[Prevalence of Hypovitaminosis D in Portugal: A Narrative Review]

25-hydroxyvitamin D is the best biomarker of vitamin D biological activity, and its serum level has been used in studies assessing vitamin D sufficiency. However, the definitions of vitamin D deficiency and insufficiency remain non-consensual. The cutoff point for serum 25-hydroxyvitamin D levels used to define vitamin D sufficiency typically ranges between 50 and 75 nmol/L. Serum levels below 30 nmol/L, indicative of severe vitamin D deficiency, may have potentially adverse effects on skeletal and overall health. Hypovitaminosis D is highly prevalent worldwide, including across Europe. Several cross-sectional studies using random or convenience sampling, conducted at national or local levels, have documented a high prevalence of hypovitaminosis D, including severe cases, in Portugal. These studies were summarized in a narrative review, confirming the magnitude of this public health problem. The causes of hypovitaminosis D include low dietary intake or malabsorption of vitamin D, as well as inadequate sun exposure and insufficient endogenous synthesis, which are influenced by multiple factors. Several strategies have been proposed to address this significant public health issue, including adopting a healthier lifestyle with increased outdoor physical activity, controlling excess weight and obesity, and consuming foods naturally rich in vitamin D. However, given the high prevalence of sedentary lifestyles and obesity in Portugal, combined with inadequate dietary intake of vitamin D, these measures alone may be insufficient. Vitamin D supplementation and food fortification are two additional strategies for correcting hypovitaminosis D. Supplementation targeted at individuals at risk or with confirmed deficiency has not proven effective in significantly reducing the prevalence of severe deficiency in the Portuguese population. Food fortification, whether mandatory or voluntary, may represent a complementary and potentially more effective approach than individualized supplementation in populations with a high prevalence of severe hypovitaminosis D, as is the case in Portugal.