Vitamin D: Benefits, Normal Levels, Deficiency Symptoms and Dosage

In short: the key facts about vitamin D

Vitamin D is a fat-soluble vitamin needed for calcium absorption and for healthy bones, muscles and immune function. In Lithuania, about 2 in 3 people have low vitamin D levels, especially from October to April, when the sun at this latitude produces almost no vitamin D in the skin. Healthy adults usually need 15–20 µg (600–800 IU) of vitamin D per day, and the safe upper limit for adults is 100 µg (4,000 IU) per day. Vitamin D status is measured with a 25(OH)D blood test; a level below 50 nmol/L (20 ng/mL) is considered deficient. Doses above preventive levels should be prescribed by a doctor.

What is vitamin D and how does it work in the body?

Vitamin D is a group of fat-soluble compounds, of which two matter most for humans: vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). D3 is made in the skin when it is exposed to UVB sunlight and is found in animal foods; D2 comes from plants and mushrooms.

Vitamin D itself is inactive. The liver converts it into 25-hydroxyvitamin D (calcidiol, 25(OH)D), the form measured by the blood test. The kidneys and other tissues then convert 25(OH)D into the active hormone calcitriol (1,25(OH)₂D).

The main functions of vitamin D:

• Calcium and phosphorus metabolism – increases calcium absorption in the gut, which is essential for mineralising bones and teeth.

• Bone health – deficiency causes rickets in children, and osteomalacia and a higher risk of osteoporosis and fractures in adults.

• Muscle function – supports muscle strength; in older people, deficiency is linked to falls.

• Immune system – many immune cells carry vitamin D receptors; deficiency is linked to more frequent respiratory infections.

• Other associations – observational studies link low vitamin D to autoimmune and cardiovascular disease, diabetes and depression, but it has not been proven that supplements prevent these conditions.

Read more: Why is vitamin D so important for our immune system? and Arthritis and Vitamin D: What's the Connection?

Vitamin D deficiency in Lithuania

Studies in Lithuania have found low blood vitamin D levels in about 67% of the population, with normal levels in just over 20%. Levels are lowest from January to April, but even in summer around half of those tested were deficient.

Why deficiency is so common:

• Geography. Lithuania lies at about 54–56° north. From roughly October to March the sun is too low for enough UVB to reach the skin.

• Lifestyle. Indoor work, clothing and sunscreen all reduce vitamin D production in the skin.

• Diet. People eat little oily fish, and foods in Lithuania are rarely fortified with vitamin D.

In 2021, the Lithuanian College of Family Physicians published the country’s first national guidelines on the diagnosis, prevention and treatment of vitamin D deficiency. They set out who should be tested and which preventive doses to use. Doctors recommend that people over 65 take vitamin D all year round.

Vitamin D deficiency symptoms and risk groups

Symptoms of vitamin D deficiency are usually non-specific, so it often goes unnoticed for a long time. Mild deficiency frequently causes no symptoms at all.

The most common signs of vitamin D deficiency:

• persistent fatigue and low energy;

• bone, joint or back pain;

• muscle weakness, cramps or aches;

• frequent colds and respiratory infections;

• low mood, especially in winter;

• slow wound healing, hair loss;

• in children – rickets (bone deformities, late tooth eruption); in adults – osteomalacia and fractures.

People at higher risk of vitamin D deficiency:

• infants, especially if exclusively breastfed;

• pregnant and breastfeeding women;

• people over 65 (ageing skin makes less vitamin D);

• people who are overweight or obese (vitamin D is stored in fat tissue);

• people who spend little time outdoors, work night shifts or keep their skin covered;

• people with darker skin;

• people with gut diseases (coeliac disease, Crohn’s disease), liver or kidney disease, or after bariatric surgery;

• people taking certain medicines (e.g. glucocorticoids, some anti-epileptic drugs).

Normal vitamin D levels: the 25(OH)D blood test

Vitamin D status is assessed by the 25(OH)D concentration in the blood. Results are given in nmol/L or ng/mL (1 ng/mL = 2.5 nmol/L). Exact cut-offs vary slightly between laboratories and guidelines.

Does everyone need a test? The 2024 Endocrine Society guideline advises against routine 25(OH)D testing in healthy people without a specific reason (Endocrine Society, 2024). Testing is useful if you have symptoms, bone disease (osteoporosis), malabsorption, kidney or liver disease, or plan to take high doses.

Recommended vitamin D dose by age

For prevention, healthy adults usually need 15–20 µg (600–800 IU) of vitamin D per day; the safe upper limit is 100 µg (4,000 IU) per day. Conversion: 1 µg = 40 IU (international units).

Doses are based on recommendations from the European Food Safety Authority (EFSA) and the US National Academy of Medicine; upper limits follow the EFSA 2023 scientific opinion.

The 2024 Endocrine Society guideline identifies four groups that may benefit from more vitamin D than the standard recommendation: children and adolescents up to 18, pregnant women, adults over 75, and adults with prediabetes (Healio, 2024). If a deficiency is diagnosed, a doctor sets the treatment dose and duration.

