Vitamin D Explained Simply

1. What Is Vitamin D?

Vitamin D is a group of fat-soluble vitamins. The most important are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). The body itself forms vitamin D3 in the skin when sunlight with UVB radiation hits the skin. In the liver and kidneys, the storage form and the active form are produced from it.

2. Where Is It Found, and What Is It Made From?

We absorb only little vitamin D from food. It is found mainly in fatty sea fish, cod liver oil, egg yolk and in mushrooms that have been exposed to UV light. In our latitudes, the sun is hardly sufficient for self-production during the winter half of the year. Vitamin D3 for preparations is mostly obtained from wool fat, for vegan preparations from lichens. Vitamin D2 is produced from yeast or mushrooms.

3. What Does Vitamin D Do? What Research Shows

Infections and Immune System

In COVID-19, mortality was lower with vitamin D in several studies, but a review warns that small studies overestimate the benefit 20307141. In children with asthma, severe attacks decreased 2667. Inflammation values decreased in type 2 diabetes and dialysis patients 2335. Vitamin D did not overall prevent respiratory infections in children 121956, but in schoolchildren in winter the number of sick days decreased 61.

Bones, Falls and Fractures

Children whose mothers received vitamin D during pregnancy had higher bone density at primary school age 53. In children and young adults with HIV, a relatively high daily dose improved bone density 28. For falls in older people, some studies found no difference, others fewer falls 94758456. Fewer bone fractures were shown only when vitamin D was given together with calcium 6.

Blood Sugar and Metabolism

In adults with type 2 diabetes and women with polycystic ovary syndrome, blood sugar and insulin resistance improved with vitamin D3 233377. In healthy people and in fatty liver disease, blood sugar and insulin values remained without difference 1734. Large studies in adults without diabetes found no difference in newly occurring type 2 diabetes 4445. It remains open which groups and which dose benefit.

Heart, Circulation and Blood Pressure

For heart, circulation and blood pressure, results without difference predominate. Several studies found no change in blood pressure 174551606572. Cardiovascular mortality also remained unchanged 435257. A meta-analysis (joint evaluation of many studies) showed less atrial fibrillation after bypass surgery 32. It remains open whether certain groups do benefit.

Mood and Depression

In adults with depression, vitamin D improved symptoms compared with placebo (sham treatment) 2959. In women with premenstrual syndrome, it improved depression, not anxiety 38. In children with ADHD, vitamin D3 plus magnesium improved emotional problems, anxiety and shyness 747675. A large study found no difference in newly occurring severe depression 48. The studies are often small and differ in dose and group.

Pregnancy and Children

A review with meta-analysis found less gestational diabetes and a somewhat higher birth weight 42. With cholecalciferol, spontaneous vaginal births occurred more often because instrumental delivery was less frequent 50. For preeclampsia, two meta-analyses found a lower risk 2240. Another review and a controlled study saw no difference there 4251.

Autoimmune Diseases

In systemic lupus, vitamin D reduced disease activity and improved complement values 31. In rheumatoid arthritis, disease activity and pain decreased 69. In clinically isolated syndrome, an early form of multiple sclerosis, vitamin D reduced disease activity, especially on MRI; relapses alone did not become less frequent 55. In a large study, vitamin D reduced the number of confirmed autoimmune diseases over five years 80. Some results showed no difference 273137.

Muscle Strength

In adults with chronic intestinal failure and vitamin D deficiency, vitamin D2 improved muscle strength 79. Children of mothers who received vitamin D during pregnancy had more lean mass at primary school age 53. Together with exercise training, better values were shown in individual strength and function tests 242564. With administration of vitamin D alone, no difference was found in muscle strength 63707849.

Cancer Risk and Course

In cancer diseases, a mixed picture emerges. A study in patients with advanced cervical cancer found more complete remissions with vitamin D 66. Another study in women with breast cancer reports pathological complete remission more often 73. The largest study in older adults found no difference in cancer incidence 46. The evidence is low, the studies are mostly small.

Other Outcomes

In this area, numerous controlled studies (with comparison group) investigated very different outcome measures. The storage form 25(OH)D in blood reliably increased 273134. In diabetic foot ulcer, wounds healed more often 36. Calcium and phosphate values mostly remained without difference 35546268. It remains open which dose and form benefits which group.

4. How Was Vitamin D Taken in the Studies?

In the studies, participants mostly took vitamin D as a capsule, tablet, drops or injection. The most common were vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol), sometimes also calcifediol. Some studies gave vitamin D together with calcium, vitamin K2, omega-3 fatty acids, magnesium, zinc or probiotics. How often and in what amount it was taken is shown in the table.

5. Is Vitamin D Safe? Side Effects in the Studies

In two joint evaluations of many studies, side effects occurred no more often with vitamin D than with placebo (sham treatment). Too much calcium in the blood also did not occur more often 1335. A review warns that very high doses can increase the calcium level in the blood too much 17. The results depend strongly on form, dose and baseline levels. Many studies were small or short.

All details with tables and sources: full expert page on Vitamin D