Population studies leave no room for doubt: vitamin D deficiency is one of the most common health problems in Poland. It is estimated that it affects as many as 90% Poles during the fall and winter months. And the consequences extend far beyond just the bones—vitamin D affects the immune system, mood, sleep, hormone balance, and the regulation of inflammation.
Why Poles, of all people?
Poland lies at a latitude of 49–54°N. Vitamin D synthesis in the skin under the influence of UVB radiation is possible here only for about 4–5 months a year—roughly from May to September, between 10 a.m. and 3 p.m. Outside of this period of sunshine, the skin produces virtually no vitamin D, regardless of how much time we spend outdoors.
Added to this are lifestyle factors: office work and a sedentary lifestyle mean that even in the summer, many people spend most of their day indoors. Sunscreen—though essential for skin protection—further limits UVB synthesis. It doesn’t help that the average Pole’s diet is low in natural sources of vitamin D (fatty ocean fish, egg yolks, and liver).
Result: According to data from the PolSenior study and other population-based studies, vitamin D deficiency (below 20 ng/ml) or insufficiency (20–30 ng/ml) affects the vast majority of Polish adults, with rates peaking during the winter months.
What is vitamin D really needed for?
Vitamin D is a steroid hormone, not a traditional vitamin—its receptors (VDR) are found in nearly every tissue in the body. This explains its “pleiotropic” (multifaceted) effects, as researchers have noted. It also means that the body can synthesize it on its own when exposed to sunlight.
- Bones and Calcium-Phosphate Metabolism – Vitamin D regulates the absorption of calcium and phosphorus in the intestines. Without it, calcium from the diet and supplements will not be properly absorbed. A deficiency leads to osteomalacia in adults and rickets in children, and in the long term increases the risk of osteoporosis and fractures.
- Resilience – Vitamin D regulates the activity of immune system cells: monocytes, macrophages, and T cells. A deficiency in vitamin D is associated with an increased susceptibility to respiratory infections and impaired modulation of the immune response.
- Mood and Mental Health – VDR receptors are present in the brain, and vitamin D plays a role in serotonin synthesis. A deficiency is associated with an increased risk of seasonal depression and a general decline in mood—this is one of the mechanisms behind the so-called “winter blues.”.
- Dream – Vitamin D influences the circadian rhythm by regulating the expression of genes associated with the biological clock. A deficiency in vitamin D is associated with shorter and less restorative sleep.
- Muscles and Recovery – Vitamin D plays a role in muscle protein synthesis and neuromuscular transmission. Physically active people with a deficiency recover less effectively after exercise and are more prone to injuries.
- The Cardiovascular System and Metabolism – Low vitamin D levels are associated with an increased risk of hypertension, insulin resistance, and metabolic syndrome, although the direction of causality is still being studied.
How can you tell if you have a vitamin D deficiency?
Vitamin D deficiency rarely causes clear, characteristic symptoms and, as a result, is often overlooked for years. Nonspecific signs that may indicate low levels include:
- chronic fatigue and weakness with no apparent cause
- recurrent infections, especially of the respiratory tract
- low mood, irritability, seasonal depression
- muscle and joint pain, especially in the long bones
- sleep problems, difficulty falling asleep
- hair loss (related to deficiencies in general)
- slow wound healing
The only reliable way to assess vitamin D levels is through a blood test: measuring the concentration of 25(OH)D (25-hydroxyvitamin D). This is a simple and relatively inexpensive test available at any laboratory. It’s a good idea to have it done in the fall or spring.
What is the optimal level of vitamin D in the blood?
Vitamin D concentration is expressed in ng/ml or nmol/l (1 ng/ml = 2.5 nmol/l). According to the 2023 Polish guidelines (Płudowski et al.):
- less than 20 ng/ml (50 nmol/l) – deficiency – requires supplementation or treatment
- 20–30 ng/ml – insufficient level – supplementation recommended
- 30–50 ng/ml – the optimal level for most adults
- 50–100 ng/ml – high level – safe with controlled supplementation
- above 100 ng/ml – potential toxicity – requires medical consultation
How Much Should You Take? Current Polish Guidelines (2023)
According to the guidelines by Prof. Płudowski et al. (Nutrients, 2023), the preventive dosage for adults depends on age, body weight, and lifestyle:
- Adults aged 18–65 – 1,500–2,000 IU per day as a preventive supplement. If sun exposure is limited, take the upper limit of the range.
- People over 65 years of age – 2,000–4,000 IU per day – skin synthesis declines dramatically with age, and intestinal absorption is less efficient.
- People with obesity (BMI > 30) require higher doses—adipose tissue sequesters vitamin D, reducing its bioavailability. Typically, twice the standard dose.
- If a deficiency is confirmed, therapeutic doses (4,000–10,000 IU) should be administered only under medical supervision and following a 25(OH)D level test.
Important: The guidelines do not recommend routine vitamin D testing in healthy adults without risk factors. However, supplementation at prophylactic doses (1,500–2,000 IU) is widely recommended due to Poland’s geographical conditions.