How to take vitamin D: D3 or D2, when and with what

Vitamin D3 (cholecalciferol) is usually preferred because it raises blood 25(OH)D more effectively and for longer than D2. D2 suits vegans, although plant-based (lichen-derived) D3 is also available.

Practical tips:

• Take it with a meal. Vitamin D is fat-soluble, so it is absorbed better with food that contains fat.

• Daily rather than occasional large doses. For people over 50, guidelines recommend a lower daily dose instead of infrequent very high doses (e.g. 50,000 IU).

• The form matters little. Drops, capsules, tablets and sprays work similarly; drops are easiest for infants.

• Vitamin K2. Often combined with D3 because it helps direct calcium to bones. There is not yet convincing evidence that the combination is necessary for healthy people. People taking warfarin should not take K2 without their doctor’s approval.

• Magnesium. Involved in activating vitamin D, so adequate dietary magnesium matters.

• Calcium. Calcium supplements with vitamin D are only needed if dietary calcium is low or in osteoporosis.

• Interactions. Some medicines (glucocorticoids, orlistat, cholestyramine, some anti-epileptics) reduce vitamin D absorption or speed up its breakdown.

Sources of vitamin D: food and sunlight

Getting enough vitamin D from food alone is hard: you would need to eat oily fish several times a week. That is why supplements are usually recommended in Lithuania during the colder months.

Amounts vary with the fish’s origin, season and preparation method.

Sunlight. In summer (roughly May to August), 15–30 minutes of midday sun on bare arms and face lets most people make a good amount of vitamin D. For longer exposure, use sun protection, as the risk of skin cancer rises. In winter, the sun in Lithuania produces practically no vitamin D in the skin, and summer stores usually do not last through the whole cold season.

For other vitamins worth checking, see 7 Essential Vitamins You Need After Age 40.

Vitamin D overdose and safety

It is practically impossible to overdose on vitamin D from food or sunlight; toxicity almost always comes from taking high-dose supplements for a long time. EFSA set the safe upper limit for adults at 100 µg (4,000 IU) per day, and adverse effects in studies were seen from around 250 µg (10,000 IU) per day (EFSA, 2023).

Too much vitamin D raises calcium in the blood and urine (hypercalcaemia, hypercalciuria). Possible symptoms:

• nausea, vomiting, loss of appetite;

• intense thirst and frequent urination;

• constipation, abdominal pain;

• weakness, confusion;

• kidney stones, heart rhythm problems.

People with sarcoidosis or other granulomatous diseases, kidney stones, hyperparathyroidism or severe kidney disease should take vitamin D with caution and only after consulting a doctor. Keep children’s supplements out of their reach and do not exceed the manufacturer’s dose.

Learn more: 5 Signs and Side Effects of Too Much Vitamin D and Too much sun? Vitamin D overdose – real threat or myth?

FAQ: common questions about vitamin D

How much vitamin D should I take per day?
For prevention, healthy adults usually need 15–20 µg (600–800 IU) per day. If a deficiency is diagnosed, a doctor may prescribe a higher dose, most often 25–100 µg (1,000–4,000 IU) per day.

Is it better to take vitamin D in the morning or evening?
Time of day makes little difference. What matters most is taking it regularly, with a meal that contains fat.

Do I need vitamin D in summer?
Most people who spend time outdoors regularly do not need supplements in summer. People over 65, those who rarely go outside, pregnant women and infants are advised to take vitamin D all year.

How quickly does vitamin D rise in the blood?
On a steady daily dose, 25(OH)D levels stabilise within about 2–3 months, so a follow-up test makes sense no earlier than 3 months.

Can you overdose on vitamin D?
Yes, but only by taking high-dose supplements for a long time, usually above 250 µg (10,000 IU) per day. At recommended doses the risk is very low.

What is the difference between vitamin D and D3?
Vitamin D is the general name. D3 (cholecalciferol) is one of its forms – the one made in human skin and most often used in supplements.

Does vitamin D help with depression, colds or COVID-19?
Deficiency is associated with these conditions, but studies have not shown that supplements treat them in people who are not deficient. Supplements help most when levels are low.

Sources

1. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907–1947.

2. Endocrine Society. Guideline recommends healthy adults under the age of 75 take the recommended daily allowance of vitamin D, 2024.

3. EFSA NDA Panel. Scientific opinion on the tolerable upper intake level for vitamin D. EFSA Journal. 2023;21(8):8145.

4. EFSA NDA Panel. Scientific Opinion on the Tolerable Upper Intake Level of vitamin D. EFSA Journal. 2012;10(7):2813.

5. Healio. Endocrine Society guideline: Most adults do not benefit from additional daily vitamin D, 2024.

6. Bonum.lt. Why is vitamin D so important for our immune system?

7. Bonum.lt. 5 Signs and Side Effects of Too Much Vitamin D

8. Bonum.lt. Too much sun? Vitamin D overdose – real threat or myth?

9. Bonum.lt. Arthritis and Vitamin D: What's the Connection?

10. Bonum.lt. 7 Essential Vitamins You Need After Age 40

Disclaimer. This article is for information only and does not replace advice from a doctor or pharmacist. Before taking higher doses of vitamin D – especially if you have a chronic condition, are pregnant or take other medicines – consult your doctor.