SQU Med J, February 2011, Vol. 11, Iss. 1, pp. 56-61, Epub. 12th Feb 11
Submitted 19th Apr 10
Revision ReQ. 22nd Jun 10, Revision recd. 24th Jul 10
Accepted 4th Aug 10

Department of Chemical Pathology, Directorate of Medical Laboratories, Royal Hospital, Muscat, Oman.
Email: manal.kalifa@moh.gov.om

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 25

 412006
 6 ± 29 ± 4518
 25
 25
 25 50
(P < 0.001 r = 0.482)(P < 0.01 r = 0.672)
 25



  25: ةمفتاح الكلمات 

aBStract: Objectives: Sunlight exposure has a vital role in vitamin D synthesis. Although vitamin D deficiency 
has been well documented in temperate zones, studies have been scarce in tropical countries where the population 
is well covered and for various reasons avoids sun exposure. The objective of this study was to investigate serum 
25-hydroxyvitamin D [25(OH)D] levels and its relationship to biochemical bone profile, exposure to sunlight and 
vitamin D intake amongst Omani women of childbearing age. Methods: 41 apparently healthy women working 
at the Royal Hospital, Muscat, Oman and aged 18–45 years, with mean ± SD of 29 ± 6 years, were included in 
this study conducted in December 2006. They completed a questionnaire regarding the duration of sun exposure, 
food intake and type of clothing worn. Blood samples were collected from them and analysed for serum 25(OH)D, 
calcium, phosphate, alkaline phosphatise and parathyroid hormone levels. Results: All the women had a 25(OH)D 
level <50 nmol/L as the cut-off for deficiency. 25(OH)D levels were strongly correlated with the lack of sun exposure 
(r = 0.672, P < 0.001) and a significant correlation was also found between 25(OH) D level and food intake (r = 0.482, 
P < 0.01). Conclusion: Subclinical 25(OH)D deficiency may be prevalent amongst Omani women. Risk factors such 
as poor sunlight exposure should be addressed in women of childbearing age and, if increased sunlight exposure is 
not possible, oral supplementation should be considered to avoid all the consequence and complications of vitamin 
D deficiency.

Keywords: Vitamin D deficiency; 25-hydroxyvitamin D; Women; Sunlight; Oman 

Vitamin D Status in Healthy Omani Women 
of Childbearing Age

Study of female staff at the Royal Hospital, Muscat, Oman
Manal K Al-Kindi

CLINICAL & BASIC RESEARCH

ADVanceS in KnoWleDGe
1. Vitamin D deficiency may be prevalent among Omani women mainly due to poor sunlight exposure.
2. This study stimulates the need for further studies to identify the prevalence of metabolic bone diseases among Omanis.



Manal K Al-Kindi

Clinical and Basic Research | 57

Vitamins D is produced endogenously by the exposure of skin to sunlight, and absorbed from food containing or 
supplemented with vitamin D. When it penetrates 
the skin, ultraviolet light is converted from 
7-dehydrocholesterol to vitamin D3. Vitamin D 
from the skin or diet is metabolised in the liver 
to 25-hydroxyvitamin D [25(OH)D], then further 
metabolisation occurs in the kidney to form the 
active form 1,25-dihydroxyvitamin D [1,25(OH)2D]. 
The renal production of 1,25(OH)2D is tightly 
regulated by plasma parathyroid hormone levels 
as well as serum calcium and phosphate levels.1 

Deficiency of vitamin D results in rickets in children 
and osteomalacia, muscle weakness and increased 
risk of fractures in adults.1

The discovery that most tissues and cells in the 
body have vitamin D receptors and some of them 
have the enzymatic capability to convert vitamin 
D to its active form, 1,25(OH)2D, may explain 
the important role of vitamin D in skeletal and 
non-skeletal health. Vitamin D also plays a role in 
decreasing the risk of chronic conditions such as 
diabetes, cancer, and autoimmune, infectious and 
cardiovascular diseases.2,3  

Vitamin D deficiency is a well-recognised 
epidemic problem worldwide. Given the significant 
role of sunlight in vitamin D synthesis, it is quite 
logical to suggest a low prevalence of vitamin D 
deficiency in tropical countries. However,  studies 
carried out in the last two decades have shown a 
high prevalence of vitamin D deficiency in many  
tropical countries such as Turkey,4 Iran,5 Saudi 
Arabia,6,7,8,9 United Arab Emirates,10  Kuwait,11 
Bangladesh12 and Tunisia.13 

Vitamin D status has so far been poorly 
documented in the Omani population. The 
present study aimed to investigate vitamin D levels 
among Omani females of childbearing age where 
a sufficient level is important to prevent neonatal 
hypocalcaemia and impaired bone growth. This 
study also investigated the prevalence of vitamin 
D deficiency among Omani women and its 
relationship to sun exposure, food intake and type 
of clothing.

