Vitamin D: Benefits, Dosage and When You Need a Blood Test

10 minutes

Advice about vitamin D can be confusing: should you take it all year or only in winter, get a blood test first or simply buy a supplement? Learn how much adults need, whether food can provide enough and how to avoid excessive doses, without promises that one supplement can solve every health problem.

15 August 2026

Unagrande Yoga Expert

252

What is vitamin D?
What does vitamin D do in the body?
Daily vitamin D requirements
Which foods contain vitamin D?
Can you get enough vitamin D from sunlight?
Who is at greater risk of low vitamin D?
Symptoms of vitamin D deficiency
Which blood test measures vitamin D?
How to take vitamin D
Choosing a dose
Combining vitamin D with K2, magnesium and omega-3
The risks of too much vitamin D
Frequently asked questions
The key points about vitamin D

There is plenty of talk about vitamin D: some people take it all year round, others hope to get everything they need from the sun, and others start with a blood test. What makes sense? Vitamin D does help the body absorb calcium and phosphorus, supports healthy bones and contributes to normal muscle function.

But more is not always better. Tiredness or a low mood alone do not prove that you are deficient, not every healthy person needs a test, and high doses without a medical reason can cause harm. Let’s look at how much vitamin D adults need, whether food can provide enough and how to take a supplement safely.

The essentials: Russian dietary reference values recommend 15 mcg, or 600 IU, of vitamin D a day for adults, rising to 20 mcg, or 800 IU, from age 65. This is the total from food and supplements, rather than a supplement dose that everyone should take. Recommendations differ between countries.

What is vitamin D?

The name “vitamin D” covers several fat-soluble compounds. Two forms are most common in food and supplements:

D2, or ergocalciferol, is mainly obtained from mushrooms and yeast;
D3, or cholecalciferol, is found in animal foods and is produced in the skin when it is exposed to UVB rays. Vegan D3 can also be made from lichen.

Both forms are absorbed in the intestine and raise blood levels of 25(OH)D. On average, however, D3 raises the level somewhat more effectively and maintains it for longer, which is why it is more commonly used in supplements.

Once it enters the body, vitamin D is converted in the liver and kidneys. This produces the active form that helps regulate calcium and phosphorus metabolism.

What does vitamin D do in the body?

The clearest and best-studied role of vitamin D is supporting bones and muscles. It:

helps the intestine absorb calcium and phosphorus;
helps maintain normal levels of these minerals in the blood;
is needed for the mineralisation and renewal of bone tissue;
contributes to muscle function and communication between nerves and muscles;
plays a part in immune function and the regulation of cellular processes.

Observational studies have linked low 25(OH)D levels with depression, cardiovascular disease, autoimmune conditions and other illnesses. An association does not mean that a lack of vitamin D caused the illness. Clinical trials have not established vitamin D supplements as a universal way to prevent depression, heart disease, cancer or colds in people without a confirmed deficiency.

Vitamin D matters, but it is not a cure-all. It cannot replace a balanced diet, movement, sleep or prescribed treatment.

Daily vitamin D requirements

The numbers can be confusing: a dietary requirement, a preventive supplement dose and a treatment plan are different things. Russian dietary reference values recommend the following daily amounts for adults:

Age	Vitamin D per day
18–64 years	15 mcg, or 600 IU
65 years and over	20 mcg, or 800 IU

1 mcg of vitamin D = 40 IU.

These values describe the average amount needed from all sources. They do not mean that every adult must take a supplement at exactly this dose each day: some vitamin D comes from food, and some may be produced in the skin.

Local guidance varies. US recommendations are 600 IU daily for adults through age 70 and 800 IU after that. UK guidance uses 400 IU (10 mcg) daily and advises considering a supplement in autumn and winter, or throughout the year for some people at higher risk. Follow the guidance for your country and any individual medical advice.

Treatment plans for a confirmed deficiency differ from ordinary dietary requirements. They are chosen individually, taking into account blood test results, health conditions, medicines and the risk of taking too much.

Which foods contain vitamin D?

There are relatively few natural food sources of vitamin D. Oily fish and fish oils contain the most. Eggs and liver provide smaller amounts, and mushrooms can accumulate D2 after exposure to ultraviolet light.

Approximate amounts per serving:

Food and serving size	Vitamin D	IU
Cooked trout, 85 g	16.2 mcg	645
Cooked salmon, 85 g	14.2 mcg	570
UV-exposed white mushrooms, ½ cup	9.2 mcg	366
Fortified milk, 240 ml	about 2.9 mcg	about 120
Fortified plant-based drink, 240 ml	2.5–3.6 mcg	100–144
Canned sardines, 2 fish	1.2 mcg	46
1 egg	1.1 mcg	44
Cooked beef liver, 85 g	1 mcg	42

Can food provide all the vitamin D you need?

