**3. Source of vitamin D**

The main sources of vitamin D are our diet, supplementation, and sun exposure [2].

Two dominant forms of dietary vitamin D are vitamins D2 (ergocalciferol) and D3 (cholecalciferol) [12]. Vitamin D2 is produced by plants and invertebrates after ultraviolet radiation exposure [13]. Vitamin D3 is naturally found in many foods (such as oily fish, egg yolks, cod liver oil, cheese, mackerel, salmon, tuna fish, and beef liver), fortified foods (margarine, breakfast cereals, dairy products, orange juice), and vitamin supplements (both vitamins D2 and D3 are available) [1, 14, 15]. Dietary vitamin D provides only 10–20% of circulating levels of vitamin D [13].

The chemical structure of these vitamins (D2 and D3) is similar but differs only in their side chains (**Figure 1**). This structural difference modifies their binding to carrier protein vitamin D binding protein (DBP) and their metabolism [16]. Vitamin D3 is significantly demonstrated more effective than D2 in increasing serum 25-hydroxyvitamin D [25(OH)D] concentrations due to several reasons including reduced vitamin D2 binding and metabolites to DBP in plasma, a non-physiological metabolism, and a shorter shelf life of vitamin D2 [13]; therefore vitamin D3 is considered the preferred choice for supplementation [14].

Sun exposure is the chief source of vitamin D via the synthesis in the skin through the action of ultraviolet B (UVB) radiation on the precursor of vitamin D3 [4, 17]. The Commission Internationale de l'Eclairage (CIE) described the efficiency of vitamin D

**5**

**Figure 2.**

*Vitamin D and Its Deficiency in Saudi Arabia DOI: http://dx.doi.org/10.5772/intechopen.88745*

**4. Synthesis and metabolism**

*The structure of vitamins (D2 and D3).*

**Figure 1.**

radiation as the efficiency of each wavelength to synthesize vitamin D in the skin. The CIE suggests the efficiency of UVB radiation that covers the spectral range (255–330 nm) with a maximum at about 295 nm [1]. A whole-body exposure to UVB radiation inducing the light pink color of the minimal erythema dose for 15–20 min is

Vitamin D, either endogenously produced (vitamin D3) or ingested (vitamin D2 or vitamin D3), must be activated in order to produce its effects [5]. This biological

Firstly, UVB radiation penetrates the epidermis and stimulates the conversion of 7-dehydrocholesterol (7-DHC) into pre-vitamin D3 [18] which undergoes thermal

Secondly, vitamin D2 or D3 is specifically translocated by DBP into circulation and then to the liver for hydroxylation at carbon-25 to form 25-hydroxyvitamin D [25(OH)D] mainly by two cytochrome P-450 enzymes (CYP2R1 and CYP27A1) [5].

able to induce the production of up to 250 μg vitamin D (10,000 IU) [1, 6].

isomerization through a sigmatropic hydride shift into vitamin D3 [13].

activation is performed in a multi-step process (**Figure 2**).

*A diagram illustrating the different sources and synthesis of vitamin D.*

*Vitamin D and Its Deficiency in Saudi Arabia DOI: http://dx.doi.org/10.5772/intechopen.88745*

*Vitamin D Deficiency*

osteoporosis, and tooth loss [6].

**2.2 Vitamin D and non-skeletal diseases**

often have low vitamin D serum level [9].

excretion [7].

others [11].

exposure [2].

**3. Source of vitamin D**

which acts by stimulating bone reabsorption and reduction of calcium urinary

Vitamin D levels are positively correlated with bone mineral density (BMD) [4]. Many observational studies have reported relations between chronic lower vitamin D concentrations and poorer lower-extremity function, lower muscle strength, lower contraction speed, and lower appendicular muscle mass [8]. Vitamin D deficiency can put people at risk because of low bone mineral density, osteopenia,

Observational studies have shown associations between the low concentration of serum vitamin D and increased risk of cancer, cardiovascular diseases, disorders of glucose metabolism, neurodegenerative diseases, and mortality [3]. Vitamin D modulates a variety of processes and regulatory systems including host defense, inflammation, and immunity and repair, especially patients with lung diseases

The biological effect of vitamin D on cardiac function is through reduced remodeling and fibrosis secondary to negative regulation of renin by vitamin D receptor (VDR)-linked gene regulation and through reduced cardiac metalloproteinase activities [10]. In addition, many indications support a relation between hypovitaminosis D and slower nerve conduction and poorer executive functions [8]. VDR are also expressed on immune cells (T and B cells, monocytes/macrophages, mast cells, and antigen-presenting cells) [10]. Moreover, vitamin D may exert positive effects

Furthermore, many studies have demonstrated that vitamin D supplementation has a beneficial effect in decreasing the mortality rate under multiple factors, by influencing the cardiovascular system, immune system, tumor progression, and

on oral health by affecting the production of antimicrobial peptides [6].

The main sources of vitamin D are our diet, supplementation, and sun

considered the preferred choice for supplementation [14].

Two dominant forms of dietary vitamin D are vitamins D2 (ergocalciferol) and D3 (cholecalciferol) [12]. Vitamin D2 is produced by plants and invertebrates after ultraviolet radiation exposure [13]. Vitamin D3 is naturally found in many foods (such as oily fish, egg yolks, cod liver oil, cheese, mackerel, salmon, tuna fish, and beef liver), fortified foods (margarine, breakfast cereals, dairy products, orange juice), and vitamin supplements (both vitamins D2 and D3 are available) [1, 14, 15]. Dietary vitamin D provides only 10–20% of circulating levels of vitamin D [13]. The chemical structure of these vitamins (D2 and D3) is similar but differs only in their side chains (**Figure 1**). This structural difference modifies their binding to carrier protein vitamin D binding protein (DBP) and their metabolism [16]. Vitamin D3 is significantly demonstrated more effective than D2 in increasing serum 25-hydroxyvitamin D [25(OH)D] concentrations due to several reasons including reduced vitamin D2 binding and metabolites to DBP in plasma, a non-physiological metabolism, and a shorter shelf life of vitamin D2 [13]; therefore vitamin D3 is

Sun exposure is the chief source of vitamin D via the synthesis in the skin through the action of ultraviolet B (UVB) radiation on the precursor of vitamin D3 [4, 17]. The Commission Internationale de l'Eclairage (CIE) described the efficiency of vitamin D

**4**

**Figure 1.** *The structure of vitamins (D2 and D3).*

radiation as the efficiency of each wavelength to synthesize vitamin D in the skin. The CIE suggests the efficiency of UVB radiation that covers the spectral range (255–330 nm) with a maximum at about 295 nm [1]. A whole-body exposure to UVB radiation inducing the light pink color of the minimal erythema dose for 15–20 min is able to induce the production of up to 250 μg vitamin D (10,000 IU) [1, 6].
