Vitamin D: what it does, deficiency, sources, and excess

Vitamin D is one of the most talked-about vitamins of recent decades, and for good reason: it plays a central role in how the body uses calcium and keeps bones firm. At the same time, it is a vitamin unlike almost any other. Most vitamins come only from food, but vitamin D can also be produced by the skin itself from sunlight. That is why it is often called the "sunshine vitamin." The thing is, modern life, with many hours indoors, clothing that covers much of the body, and sunscreen (important for skin health), makes deficiency common in many parts of the world.

This guide is meant to be a complete and careful reference. You will understand what vitamin D is, what it does in the body in factual terms, how to get it from the sun and from food, which foods are a source, what deficiency is and who is at higher risk, which signs are usually associated with it, how it is assessed by testing, how guided supplementation works, and, above all, why excess is a concern. One caveat from the start: this content is informative and does not replace a consultation. The decision to get tested or to supplement always belongs to your doctor. All the information below is based on official sources such as MedlinePlus (from the U.S. National Library of Medicine), the NHS (National Health Service of the United Kingdom), and the MSD Manuals.

What vitamin D is

Vitamin D is a fat-soluble vitamin, meaning it dissolves in fat and can be stored in the body. This characteristic matters on two fronts. On one hand, the body can keep reserves to use when supply drops, as tends to happen in winter in cold-climate countries. On the other hand, precisely because it is stored, it is not easily eliminated when there is excess, and that is why taking exaggerated amounts can be a problem, unlike some vitamins that the body simply flushes out through urine.

There are two main forms of vitamin D. MedlinePlus explains that supplements appear as vitamin D2 and vitamin D3, and notes that D3 may raise blood levels higher and for longer. Both forms are used, but which one and in what way is a clinical decision, not a shelf choice.

What vitamin D does

The best-established function of vitamin D, and the one that appears in all official sources, is helping the body absorb calcium. MedlinePlus sums it up well: vitamin D helps the body use calcium, a mineral essential for normal bone formation. Without enough vitamin D, the gut absorbs less calcium, and the body tends to draw calcium from the bones themselves to keep blood levels steady, which weakens the bone structure over time. It is through this pathway that deficiency connects to bone diseases, as we will see ahead.

The NHS, in the United Kingdom, describes vitamin D's role as helping to regulate the amount of calcium and phosphate in the body, nutrients needed to keep bones, teeth, and muscles healthy. The phosphate side connects to the mineral covered in our guide on foods high in phosphorus. In other words, it is not just "fixing calcium," it is taking part in a broader mineral balance.

Beyond the bones, MedlinePlus points out that vitamin D has a role in the functioning of muscles, which need it to move, of nerves, which use it to carry messages between the brain and the rest of the body, and of the immune system, which relies on it to fight off bacteria and viruses. Here a note of balance matters: taking part in the functioning of a system is not the same as preventing or treating diseases of that system. Vitamin D is part of the body's healthy functioning, not a medicine that, on its own, prevents disease.

How the body gets vitamin D

There are three main paths, and they complement each other.

Sun exposure

The skin produces vitamin D when exposed to direct sunlight. That is the reason for the "sunshine vitamin" nickname. The catch is that this production depends heavily on the angle of the sun's rays, and therefore on the time of year and the latitude. The NHS notes that, in the United Kingdom, the skin can produce vitamin D mainly from late March through September, and that in autumn and winter sunlight is not enough for this production. In Brazil, with more sun throughout the year, the situation is different, but the principle is the same: the amount produced varies with the season, the time of day, the latitude, the skin tone, and how much of the skin is exposed.

Here lies a delicate balance. The sun is a natural source of vitamin D, but too much exposure ages the skin and increases the risk of skin cancer, as MedlinePlus reminds us. There is no contradiction: the recommendation is not to expose yourself more to "make vitamin D," but to keep a balanced and safe exposure, following guidance, especially according to your skin tone and your history.

Food

Few foods naturally contain vitamin D, and that is one of the reasons deficiency is common. The dietary sources are described further below, in the table.

Guided supplementation

When sun and food are not enough, supplementation may be advised, always with professional follow-up. The NHS even suggests that, in the UK context, people consider a daily supplement in autumn and winter, precisely because production by the skin drops sharply in those seasons. This is a public health recommendation for a specific reality of climate and latitude, not an automatic rule for everyone. The decision about supplementing should take into account your case and your doctor's assessment.

Dietary sources of vitamin D

Bringing together information from MedlinePlus, the MedlinePlus Medical Encyclopedia, and the NHS, the table below organizes the main sources. Remember that the exact amount varies with the food, the preparation, and fortification, and that the table is informative.

