Since the latitude of Amman, Jordan is 31°N, cutaneous Vitamin D3 synthesis should be sufficient all year round . However, this is not the case in Jordanian males or Western dress style females. Cutaneous Vitamin D3 synthesis starts with UVB (290-315 nm) exposure. UVB converts 7-dehydrocholesterol to pre-vitamin D3 which is then converted to vitamin D3 by thermal isomerization. Continued skin exposure to UVB radiation, however, can result in the breakdown of pre-vitamin D3 and vitamin D3 to inactive photoproducts . This process is thought to be a regulatory process of vitamin D synthesis . Therefore, excess sun exposure can lead to inactive photoproduct and a decrease in vitamin D status. In addition, levels of UVB reaching the skin site for vitamin D synthesis can be attenuated by clothing, sunscreen, or skin's melanin content [21–24]. In the present study, although Jordanian males and Western dress style females were shown to possess significantly higher vitamin D levels than those of low sun exposure females (Hijab and Niqab dressed females), nonetheless, 80% of the former groups have insufficient levels (<50 nmol/l). Therefore, several reasons other than sun exposure could be involved in such reduction of vitamin D levels.
Jordanians and Middle-Eastern in general have darker skin textures i.e reflecting higher melanin production in response to UV radiation. Melanin absorbs electromagnatic radiation and competes with 7-dehydrocholesterol for UVB photons . The latter may be one of the reasons of low vitamin D status among Jordanians and other Middle-Eastern countries that are located below 35°N of the equator [26–30].
Vitamin D status is also affected by seasons especially in areas of high latitude (>40°N). It has been demonstrated that measuring 25(OH) D levels in the summer is relatively higher than winter times [21, 31]. However, this is also associated with the latitude of the city or the country where the study is performed. For instance, people living at 23.5-66.5° latitude may lack sufficient UVB to synthesize vitamin D during one month of the whole year, whereas other studies reported that below 35° the UVB exposure is enough for vitamin D synthesis throughout the year [28, 31]. In addition, Jordan wintertime starts mid of January [28, 29]. Therefore, the timing of the current study should not be the reason of the low levels seen for 25(OH) D.
Since clothing is a major blocker to sun exposure and thus vitamin D synthesis and status, this study has shown that females dressing Hijab (uncovered face and hands) or Niqab (i.e. covering all their bodies) have less 25(OH)D plasma levels than their counterparts Western style-dressed females living in Jordan. In addition, sun exposure to uncovered face and hands as in Hijab- dressed females is not sufficient for vitamin D synthesis. It has to be mentioned that the texture of clothing such as non-synthetic fibers (cotton or lenin) are less effective in blocking UV radiation than wool, silk, nylon and polyester. In addition, a lighter color of cotton clothing such as white is less effective in blocking ultraviolet light than black [23, 24]. These factors might contribute to some variations herein; however, such clothing factors were not addressed in the study.
The present study supports the fact that the frequency of dairy consumption is a good source of vitamin D. Although males in Jordan consumes less dairy products than females, their 25(OH)D levels were significantly dependent on dairy consumption. Thus, it is concluded that sun exposure and dairy consumption are the main factors that could keep vitamin D levels close to the recommended values as much as possible.
Vitamin D (D2 or D3) is absorbed in the gastrointestinal tract, binds to chylomicrons, and is transported via lymphatic system to blood circulation. Circulating vitamin D, whether from diet or sun exposure, enters the circulation, binds to vitamin D-binding protein, and transported to liver for metabolism or adipose tissue for storage. In the liver vitamin D is metabolized to 25(OH)D by the vitamin D hydoxylase and 25(OH)D which is further modified to the active form 1,25(OH)2D in the kidneys. The rate and extent of the elevation of plasma 25(OH)D levels are dependent on the vitamin D hydroxylase. In addition, 25(OH)D goes through inactivation by cytochromes P-450 CYP24 (or 25(OH)D-24-hydroxylase) and CYP3A4. In addition, vitamin D-binding protein (group-specific component; GC) is also cofactor in the plasma levels of 25-(OH)D. Therefore, single nucleotide polymorphism (SNP) markers in the genes, namely CYP2R1 and GC, might contribute to the variations of 25(OH)D levels in healthy Caucasians [32, 33]. Therefore, it is better that optimal concentrations of 25(OH)D in different populations should be defined in order to reduce certain diseases due to high vitamin D. Further research is required in order to clarify the genetic architecture underlying 25(OH)D plasma concentrations among Jordanians.
Plasma calcium concentration is regulated by 1,25(OH)D and parathyroid hormone (PTH). 1,25(OH)D increases calcium absorption from the intestines and PTH increases plasma calcium levels by inducing renal calcium reabsorption and stimulating bone resorption. Thus, it is essential to keep vitamin D levels sufficient in order to keep calcium level normal within the body and reduce PTH action on the bones. The present study demonstrates that the mean plasma calcium levels are less in the groups with less 25(OH)D levels. Therefore, it is essential to supplement females with vitamin D to reduce the process of osteoporosis as early as possible of their lives .
The daily needs of vitamin D is still under debate and controversial. The Institute of Medicine (IOM) published its recent report regarding the dietary reference intakes and revealed that 600 IU/day is the recommended daily allowance for individuals between 1-70 years of age . These recommended daily allowances, however, are less than other studies have shown and recommended [34, 36]. The latter studies reported that in order to keep plasma 25(OH)D levels > 75 nmol/L, it is recommended that the daily allowance of vitamin D be 1000-2000 IU/day and may go up to 6000 IU/day in case of pregnant women .