Vitamin D – Why Is This Sunshine Vitamin So Important for Us?

Affectionately called the sunshine vitamin, vitamin D plays many vital roles in the body. Without it, bones, teeth, and even the immune system cannot function properly. And that’s just the tip of the iceberg.

In Estonia, vitamin D levels among the population have unfortunately been studied very little. The most widely referenced data comes from orthopedic doctor Leho Rips’s doctoral dissertation at Tartu University Hospital, published in 2024. His research was conducted at three different time points among conscripts. The results were rather discouraging.

In an observational study where participants did not receive vitamin D supplements, 36% had a critical deficiency (<25 nmol/L) by spring. The second and third parts of the study showed that supplementation with 600 IU was not sufficient, 1200 IU gave only a small effect, while 4000 IU was significantly more effective at maintaining healthy blood levels.

We are not all conscripts, but we live under the same Estonian sky, touched by the same sun and influenced by the same factors. People seem to know they should take vitamin D, yet deficiency remains widespread. Why? The reasons may be more surprising than you’d think.

What Is Vitamin D and Why Do We Need It?

Vitamin D is a fat-soluble vitamin that performs many essential functions in the human body:

• Helps absorb calcium and phosphorus, vital for the development and strength of bones and teeth.
• Strengthens the immune system and helps protect against bacterial and viral infections.
• Reduces inflammation and supports chronic inflammatory conditions.
• Helps maintain a healthy nervous system.
• Supports blood clotting and heart function.
• Assists in proper muscle contraction strength.

Long-term deficiency can have very serious consequences. Infants and young children are especially at risk of rickets, a disease that causes bones to soften and bend. Older adults are another high-risk group, suffering from osteoporosis (bone thinning) and osteomalacia, which leads to weak bones, bone pain, and muscle weakness.

What Causes Vitamin D Deficiency in the Body?

Vitamin D deficiency in the body can be caused by several factors or a combination of them. The most important are (A5, A8, A6):

• Low sun exposure
• Conscientious use of sunscreen
• Older age
• A diet poor in vitamin D
• Several diseases (e.g., kidney failure, Crohn’s disease, chronic inflammation, celiac disease)
• Darker skin pigmentation
• Excessive alcohol consumption and regular smoking

If you’re now sitting with a glass of wine, realizing you haven’t been out in the sun for a while, it may be time to book a vitamin D test and, for prevention, take that vitamin D supplement out of the kitchen cupboard. And if you don’t yet have a vitamin D supplement at home, check here.

Who Suffers Most from Vitamin D Deficiency?

The best natural source of vitamin D is sunlight, and looking out the window in Estonia, it’s easy to understand why vitamin D deficiency can develop so easily.

Vitamin D deficiency most threatens:

• All people in Estonia every year from September to May
• People who spend a lot of time indoors
• People diagnosed with osteoporosis
• Older adults
• Frequent sunscreen users
• Regular smokers and alcohol consumers
• Obese people
• People with chronic inflammatory conditions
• Pregnant and breastfeeding women

Even office workers and others who spend most of their waking hours indoors under artificial light are not spared from the higher risk of deficiency. Sunlight shining through the window doesn’t help either, since glass blocks UVB rays, which are essential for vitamin D synthesis in the body. So, it’s worth spending your lunch break outdoors, lying on a picnic blanket, and turning your face toward the sun.

For the same reason, those who spend little time outdoors in summer (e.g., older adults, people with mobility limitations) are also at higher risk. In addition, older adults’ skin has a reduced ability to produce vitamin D, which further increases the risk (A1).

The graph is based on a study by Young et al. (2018), which found that aging reduces vitamin D synthesis by approximately 13% every decade. The graph was created using modeling.

The risk of deficiency is also higher for indoor athletes (ball games, swimmers) not only because of reduced sun exposure but also because their muscle cells consume more vitamin D reserves during exercise (A1).

Surprisingly, obese individuals are also at risk because some of the vitamin D absorbed through the digestive system or produced in the skin is stored in fat tissue. The greater the body fat, the more vitamin D binds to it, leaving less available in the bloodstream as 25(OH)D vitamin (A5).

Several studies have shown that smokers have about 10–15% lower vitamin D levels than non-smokers. Tobacco smoke causes vitamin D to break down faster in the liver and kidneys, leading to lower blood levels. There is also evidence that smokers may have intestinal mucosal damage, which disrupts the absorption of fats and fat-soluble vitamin D (A14, A15).

Sunscreen use: According to international dermatology organizations, SPF15 sunscreen blocks ~93% of UVB rays and SPF30 blocks ~97% in laboratory conditions, thereby inhibiting vitamin D synthesis in the skin. In real life, the percentage is lower since it’s difficult to cover the skin perfectly evenly. Still, sunscreen use is very important to prevent premature skin aging and skin diseases. From this perspective, we recommend protecting yourself with sunscreen whenever you spend time outdoors in the sun.

