Medical professionals are increasingly focusing on insufficient Vitamin D levels, with Dr. Amjad El-Haddad, an allergy and immunology consultant, highlighting a strong connection between Vitamin D deficiency, increased susceptibility to infections, and depressive moods. Common symptoms associated with deficiency include persistent fatigue, exhaustion, unexplained bone and muscle pain, and recurring viral infections and colds.
Investigations suggest a close link between low Vitamin D levels and stress and depression, though deficiency is not considered the sole cause. Dermatologist Dr. Ayda Kart A?kar also noted that a deficiency correlates with mood swings, anxiety, depression, and an elevated cardiovascular risk. Additionally, winter months, when Vitamin D levels typically decline, can see an increase in conditions such as psoriasis, eczema, and hair loss.
Further research underscores the presence of Vitamin D receptors in the hippocampus and prefrontal cortex, areas crucial for cognition and emotional regulation. The micronutrient also impacts the synthesis of neurotransmitters like serotonin and dopamine. Consequently, a deficit is associated with seasonal affective disorder, anxiety, stress, and an increased risk of dementia in old age. Research results from 2026 suggest that targeted supplementation for diagnosed Mild Cognitive Impairment may slow the progression of cognitive decline, although it offers neither a cure nor complete protection.
However, regarding physical immune defense, research differentiates between statistical observations and concrete treatment effects. The German Nutrition Society (DGE) referenced an umbrella review of 73 studies, indicating that a lower Vitamin D status correlates with a higher risk for acute respiratory infections, but noted that no proven protective effect exists. An updated meta-analysis of 46 randomized controlled studies involving 64,086 participants found no statistically significant protection against respiratory infections through Vitamin D supplementation. Another meta-analysis of 18 studies with 3,648 participants similarly showed no significant benefit for therapeutic administration during acute infections.
Cell biologist Stephan Barth emphasized that for individuals already adequately supplied with Vitamin D, there is no evidence that additional micronutrients enhance immune defense. Professor Tine Jess of Aalborg University highlighted that Vitamin D might be relevant for winter depression but is not proven as a sole treatment. Professor Jess also investigates how a deficiency may alter the gut microbiome, thereby affecting immunity.
Natural sources like fish, meat, egg yolk, or liver only partially cover the body's Vitamin D requirements. The primary source remains the body's synthesis through UVB radiation. Dr. El-Haddad advises 10 to 15 minutes of direct sun exposure around midday, as windowpanes block the necessary radiation. Dr. Kart A?kar recommends 15 to 20 minutes of sunlight on the face and arms three to four days per week. However, in Central Europe, UV radiation is often insufficient from October to March.
Before taking supplements, a laboratory determination of 25-hydroxyvitamin D levels is recommended. The DGE classifies values below 25 to 30 nmol/l as deficient and advises a daily intake of 20 micrograms, or 800 International Units (IE), if needed. Internist and endocrinologist Dr. Stephan Scharla underscored the clear advantages of daily administration over high-dose pulse therapies, stating that preparations with high weekly depots have no proven benefit for prevention. The Federal Institute for Risk Assessment warns that daily doses exceeding 4000 IE can cause dangerous hypercalcemia. Studies have also shown that consistently high dosages can even reduce bone density. An investigation by Öko-Test also criticized numerous over-the-counter preparations for being excessively dosed. Experts therefore uniformly advise that supplements should only be taken after medical consultation.