Vitamin D is an essential nutrient attracting attention due to its increasing deficiency in children and adults worldwide. One of the best known functions of vitamin D is its involvement in the homeostasis (balance) of calcium and phosphorus, which helps maintain a healthy skeleton and good muscle contraction. However, its recently discovered relationship with the immune system has prompted the study of other consequences its deficit may have on health.
Vitamin D comes in two forms: vitamin D2 or ergocalciferol, which is only obtained through food, and vitamin D3 or cholecalciferol, which can also be obtained through the diet but is synthesised in the skin via sunlight. Both forms of this nutrient have the same effects on the body. It is considered we can obtain up to 90% of the vitamin D a person needs through the sun's rays. The main sources of vitamin D in food are of animal origin. It is found in fish liver oil, in oily fish, - such as salmon, sardines, herring and mackerel, - in egg yolk and in whole or enriched dairy products. The absorption or synthesis capacity of vitamin D can vary according to numerous factors, such as the person’s age, certain diseases and any drugs taken.
By consensus, the inactive form of vitamin D (25(OH)D) is the marker of choice to measure its levels in blood. Values greater than 30 ng/mL are considered optimal; 20-30 ng/mL is considered insufficient; and values less than 20 ng/mL are deficient. If levels are low, despite exposure to the sun and a diet rich in these foods, optimal levels can be obtained by supplementing with synthetic vitamin D, provided this is indicated by a health professional.
Vitamin D deficiency is known to cause osteoporosis in adults and rickets in children, which is a skeletal disorder that causes soft and weak bones. Also, its involvement in the regulation of the immune system has been studied in recent years. Vitamin D receptors in all the cells of this system have been observed, which could indicate that this vitamin is involved in its regulation.
Vitamin D deficiency is considered a risk factor for immune system dysfunction, respiratory tract infections and viral infections. Thus, it has been postulated that a deficit of this vitamin can increase the degree of severity of COVID-19 infection. The immune dysregulation associated with the increase in the inflammatory condition during COVID-19 could be adequately corrected with the immunomodulatory activity of vitamin D. Also, supplementary vitamin D, especially where deficiency or insufficiency exists, can reduce the development of severe symptoms. However, the evidence for this is based mainly on observational studies, so randomised clinical trials with larger sample sizes are needed to find more conclusive evidence on the role of vitamin D in COVID-19.
Awareness of the function of vitamin D in other organs and systems could help establish the relationship of this nutrient with the development of these pathologies and even open up new lines of treatment.
Information documented by: Alba Andreu, nutritionist in the Endocrinology and Nutrition Service of the Clinical Institute of Digestive and Metabolic Diseases (ICMDM) at Hospital Clínic de Barcelona.