Health A-Z


Vitamin D is the only vitamin that can be synthesized in the skin when exposed to sunlight, more specifically ultraviolet (UV) radiation. After this first synthesis process, vitamin D3 (cholecalciferol) undergoes a first hydroxylation in the liver and then a second activation in the kidney, originating 1,25-dihydroxyvitamin D (calcitriol), the biologically active form.

Although vitamin D is widely known for its role in calcium homeostasis, bone formation, and absorption of phosphorus from the diet, several pieces of evidence have emerged in recent years that relate this nutrient to other physiological systems, such as the immune system, cardiovascular system, and others.

The interaction of vitamin D with immune system has been the subject of an increasing number of publications, some of them showing that receptors for this vitamin have been identified in different types of immune cells, which are able to convert vitamin D into its active form. This activation leads to a set of immunomodulatory actions on the immune system cells, namely T lymphocytes, monocytes and neutrophils, as well as on the production and action of several cytokines, substances that alert the body to the presence of an invader (such as viruses, bacteria or fungi), allowing it to activate its defense cells. In this regard, A deficiency in vitamin D has been associated with increased susceptibility to various infections.

A study published in March 2020 in the scientific journal Archives of Osteoporosisrevealed the vitamin D deficiency in the Portuguese adult population as a whole. According to this study, "vitamin D deficiency / insufficiency is highly prevalent in Portugal, affecting more than 60% of all Portuguese adults, with strong geographical and seasonal variation". It also highlights the "need to evaluate the relevance of vitamin D deficiency as a public health problem" and the need to define "strategies to prevent vitamin D deficiency in Portugal".

Another way to get vitamin D is through food, which is only 10-20% of the daily requirement. The richest foods are fatty fish (e.g. sardines and salmon), egg yolk, liver, butter and milk fat (e.g. cream). The cutaneous synthesis of vitamin D after sun exposure then contributes to a high percentage of your daily intake. However, it depends on several factors such as the season, the time and area of exposure without sun protection, and skin tone (people with darker skin synthesize less than those with lighter skin). In addition, with advancing age, the ability to synthesize vitamin D decreases.

Increased exposure to sunlight is not a viable option for most individuals, due to the phototoxicity of UVB rays. Vitamin D3 supplementation is the safest and most practical way to reverse the deficiency of this nutrient. Most people who are deficient in vitamin D have no specific complaints, unless the deficiency is very severe. It is therefore very important to focus on prevention!

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Duarte, C., Carvalheiro, H., Rodrigues, A.M. et al. Prevalence of vitamin D deficiency and its predictors in the Portuguese population: a nationwide population-based study. Arch Osteoporos 15, 36 (2020).

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