**8. Vitamin D deficiency**

Most studies have identified the vitamin D insufficiency (VDI) at concentrations of [25(OH)D] less than 75 nmol/L (30 ng/mL) and vitamin D deficiency (VDD) at concentrations below 50 nmol/L (20 ng/mL) [1, 4, 10, 17].

Vitamin D deficiency is still a highly prevalent disorder. It is estimated that ~1 billion people are deficient or have insufficient levels of vitamin, in spite of foods fortified with vitamin D and wide supplement intake [2].

VDD is widespread in the whole world as well as predominant in Asia and in the Middle East (more than 50% of the population is VDD and about 75% is VDI) [4]. VDD is found in 30–50% of otherwise healthy middle-aged to elderly adults [11].

Deficiency of vitamin D can result from many reasons such as dietary inadequacy of vitamin D, poor absorption and use, increased requirement, increased excretion and catabolism, limited sunlight exposure, and inefficient production in the skin. Dietary deficiency of vitamin D is associated with milk allergy, lactose intolerance, ovo-vegetarianism, and veganism [1, 4]. In addition, various diseases affect the bioavailability of vitamin D, such as gastrointestinal disorders which limit its absorption; kidney and liver diseases can prevent the activation of the parenteral vitamin D or impair the conversion of vitamin D into its active metabolites [4].

Severe VDD in adults leads to osteomalacia while in children leads to rickets, defective bone mineralization, increased bone turnover, increased risk of fractures [4], impaired reproductive function, and production of gonadal hormone that may affect other organs, e.g., gastrointestinal and renal calcium handling, renal CYP27B1 activity, and bone function [20].

In the critical care condition, VDD has been associated with adverse outcomes such as infections, longer length of stay, acute kidney injury, and higher mortality [10].

#### **9. Vitamin D deficiency in Saudi Arabia**

A total of 132 articles studying the deficiency of vitamin D in Saudi Arabia were identified based on the initial PubMed search criteria. About 20 studies have investigated the vitamin D deficiency in healthy individuals living in Saudi Arabia during

