☀️ Vitamin D3 (cholecalciferol) and D2 (ergocalciferol): differences, benefits and dosage toxicity
The "sunshine vitamin": from rickets to immunity — why form and dose matter
Vitamin D is not really a single vitamin but a whole group of fat-soluble compounds, and in its action it resembles a hormone more than a classic vitamin. It is not just "needed for bones" — the work of the immune system, mood, muscle strength and even vessel health depend on it. At the same time, vitamin D deficiency is widespread: according to various data, 40–60% of adults have a below-normal level, especially in northern latitudes where there is little sun.
In pharmacies and supplement stores you will meet two main forms — D2 (ergocalciferol) and D3 (cholecalciferol). They are similar but not identical, and the difference between them directly affects how well the body "stores" the vitamin. Let's go through everything in order — from how humanity learned about this substance to exactly how much to take and where to find it in food.
History of discovery: how humanity learned about vitamin D
For a long time people suffered from rickets without understanding its cause. As early as the 17th century doctors described the "English disease" — bowed legs and weak bones in children. By the early 20th century rickets was a real epidemic in industrial cities where children barely saw the sun and buildings blocked the streets with factory smoke.
1919 — Mellanby's experiment
British doctor Edward Mellanby kept puppies indoors without light and fed them different diets. It turned out that adding cod liver oil prevented rickets. This was the first realization that the problem lay in some fat-soluble dietary factor.
1922 — the name is born
American biochemist Elmer McCollum, who had already discovered vitamin A, isolated from cod liver oil a substance that did not cure "night blindness" but did cure rickets. He named it "vitamin D" — as the next letter of the alphabet after the already known A, B, C.
1928 — Nobel Prize
German chemist Adolf Windaus received the Nobel Prize in Chemistry for studying sterols and their relation to vitamins. His work laid the foundation for understanding how ultraviolet turns provitamin into active vitamin D.
1924 — "sunshine in a bottle"
American biochemist Harry Steenbock discovered that ultraviolet irradiation of foods (and skin) enhances their anti-rickets action. This made it possible to irradiate milk with ultraviolet and effectively defeat mass rickets in the US.
1936 — isolation of D2 and D3
Windaus finally isolated and described ergocalciferol (D2) from irradiated yeast and cholecalciferol (D3) from animal sources. Since then both forms have been used in supplements, but the question "which is better" became the subject of scientific debate.
How vitamin D benefits humans
Vitamin D is a fat-soluble vitamin that in its active form works as a hormone. It influences hundreds of genes in our body. Here are the main areas where it plays an important role:
Phosphorus-calcium metabolism
Helps absorb calcium and phosphorus from the gut. Without enough vitamin D, calcium is not absorbed even if you eat enough of it. Bones become fragile — this is rickets in children and osteomalacia/osteoporosis in adults.
Protection from infections
Participates in innate immunity. Research links D deficiency with more frequent colds and respiratory infections. Vitamin D helps protective cells produce antimicrobial peptides.
Strength and fall prevention
Needed for muscle function. In older people, correcting D deficiency reduces the risk of falls and fractures. Muscle weakness is one of the symptoms of deficiency.
Psyche and seasonal swings
D deficiency is linked to low mood and seasonal affective disorder ("winter depression"). There are many vitamin D receptors in the brain.
Blood pressure and risk
Supports the vascular endothelium and regulates the renin-angiotensin system. Chronic D deficiency is associated with a higher risk of hypertension and cardiovascular events.
Pregnancy
Important when planning and during pregnancy. A sufficient D level reduces the risks of some pregnancy complications and problems in the newborn.
Where vitamin D is found: foods with numbers
The main source of vitamin D is not food but the sun. Under ultraviolet-B, the skin produces cholecalciferol (D3). However, in northern latitudes, in winter and when using sunscreens, synthesis drops sharply. So food and supplements are an important "safety net." The problem is that foods contain very little vitamin D, and it is hard to meet the need with diet alone.
| Food | D content (IU per 100 g) | Comment |
|---|---|---|
| Cod liver oil | ~10,000 IU | Record holder; 1 tsp ~ 1,300 IU |
| Herring | ~1,600 IU | Excellent source |
| Mackerel | ~1,000 IU | Good source |
| Wild salmon | ~600–1,000 IU | Higher in wild |
| Farmed salmon | ~250 IU | Less than wild |
| Sardines | ~200 IU | Average source |
| Tuna | ~100–230 IU | Depends on the species |
| Egg yolk | ~40 IU (1 pc) | Little; sun/bird feed matters |
| Mushrooms (UV-irradiated) | ~400–1,000 IU (D2) | D2 source for vegans |
| Fortified milk | ~100 IU/glass | Fortified artificially |
Daily and monthly vitamin D norms
You need to distinguish between the daily intake norm (how much to take) and the target blood level (25(OH)D). Official norms are set by the US Institute of Medicine (IOM, 2011). Many doctors and societies recommend somewhat higher "target" values for prevention.