Vitamin D2 or D3? Which form should you choose?
There are two forms of vitamin D available on the market in supplement form: D2 (ergocalciferol, plant-derived) and D3 (cholecalciferol, synthesized in the skin of animals). Meta-analyses clearly show that vitamin D3 is more effective at raising and maintaining blood levels of 25(OH)D than D2 at equivalent doses. Polish guidelines recommend D3 as the form of choice.
Synergy with Vitamin K2 and Magnesium—Why Combine Them?
Vitamin D does not work in isolation. Its effectiveness and safety depend on two key cofactors that are often overlooked when taking supplements.
Vitamin K2 (MK-7) – Vitamin D increases calcium absorption, but K2 determines where that calcium ends up. It activates osteocalcin (which directs calcium to the bones) and inhibits the calcification of blood vessels. High-dose D3 supplementation without K2 could theoretically increase the risk of calcium deposition in the arteries. The MK-7 (menaquinone-7) form has the longest half-life and the best bioavailability. Typical preventive dose: 100–200 µg per day.
Magnesium – is essential for the conversion of vitamin D into its active form (calcitriol) in the liver and kidneys. Without sufficient magnesium, supplemented vitamin D may not be activated. Magnesium deficiency is just as common as vitamin D deficiency—this combination explains why many people do not see the benefits of D3 supplementation.
Practical tip: When taking a D3 supplement, make sure to also get enough K2 (MK-7 form) and maintain adequate magnesium levels.
How should you take vitamin D supplements to make them effective?
A few practical tips:
- Take it with fat—vitamin D is fat-soluble, and when taken with a meal containing fat (avocado, nuts, olive oil), it is absorbed up to 50% better than on an empty stomach.
- In the morning or at noon—taking this supplement in the evening may disrupt sleep in some people (vitamin D can stimulate cortisol production). Take it in the morning or at lunch.
- Consistency – the effects of vitamin D supplementation build up over weeks. Daily consistency is more important than occasional high doses.
- Check your levels after 3 months—if you’re starting supplementation after being diagnosed with a deficiency, have a follow-up 25(OH)D test after about 3 months to assess the effectiveness and adjust the dose if necessary.
How much vitamin D does the diet provide?
Your diet can provide some vitamin D, but it rarely meets your full daily needs. The best dietary sources:
- salmon (about 500–900 IU per 100 g) – the best natural source
- herring and mackerel (about 300–500 IU per 100 g)
- sardines in oil (about 270 IU per 100 g)
- egg yolk (about 40 IU per egg)
- beef liver (about 50 IU per 100 g)
- sun-dried mushrooms – the only significant plant source of vitamin D2
Even regular consumption of saltwater fish covers only part of the daily requirement. In Poland’s climate, supplementation is a necessity for most people, not just an option.
Sources
Płudowski et al. (2023), “Guidelines for Preventing and Treating Vitamin D Deficiency: A 2023 Update in Poland,” Nutrients. https://pubmed.ncbi.nlm.nih.gov/36771403/
PubMed: https://pubmed.ncbi.nlm.nih.gov/?term=vitamin+D+immunity
Frequently Asked Questions
Is it possible to overdose on vitamin D?
Yes, although an overdose is very unlikely at prophylactic doses (up to 4,000 IU). Toxicity manifests as hypercalcemia: nausea, weakness, headaches, heart rhythm disturbances, and calcium deposits in the kidneys and blood vessels. The risk increases with doses above 10,000 IU per day taken over an extended period without laboratory monitoring. Therefore, high therapeutic doses require medical supervision.
When is the best time to start taking supplements—in the fall or year-round?
Ideally, year-round, except during the months when you regularly spend time in the sun with your arms and legs exposed for at least 15–20 minutes a day (in Poland, approximately May–August, only between 10 a.m. and 3 p.m.). In practice, for most Poles, it is safer to take a preventive dose of the supplement for 12 months than to take breaks in the summer and experience a severe deficiency in the winter.
Is vitamin D from sunscreen absorbed through the skin?
No—sunscreen applied to the skin blocks UVB radiation but does not provide vitamin D on its own. Vitamin D in creams (cosmetics) has marginal bioavailability through the skin and is not a substitute for oral supplementation.
I have dark skin. Does that matter?
Yes, and it's a big one. Melanin in the skin absorbs UVB radiation, reducing vitamin D synthesis. People with dark skin need up to 3–5 times longer exposure to the sun to produce the same amount of vitamin D as people with fair skin. Supplementation is particularly important for them.
Do children need supplements, too?
Yes. Infants should receive 400 IU per day starting from the first days of life, regardless of their feeding method (breast milk contains trace amounts of vitamin D). Older children and adolescents: 600–1,000 IU per day, depending on age, season, and sun exposure. Specific doses are determined by a pediatrician.
This article is for informational purposes only. It is not a substitute for medical advice—the dosage for a diagnosed vitamin D deficiency should be discussed with a doctor.