Symptoms of Vitamin D Deficiency

Vitamin D deficiency often sneaks up with symptoms that go unnoticed or appear vague, resembling other conditions. The most common possible symptoms (A6, A7, A8):

• Persistent fatigue or low mood
• Bone pain or history of fractures
• Weakening or decaying teeth
• Muscle pain
• Tingling or needle-like sensations in hands or feet
• Hair loss
• Increased susceptibility to infections
• Pale skin
• Muscle twitches or tremors

Daily Vitamin D Intake Needs and Blood Levels

Before starting active supplementation, it is worth checking your blood vitamin D levels. The optimal level is considered to be above 75 nmol/L year-round (A1, A5).

Reference values for vitamin D levels in blood:

• <25 nmol/L – severe deficiency
• 25–50 nmol/L – moderate deficiency
• 50–75 nmol/L – suboptimal
• 75–150 nmol/L – optimal
• 250 nmol/L – toxic

How Much Vitamin D Is Safe to Consume Daily?

Daily vitamin D needs depend largely on blood levels, body weight, lifestyle, age, and other health conditions. Typically, recommendations are based on a 70 kg adult, for whom the safe upper limit is 4000 IU (57 IU/kg). However, this does not mean that a 100 kg person should take 5700 IU. Larger doses should always be discussed with a doctor, and blood levels should be tested before and after a supplementation course to assess whether 4000 IU daily has been sufficient (the trend should be upward). This way, you’ll learn what your body truly needs (A16).

According to the European Food Safety Authority (EFSA), which Estonia also follows, the safe daily upper intake levels are (A10, A16):

• Infants up to 12 months: 25 µg / 1000 IU
• Children 1–10 years: 50 µg / 2000 IU
• Adolescents and adults (11 years and older): 100 µg / 4000 IU
• Pregnant and breastfeeding women: 100 µg / 4000 IU

Remember: 1 µg = 40 IU.

Sources of Vitamin D

Sunlight and Synthesis in the Skin

Vitamin D synthesis in the skin occurs through ultraviolet radiation, specifically UVB rays, which convert 7-dehydrocholesterol in the skin to vitamin D3. Factors influencing synthesis include:

• Season and time of day – spring and summer sunlight is stronger than autumn and winter; midday sun is most effective.
• Weather – cloud cover reduces synthesis.
• Skin type and age – darker skin and older age reduce synthesis.
• Windows – UVB rays do not pass through glass.
• Sunscreen – SPF 8 and above reduces vitamin D production significantly.

In Estonia, the sun is only a reliable source of vitamin D for 3–4 months a year (mainly in summer). From September to May, UVB levels are too low for meaningful synthesis (A1).

Vitamin D–Rich Foods

Daily needs cannot realistically be met through food alone. For example:

• Fatty fish (e.g., farmed salmon, mackerel, sardines): 100–300 IU per 100 g
• Egg yolk: ~40 IU per egg
• Milk or kefir: ~40 IU per liter
• Fortified foods: 80–160 IU per 200 ml glass

Food usually covers only 1–10% of daily needs.

Supplements

Because diet and sunlight are insufficient in Estonia, supplements are the most effective way to ensure adequate intake.

Vitamin D Dosage – How Much and How to Take?

First, check your serum 25(OH)D level. Optimal is above 75 nmol/L year-round. Daily needs depend on age, weight, and risk group.

• Infants up to 1 year: ~400 IU

• Children 1–2 years: 400–800 IU

• Older children (e.g., 12 years): 800–1600 IU

• Adults: depends on weight and lifestyle; safe daily upper limit is 4000 IU for a 70 kg person

Risk groups (e.g., osteoporosis patients, older adults) may need more.

Vitamin D Overdose and Safety

Vitamin D is essential, but too much can cause hypervitaminosis D – rare but possible. Serious cases usually occur from taking >10,000 IU daily for months (A18).

Symptoms of toxicity are linked to hypercalcemia (excess blood calcium):

• Fatigue, weakness, dizziness
• Frequent urination and thirst
• Nausea, vomiting, constipation, abdominal pain

How and When to Take Vitamin D?

Vitamin D is fat-soluble, so it should be taken with a fat-containing meal (preferably healthy fats). Morning or evening does not matter – consistency is key (A12).

For deficiency treatment, ensure adequate calcium intake as well, since together they reduce fracture risk (A6).

Q&A

Does more sun mean more vitamin D?

No. After about 10–30 minutes of midday sun exposure (longer for darker skin), the body begins to break down excess vitamin D to avoid overproduction (A11).

What’s the difference between vitamin D2 and D3, and which should you choose?

D2 (ergocalciferol) is plant-based, found mainly in fortified foods. D3 (cholecalciferol), usually animal-based and also produced by the skin under UVB rays, maintains blood levels more effectively and for longer (A4).