| Category | Recommended norm | Safe upper limit |
|---|---|---|
| Adults 19–70 years | 600 IU/day (15 mcg) | 4,000 IU/day |
| Adults 71+ years | 800 IU/day (20 mcg) | 4,000 IU/day |
| Pregnancy | 600 IU/day | 4,000 IU/day |
| Deficiency correction | 2,000–5,000 IU/day (as prescribed) | Under 25(OH)D control |
| Megadose (prescribed by doctor) | 50,000 IU once/week × 6–8 weeks | Only as prescribed |
How to calculate the monthly norm
If you calculate a month's supply: 1,000 IU/day ≈ 30,000 IU/month. Some take vitamin D in courses or weekly doses. For example, instead of daily 1,000 IU you can take 7,000 IU once a week. Such a regimen is also acceptable and convenient. The main thing is not to grab giant single doses without control.
D3 vs D2: the difference and why D3 is better
Both forms can prevent rickets and raise the vitamin D level, but they differ in origin and metabolism.
| Parameter | D2 (ergocalciferol) | D3 (cholecalciferol) |
|---|---|---|
| Source | Yeast, mushrooms (plant) | Human skin, animal products |
| Skin synthesis | No | Yes, under UV-B |
| Affinity for carrier protein | Lower | Higher |
| Half-life | Shorter | Longer |
| Maintaining 25(OH)D | Weaker long-term | Stronger |
| Suitable for vegans | Yes | Only lichen-based |
The key comparative study — Armas, Hollis & Heaney (2004): after a single dose of D2, the 25(OH)D level rose faster but also fell faster, while D3 gave a more sustained rise. The meta-analysis by Tripkovic et al. (2012) confirmed that D3 is more effective than D2 at maintaining the level.
Which form to choose
Vitamin D3
Better absorption and a stable level. From lanolin (sheep) — traditional; from lichen — the vegan option.
D2 or lichen D3
D2 from mushrooms or D3 from lichen — plant-based. With D2, closer monitoring of the level is important.
Toxicity: when vitamin D becomes dangerous
Vitamin D is fat-soluble, it accumulates in tissues, so you can genuinely poison yourself with it. Hypervitaminosis D develops with long-term use of large doses — usually well above 10,000 IU/day over months. It manifests as hypercalcemia — too much calcium in the blood, which the body "pulls" from bones and absorbs from food in excess.
Reasonable doses
800–5,000 IU/day under test control. Usually well tolerated; the overdose risk is minimal.
Megadoses
Tens of thousands of IU per day for weeks. Real risk of hypercalcemia. Only as prescribed by a doctor with blood control.
Which form of vitamin D is better — D3 or D2?
For most people D3 is better: it raises and holds the 25(OH)D blood level better. D2 is a plant-based alternative for vegans, but requires closer monitoring.
How much vitamin D do I need per day?
The official norm is 600–800 IU/day. For deficiency correction, 2,000–5,000 IU/day is often used as prescribed. The safe upper limit is 4,000 IU/day for adults.
Can you poison yourself with vitamin D?
Yes, with long-term use of large doses (usually above 10,000 IU/day for months) hypervitaminosis with hypercalcemia develops — a serious condition requiring a doctor.
Which foods contain the most vitamin D?
Cod liver oil (~10,000 IU/100 g), herring (~1,600), mackerel (~1,000), wild salmon (~600–1,000), sardines (~200). Eggs and milk contain little.
How is vitamin D beneficial for humans?
Strengthens bones and teeth, supports immunity, muscle strength, mood, heart and vessel health. Receptors for it are present in almost all tissues.
What vitamin D level is considered normal?
Target 25(OH)D is 30–60 ng/mL (75–150 nmol/L). Deficiency is below 20 ng/mL. Toxicity risk is above 100 ng/mL.
Can you get vitamin D from the sun?
Yes, under UV-B the skin synthesizes D3. About 15–20 minutes of sun a day is enough. In winter and northern latitudes synthesis drops sharply, so food and supplements are needed as a backup.
How to take vitamin D — with food or on an empty stomach?
Better with food containing fat: vitamin D is fat-soluble, and fats improve its absorption from the gut.
Vitamin D3 from lanolin or lichen — is there a difference?
Chemically it is the same cholecalciferol, the only difference is the source of raw material. Lichen D3 is an option for vegans; the effect is identical.
How much vitamin D do I need per month?
At 1,000 IU/day this is ~30,000 IU/month. It is acceptable to take 7,000 IU once a week instead of daily 1,000 IU. The exact scheme is selected based on a test.