure, an unvaried diet, conditions that impair intestinal absorption or diseases that interfere with vitamin D metabolism. Some medicines can also affect vitamin D levels. In these situations, it may be appropriate to consult a doctor to assess the risk of deficiency and whether supplementation is necessary.
In older people
As people age, the skin's ability to produce vitamin D in response to sunlight tends to decrease. This may be accompanied by spending less time outdoors, a less varied diet and health conditions that make it harder to maintain adequate nutrient levels. For this reason, it is important to consider supplements for older people.
Vitamin D is particularly important in older age because it contributes to the maintenance of bone health and normal muscle function. An insufficient intake can impair bone mineralisation and, in the presence of other risk factors, contribute to increased skeletal fragility.
According to EFSA reference values, the adequate intake for adults, including those over 70, remains 15 µg per day (600 IU), under conditions of minimal skin synthesis. This does not mean that every older person should take the same amount through supplements: the need for supplementation depends on diet, sun exposure, clinical conditions and individual risk.
Deficiency and the use of supplements
Vitamin D deficiency can develop when skin synthesis and dietary intake are insufficient to meet the body's needs, or when conditions impair its absorption or metabolism. Risk factors include limited sun exposure, older age, certain intestinal, liver or kidney diseases, obesity and the use of specific medicines.
A significant and prolonged deficiency can affect bone and muscle health. In adults, it can contribute to osteomalacia, while in children it can cause rickets. In some cases, muscle weakness and bone pain may occur, but these symptoms are not specific and can be associated with many other conditions.
Choosing a vitamin D product should therefore start with an assessment of individual needs, avoiding the indiscriminate use of high doses.
Checking 25-OH levels
The reference test for assessing vitamin D status in the body is a blood test measuring 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. This is the main circulating form of vitamin D and the parameter most commonly used to estimate the body's stores.
The 25(OH)D result is generally expressed in ng/mL or nmol/L. To convert the values, it is useful to remember that 1 ng/mL corresponds to 2.5 nmol/L. The result should be interpreted by a doctor, who will consider it alongside medical history, risk factors and any conditions that may affect the metabolism of this nutrient.
Once the result is available, a healthcare professional can determine whether dietary and lifestyle changes are sufficient or whether supplementation is recommended. The dose and duration of any supplementation should also be established according to individual circumstances.
Contraindications and excessive intake
Vitamin D is fat-soluble and can accumulate in the body. For this reason, excessive and prolonged intake through supplements can cause vitamin D toxicity, a condition mainly associated with increased calcium levels in the blood, known as hypercalcaemia.
Excess vitamin D is generally linked to taking excessively high doses of supplements, rather than normal sun exposure or the usual consumption of foods containing it. Possible signs of toxicity include nausea, vomiting, loss of appetite, weakness, intense thirst, increased urination and, in more severe cases, kidney damage and heart rhythm disturbances.
Particular care is advisable in the presence of certain conditions, including:
- Kidney disease: this can affect vitamin D metabolism and calcium balance;
- High blood calcium levels: supplementation must be assessed with particular caution;
- Certain endocrine or granulomatous conditions: these can alter vitamin D metabolism and increase the risk of hypercalcaemia;
- Medication use: some medicines, such as thiazide diuretics, may require specific assessment when considering supplementation;
- Concurrent use of multiple products: it is important to check that multivitamins, calcium supplements and other preparations do not result in an excessive total intake.
If you have a medical condition, are taking medication or are unsure whether you need vitamin D, it is advisable to consult a doctor or pharmacist before starting supplementation. Do not increase the dose on your own or combine several products containing vitamin D without checking their composition.
Frequently asked questions
Does vitamin D help you lose weight?
No, vitamin D is not a weight-loss supplement, and there is no evidence that taking it directly causes a reduction in body weight.
Weight management mainly depends on energy balance, diet, physical activity and numerous individual factors.
Why is vitamin D considered a hormone?
Vitamin D is often described as a hormone or hormone precursor because, after being produced by the skin or obtained from food, it undergoes transformations in the body. In the liver, it is converted into 25-hydroxyvitamin D, while in the kidneys it can be transformed into its biologically active form, calcitriol. The latter binds to specific receptors found in various tissues and helps regulate processes such as calcium and phosphorus metabolism. For this reason, vitamin D has characteristics similar to those of a hormone.
Who should take vitamin D supplements?
Supplementation may be recommended in the presence of a confirmed deficiency, risk factors or conditions that reduce the skin's synthesis, absorption or metabolism of the nutrient. The need should be assessed by a doctor, who may consider age, diet, sun exposure, health conditions and any test results.
When supplementation is recommended, it is important to choose a formulation and amount suited to individual needs, without exceeding the recommended doses.
What is the purpose of combining vitamin D with vitamin K2?
Vitamin D and vitamin K2 are involved in different but related processes in calcium metabolism. Vitamin D promotes intestinal calcium absorption and contributes to the maintenance of normal bones, while vitamin K contributes to normal blood clotting and the maintenance of normal bones.
Vitamin K is involved in the activation of certain proteins, including osteocalcin and matrix Gla protein, which take part in processes related to bone tissue and the regulation of mineralisation. These mechanisms explain the interest in formulations combining vitamin D3 and K2, but they do not demonstrate that the combination guarantees that calcium is directed exclusively towards the bones or prevents deposits in blood vessels.
Vitamin D is important for maintaining bone health, normal muscle function and the immune system. The body can produce it through exposure to UVB rays, while certain foods contribute to daily intake. Age, health conditions and individual habits can influence the risk of deficiency: for this reason, any supplementation should be carefully assessed, avoiding high doses without medical advice and taking possible contraindications into account.