In principle, yes, although not everyone manages it regularly. An 85 g serving of cooked trout provides around 645 IU, meeting the Russian dietary requirement for an adult aged 18–64. The same-sized serving of salmon provides around 570 IU, almost the full amount.

Without fish, it is more difficult. For example, half a cup of UV-exposed mushrooms, a glass of fortified plant-based drink and an egg together provide approximately 510–550 IU. Vitamin D levels in mushrooms and fortified foods vary considerably, so check the label.

It is possible to obtain enough from food alone, particularly if your diet includes oily fish or fortified foods. An egg, cheese and ordinary unfortified milk will not meet the requirement on their own. If you eat few vitamin D sources, consider your diet and risk factors rather than automatically starting a supplement.

Can you get enough vitamin D from sunlight?

The honest answer is that sunlight may sometimes be enough, but there is no universal number of minutes that works for everyone. Vitamin D production in the skin depends on:

the season and latitude;
the time of day and cloud cover;
how much skin is exposed;
age and skin pigmentation;
clothing and sunscreen;
individual differences.

UVB rays barely pass through window glass, so sunlight indoors does not produce vitamin D in the same way.

Do not deliberately sunbathe or stop using SPF to obtain vitamin D. Excessive ultraviolet exposure damages the skin and increases the risk of cancer. The World Health Organization recommends avoiding excessive sun exposure, checking the UV index and using clothing, shade and sunscreen.

Who is at greater risk of low vitamin D?

This is not a checklist for self-diagnosis, but low levels are more likely in people who:

spend little time outdoors or usually cover their skin with clothing;
have conditions that impair the absorption of fats and vitamins;
have had bariatric surgery;
have certain liver or kidney conditions;
eat a diet without fish or fortified foods;
are older;
take medicines that affect vitamin D metabolism;
have osteoporosis, osteomalacia or recurrent fractures after minor injuries.

Obesity is often associated with a lower 25(OH)D concentration, but it does not automatically mean that a person needs a test or a high-dose supplement.

Symptoms of vitamin D deficiency

A mildly low vitamin D level often causes no noticeable symptoms at all. Tiredness, sleepiness, hair loss, frequent colds or a low mood are nonspecific: they have many possible causes and cannot be used to diagnose a deficiency.

Severe, prolonged deficiency in adults can lead to osteomalacia, a condition in which bones do not mineralise properly. Possible effects include:

bone pain;
muscle weakness;
difficulty climbing stairs or rising from a chair;
an increased risk of falls and fractures;
disturbances in calcium metabolism.

In children, severe deficiency can cause rickets. Children’s requirements and preventive doses differ from those for adults and should be discussed separately with a paediatrician.

Which blood test measures vitamin D?

Vitamin D status is assessed by measuring 25-hydroxyvitamin D, or 25(OH)D, in blood serum. A test for the active form, 1,25(OH)₂D, is not suitable for a routine assessment of vitamin D stores and is used for specific medical reasons.

Does everyone need a test?

No. The Endocrine Society’s 2024 clinical guideline advises against routine 25(OH)D screening in healthy adults without a specific indication. There is no established evidence that testing everyone without symptoms improves health outcomes.

A doctor may discuss testing when osteomalacia is suspected, when calcium metabolism is abnormal, for conditions that impair absorption, after bariatric surgery, for certain kidney or liver conditions, for osteoporosis or when treatment needs monitoring.

What is a normal blood vitamin D level?

There is no single universal threshold. Professional organisations take different approaches:

the 2016 Russian recommendations for adults define deficiency as a level below 20 ng/ml and insufficiency as 20–30 ng/ml;
the US Food and Nutrition Board considers 20 ng/ml or above adequate for most people, with a substantially increased risk of deficiency below 12 ng/ml;
in 2024, the Endocrine Society stopped defining a universal target level for disease prevention in healthy people.

Your result should therefore be interpreted with a doctor, taking into account the reason for testing, your symptoms, bone health, calcium level and other findings. There is no need to try to raise it to the top of the laboratory’s reference range on your own.

How to take vitamin D

If you need a supplement, you do not have to search for the strongest or trendiest product. An appropriate dose, clear instructions and taking it consistently matter more.

D2 or D3?

Both forms work, but D3 usually raises 25(OH)D somewhat more effectively and maintains the level for longer. D2 and lichen-derived D3 are suitable for vegans.

Drops, capsules or tablets?

The formulation alone does not determine effectiveness. The important factors are an accurate dose, clear instructions, product quality and how easy it is to take regularly. With drops, check how many IU are in a single drop, because concentrations vary between products.

Morning or evening?

There is no convincing evidence that one time of day is better. You can take vitamin D whenever you are most likely to remember it.

Before or after food?

Vitamin D is fat-soluble. Fat in the intestine improves its absorption, so it is convenient to take a supplement during or after a normal meal containing some fat. Some vitamin D is absorbed even without it.

Every day or a large dose once a week?