Type of source	Examples	Notes
Fatty fish	Salmon, tuna, sardines, mackerel, trout, herring	Among the richest natural sources
Organ meats	Beef liver	A concentrated natural source
Eggs	Egg yolk	The vitamin is in the yolk, not the white
Dairy	Cheese, some yogurts	Partly naturally present, partly fortified
Mushrooms	Mushrooms exposed to ultraviolet light	Contain more vitamin D when given UV
Fortified foods	Some milks, breakfast cereals, juices, soy beverages, margarines	Fortification varies between brands and countries

The MedlinePlus Medical Encyclopedia gives a concrete example of fortification in the United States: much of the milk there is fortified with about 400 IU per quart. In Brazil, fortification follows its own rules and varies between products, so it is worth reading the label. For people who eat neither fish nor eggs, fortified foods gain weight in the diet, although they rarely fully replace the role of the sun and, in some cases, of guided supplementation.

What vitamin D deficiency is

Deficiency is when the body does not have enough vitamin D to maintain its functions, especially calcium absorption. The MSD Manual points out that the most common cause is the combination of little sun exposure with a diet poor in the vitamin. There are also secondary causes, such as diseases that impair fat absorption in the gut, problems in the liver or the kidneys (organs involved in activating vitamin D), and some medications. Nutritional gaps in general are also among the factors linked to other concerns, such as some types of hair loss.

A point that often surprises people is that deficiency, in its early stages, is silent. A person can have low levels for quite a while without noticing. That is exactly why assessment is not based on "how you feel," but on risk factors and, when the doctor judges it necessary, on a blood test.

Who is at higher risk of deficiency

Some people deserve extra attention. Bringing together what MedlinePlus, the NHS, and the MSD Manual say, the higher-risk groups include:

• Older adults, especially over 70, whose skin produces less vitamin D and who tend to go out less
• People who spend a long time indoors, are housebound, or live in long-term care institutions
• Infants fed exclusively on breast milk, since breast milk has little vitamin D
• People with darker skin, because more melanin reduces the skin's production of vitamin D
• People who cover much of their body when going out, by habit or for protection
• People with conditions that impair fat absorption, such as Crohn's disease, celiac disease, and ulcerative colitis
• People with obesity or who have had stomach reduction surgery (such as gastric bypass)

Belonging to one of these groups does not, on its own, mean you have a deficiency. It means it makes sense to discuss the topic with your doctor, who will decide whether there is reason to investigate.

Symptoms usually associated with deficiency

This section calls for caution, because isolated symptoms do not confirm a diagnosis, and the signs below can have many other causes. Still, it is useful to know what the sources associate with the vitamin's lack, always as information, never as self-diagnosis.

In adults, when deficiency is prolonged and enough to weaken the bone, the MSD Manual describes a condition called osteomalacia, which can involve bone pain, muscle aches and weakness, and a greater risk of fractures, because the bones become less mineralized. In children, severe deficiency is linked to rickets, in which the forming bone does not mineralize properly, leading to growth changes, softer bones, and, in more serious cases, deformities. The MedlinePlus Medical Encyclopedia also connects deficiency to osteoporosis and rickets, through the same calcium-and-bone mechanism.

The practical message is twofold. First: many of these symptoms, such as fatigue (which might instead reflect poor sleep or insomnia), diffuse aches, and weakness, are nonspecific and can come from countless causes, so you cannot "conclude" that they are low vitamin D. Second: if symptoms like these persist, whether it is fatigue, aches, or a recurring headache, the right path is to seek assessment, not to start taking a supplement on your own trying to guess.

How vitamin D is assessed

You do not find out your vitamin D level from symptoms, but from a specific blood test. The MSD Manual and other sources explain that the test measures 25-hydroxyvitamin D, written in abbreviated form as 25(OH)D, which is the circulating form used to estimate the body's reserves. It is a different test from a routine complete blood count, though a doctor may order them together. The result is interpreted by the doctor, who considers age, risk factors, symptoms, and the overall context before drawing conclusions. In some cases of suspected rickets or osteomalacia, imaging tests such as X-rays may show characteristic changes in the bone.

Two important points. First: ordering the test comes from the professional. Requesting the test on your own, without context, can lead to mistaken interpretations, because reference values and what is considered adequate can vary with the source and the clinical situation. Second: a single result is a snapshot of one moment, and the course of action depends on the whole picture, not on an isolated number.

Supplementation: what is known, with caution

Vitamin D supplementation only makes sense within a plan defined by a health professional. This guide, on principle and by responsibility, does not recommend a dose or a regimen: that is the doctor's task, adjusting everything to your case, your age, your test results, and your history.

What can be stated factually is the following. Authorities publish reference intake values that serve as general information, not as an individual prescription. MedlinePlus lists reference daily intakes that vary with age, around 400 IU for infants, 600 IU for most children and adults, including pregnant and breastfeeding women, and 800 IU from age 71. The NHS, in the United Kingdom, works with a reference of 10 micrograms a day for children over 1 year and adults, and notes that 10 micrograms equal 400 IU. Note that these are population reference intake values, released by the authorities as information, and not guidance for you to take such an amount on your own.

