Fact Check: Vitamin D - A Pill of Sunshine Against Depression?

"What is vitamin D? - The best antidepressant"¹ and "With a smile through the day? Easy!"²
Scientifically questionable sentences like these promote the use of vitamin D supplements, nowadays a common practice in society. According to such statements, vitamin D has preventive properties against diseases like the flu, cancer and depression.

In 2019, around 280 million people worldwide were suffering from depression³. It is a common mental illness that affects people of all ages, genders and ethnic groups. Depression can cause a feeling of apathy and can make the handling of day-to-day activities difficult. Depressed people have an increased risk of physical health problems and even of committing suicide^3,4. In connection with depression, vitamin D gained scientific and public attention in recent years^5,6.

So, is there really an association between low vitamin D levels and depression? Or the other way around, can vitamin D supplements help with depression?

Vitamin D deficiencies are widespread and mainly caused by the lack of sunlight in winter. Increasing indoor lifestyle and other individual factors like age or skin pigmentation also determine the amount of vitamin D the body produces^5,7,8,9. Vitamin D is mainly produced in the skin during direct sun exposure and has many important functions in the body. Vitamin D is essential for keeping up normal levels of serotonin, which is a hormone in the brain. Normal serotonin levels as well as increased expression of vitamin D receptors in the brain are crucial for mood regulation. Serotonin can improve the mood by reducing feelings of anxiety and depression¹⁰.

Studies have investigated if low vitamin D levels and risk and severity of depression are connected. Some of them found an association^7,11,12 , others did not^13,14,15 . A major challenge in assessing the relationship between vitamin D levels and depression is to disentangle cause from effect. It is possible that depression causes the deficiency of vitamin D, since depressed people might not feel the urge to go outside^7,16 . This reverse causality needs to be considered.

Given the unknown cause-effect relationship, studies on vitamin D supplementation were looked into for potential treatment options. However, the results were contradictory, ranging from no beneficial effects to reducing depressive symptoms^8,11,16. Some of the factors that likely caused these very different conclusions are different study designs, small sample sizes, differences in the used vitamin D dosage and the duration of vitamin D supplementation^6,17,18.

A common problem were differences in the study population^6,17,18. When choosing study participants, it is important to consider that vitamin D levels depend on a variety of factors. Factors that may interfere with the efficacy of vitamin D supplements include socio- demographics (age, sex, ethnicity), sunlight, urbanization or health status (physical activity, BMI, smoking, drinking, chronic diseases)^{6,11,12,16,17,18}. Most studies did not report or analyse their flaws or such interfering factors, even though they influence the findings^6,17,18.

However, a tendency was visible for specific conditions. For example, in two types of participants vitamin D supplements showed noticeable positive effects: 1) severely depressed people with normal vitamin D levels and 2) slightly depressed people with vitamin D deficiency. These people were statistically less depressed when they consumed vitamin D supplements^6,17,18. By contrast, for slightly depressed individuals with normal vitamin D levels, vitamin D supplementation did not reduce depressive symptoms⁶.

All in all, there is no clear evidence that high vitamin D levels can alleviate depression. However, due to the variety of neurological effects of vitamin D, a link between the two is reasonable. For clinically depressed patients, vitamin D supplementation seems to work. However, even people with light depressions are advised to check possible vitamin D deficiencies with a doctor, since in this case supplementation has been shown to alleviate symptoms.  So, with the current evidence at hand, only for those people does vitamin D mean “with a smile through the day”. Vitamin D supplementation without medical advice should be used with caution at this time.

Issue 12 (PDF)
Lilli Kratzer, Linda Krause, Hanna-Margareta Schwarzbach