When supplementation is indicated, the schedule depends on the purpose and the doctor’s prescription. For adults aged 50 and over who need supplementation or treatment, the Endocrine Society favours lower daily doses over infrequent high doses.

1,000, 2,000, 5,000 or 10,000 IU: which dose should you choose?

A larger number on the packaging does not mean a product is better. It also does not tell you what dose a particular person needs.

600–800 IU is the Russian daily dietary requirement for adults, depending on age, rather than a mandatory supplement dose;
1,000–2,000 IU is common in supplements, but choosing a dose still depends on diet, medical indications and professional advice;
5,000–10,000 IU should not be used as a routine preventive dose without medical supervision.

NASEM sets the tolerable upper level for long-term daily vitamin D intake in adults at 4,000 IU, or 100 mcg, from all sources. This is a safety limit, not a recommended target. A doctor may temporarily prescribe a higher dose for treatment, but this requires a specific medical reason and monitoring.

Should you take vitamin D with K2, magnesium or omega-3?

There is usually no need to build a collection of supplements around vitamin D. Each supplement should have its own reason for use.

Vitamin K2

K2 does not improve vitamin D absorption, and there is no universal recommendation to take them only together. People taking vitamin K antagonist anticoagulants should not change their vitamin K intake or start vitamin K supplements on their own.

Magnesium

Magnesium is involved in many enzyme processes, including vitamin D metabolism. This does not mean everyone taking D3 also automatically needs magnesium. Diet and medical indications should be assessed first. For more about food sources and supplements, see “Magnesium: A Key to Energy, Calm and Health”.

Omega-3

You can take omega-3 and vitamin D with the same meal if you need both. There is no essential combination in which one must be taken to enhance the other.

Calcium

Vitamin D helps calcium absorption, but not everyone needs a calcium supplement. In most cases, it is better to start by assessing how much calcium your diet provides. Read more in “Calcium: How Much You Need and Where to Find It”.

Vitamin D generally does not need to be taken at a separate time from vitamin C, iron or zinc. If you take several supplements and medicines, however, it is best to discuss the schedule with a doctor or pharmacist.

The risks of too much vitamin D

Vitamin D toxicity is almost always caused by excessive supplement intake rather than food or sunlight. Too much vitamin D increases calcium absorption and can cause hypercalcaemia, an abnormally high blood calcium level.

Possible signs of toxicity include:

nausea and vomiting;
marked weakness;
loss of appetite;
intense thirst and frequent urination;
dehydration;
kidney stones and impaired kidney function;
abnormal heart rhythms in severe cases.

The risk can be higher when high doses are combined with calcium supplements, or with certain kidney or parathyroid conditions. Vitamin D can also interact with thiazide diuretics, orlistat, glucocorticoids and some other medicines.

Frequently asked questions

Can I take vitamin D without a blood test?

A healthy adult does not necessarily need a blood test before any preventive intake. But do not start high doses or treatment for a suspected deficiency on your own. If you have a medical condition, symptoms or take medicines regularly, discuss the decision with a doctor.

Do I need vitamin D in summer?

There is no universal answer. Summer production depends on your region, time outdoors, clothing, age, skin pigmentation and other factors. Do not deliberately sunbathe or stop using sun protection. If a doctor has prescribed vitamin D, do not stop it simply because the season has changed.

Is oil-based or water-based vitamin D better?

For most adults, the dose and consistency matter more than the type of solution. Absorption may be important in certain digestive conditions; in that case, choose the product with a doctor.

Can I take vitamin D in the evening?

Yes. There is insufficient reliable evidence that taking it in the evening worsens sleep or absorption. Choose a convenient time and follow the instructions.

How long should I take vitamin D?

It depends on the purpose. The body needs an adequate vitamin D intake on an ongoing basis, but how long you need a supplement depends on your diet, the season, your age, health conditions and the prescribed dose. A doctor decides the length of a treatment course and when to review it.

What if I miss a dose?

You usually do not need to double the next dose. Continue your normal schedule unless the product instructions or your doctor say otherwise.

The key points about vitamin D

The main established role of vitamin D involves calcium and phosphorus metabolism, bones and muscles.
Russian dietary reference values recommend 600 IU daily for adults and 800 IU from age 65, from all sources. Recommendations in other countries differ.
Tiredness, hair loss or frequent colds cannot be used to diagnose a deficiency.
A 25(OH)D test is used when medically indicated, rather than automatically for every healthy person.
D3 usually maintains vitamin D levels for longer, but D2 also works.
The time of day is not crucial; taking a supplement with a normal meal is convenient.
K2, magnesium, omega-3 and calcium do not need to be added automatically to D3.
Doses of 5,000–10,000 IU are unsuitable for self-directed daily prevention.

Vitamin D is important for health, but looking for the strongest supplement is not the best starting point. First consider your diet and risk factors. If there is a medical reason, discuss testing and an appropriate dose with a doctor.