There are also the D2 and D3 forms, already mentioned, and the choice between them, as well as the frequency and duration of any supplementation, is a clinical decision. If there is an indication to supplement, it is because a professional assessed that the gains outweigh the risks in your specific case, and they will monitor the result.

Why excess is a concern: toxicity and limits

This is one of the most important parts of the guide, and the main reason not to supplement on your own. Because vitamin D is fat-soluble and stored, excess is not easily eliminated, and the buildup can become toxic.

MedlinePlus explains that excessive vitamin D intake can lead to excessive calcium absorption, raising the levels of calcium in the blood, a condition called hypercalcemia. The described consequences include a risk of kidney damage, formation of kidney stones, mental confusion, and digestive problems. In other words, the excess brings no extra benefit and can even cause harm.

That is why authorities define upper intake limits, which act as signs of caution. The MedlinePlus Medical Encyclopedia cites a tolerable limit of 4,000 IU a day for children over 9 and adults, and lower limits for infants. The NHS, in the United Kingdom, lists a maximum of 100 micrograms a day for adults and children over 11, with progressively lower ceilings for younger children and infants. These numbers are informative and attributed to the authorities: the point of citing them here is to show that there is a safety ceiling, not to guide you to take up to that limit. Exceeding these values on your own is precisely the kind of situation that can lead to toxicity.

Vitamin D and other diseases: what the evidence shows (and what it does not yet show)

It is common to hear that vitamin D prevents or treats a huge list of diseases, from immunity to mood. Here it is necessary to separate what is a known function from what is still ongoing study.

What is well established is the calcium-and-bone axis: vitamin D is needed for calcium absorption and for bone health, and severe deficiency leads to rickets and osteomalacia. This appears consistently in the sources. MedlinePlus also describes the vitamin's role in the functioning of muscles, nerves, and the immune system.

For many other outcomes beyond bone health, from mood to complaints such as PMS, however, the relationship between supplementing vitamin D and obtaining a clinical benefit is still being studied and is not a settled conclusion. As a matter of responsibility and respect for health content rules, this guide does not claim that vitamin D prevents or cures diseases beyond what the authorities cautiously describe. If you have seen some very broad promise out there, especially from those who sell the product, the best thing to do is to be skeptical and take the question to your doctor.

Myths and facts about vitamin D

"The more vitamin D, the better." A myth, and a dangerous one. Because it is fat-soluble and stored, excess can cause hypercalcemia and kidney damage, as MedlinePlus describes. More is not better.

"I get sun every day, so I do not need to worry." It depends. Production by the skin varies with the season, the time of day, the latitude, the skin tone, and how much skin is exposed. In some situations the sun is not enough, and too much sun brings other risks. Assessment decides, not assumption.

"Vitamin D strengthens immunity and protects me from colds and infections." A half-truth that turns into exaggeration. The vitamin takes part in the functioning of the immune system, but that is not the same as preventing diseases such as the flu. For these outcomes, the evidence is still being studied.

"I can find out my vitamin D from symptoms." A myth. Deficiency is usually silent, and the associated symptoms are nonspecific. Assessment is by blood test, ordered by the doctor.

"Vitamin D is only for older people." A myth. Older adults are a higher-risk group, but breastfed infants, people with dark skin, those with little sun exposure, and people with certain gut diseases also deserve attention.

"I can buy it and take it on my own, vitamins do no harm." A myth. Vitamin D is not harmless in excess. Supplementation should be guided, and there are upper intake limits precisely because of the risk of toxicity.

What to bring and ask at the appointment

To make the most of the assessment, go prepared. It is worth noting: what your day-to-day sun exposure is like, what your diet is like (whether you eat fish, eggs, dairy), whether you fall into any risk group, what symptoms you have been feeling and for how long, and which supplements or medicines you already use. Useful questions to ask the doctor include: does it make sense to investigate my vitamin D, do I need a test, what does the result mean in my case, is there an indication to supplement and, if so, what the follow-up will be like. This dialogue is what turns generic information into a safe course of action for you.

Summary: what to take from this guide

Vitamin D is essential for the body to absorb calcium and keep bones, teeth, and muscles healthy, and it also takes part in the functioning of the nerves and immunity. The body gets it from the sun, from a few foods (fatty fish, yolk, liver, cheese, fortified items), and, when necessary, from guided supplementation. Deficiency is common and tends to be silent, with well-defined higher-risk groups, and when prolonged it is associated with rickets in children and osteomalacia in adults. The correct assessment is by a blood test ordered by the doctor, not by symptoms. The non-negotiable point is caution with excess: because it is fat-soluble, vitamin D in exaggerated amounts can be toxic and cause hypercalcemia, which is why authorities define upper limits and why supplementation should always be guided by a health professional. More than chasing a number, the sensible path is a balanced diet, good hydration, safe sun exposure, and a good conversation with your doctor.