Methods
Forty-one apparently healthy women aged 18–45 
years with (mean ± standard deviation (SD), 29 ± 6 
years), working in various departments of the Royal 
Hospital, Muscat, Oman, volunteered for this study. 
Exclusion criteria included known hepatic, renal 
or gastrointestinal disease, pregnancy, lactation 
and medication influencing bone metabolism such 
as calcium or vitamin D supplements. The study 
was approved by the Research Ethical Committee 
at the Royal Hospital. Informed consent was taken 
from all subjects who also were asked to complete 
a questionnaire before blood collection. The study 
was conducted in December 2006 (winter-autumn 
in Oman) where the temperature ranges from 15 
to 28 ºC.

All subjects completed a questionnaire on 
medical history, exposure to sunlight (estimated 
in minutes per week), skin type, sunscreen usage 
and type of clothing (i.e. exposure of hands and 
face). The questionnaire also estimated their 
dietary calcium and vitamin D intake, covering 
the commonest food preparations consumed by 
Omanis. Data on frequency of consumption (daily, 
weekly, monthly) as well as quantities consumed 
were collected and analysed.

The relation of vitamin D levels, sun exposure, 
dietary intake and application of sun screen were 
assessed. The sun exposure was calculated from 
the questionnaire and expressed in minutes per 
week. The dietary intake was also calculated using 
different parameters. In the questionnaire, the 
frequency of food consumption was quantified 
per day, per week and per month. The amount of 
food, weight of the product, vitamin content in 
the product, total vitamin D intake in that product 
and finally the total vitamin D consumed were also 
calculated. Then, vitamin D amounts per product 
were calculated from different sources (UK tables, 
Danish tables, and product ingredient information 
and the subjects’ estimations for home cooked 
food).

Blood specimens were collected in Greiner 
Vacuette tubes (Greiner Bio-One Gmbh, Baden-
Württemburg, Germany) from each subject for the 

Application to Patient Care
1. This study highlights important public health issues such as the need for adequate maternal vitamin D levels in pregnancy/lactation  

and the promotion of outdoor activities.



Vitamin D Status in Healthy Omani Women of Childbearing Age 
Study of female staff at the Royal Hospital, Muscat, Oman

58 | SQU Medical Journal, February 2011, Volume 11, Issue 1

measurement of the following analytes: serum total 
calcium, phosphate, albumin, alkaline phosphatase 
(ALP), parathyroid hormone (PTH) and 25(OH)D. 
For PTH analysis, serum samples were separated 
within 30 minutes following blood collection and 
kept frozen until analysis. Serum total calcium, 
phosphate, ALP and albumin were measured 
by Synchron LX20 PRO (Beckman Coulter, 
Inc., CA, USA); PTH was measured by Access 2 
immunoassay (Beckman Coulter, Inc., CA, USA) at 
the Clinical Biochemistry Laboratory in the Royal 
Hospital. Frozen samples unprotected from light 
14,15 were sent to Melbourne Pathology, Australia 
for the measurements of 25(OH)D using the 
LIAISON assay (DiaSorin, Inc., Minnesota, USA). 
This is a direct competitive chemiluminescent 
immunoassay for the quantitative measurement of 
total 25(OH)D. Using Biostat software, Spearman’s 
rank correlation coefficient was calculated for the 
correlation between serum 25(OH)D levels; sun 
exposure and dietary intake.16

Results
This study investigated vitamin D levels among 
childbearing Omani women and the prevalence 
of vitamin D deficiency and its relationship to sun 
exposure, food intake and type of clothing. The 
mean ± SD of age was 29 ± 6years, 25(OH)D levels 
were 25 ± 6 nmol/L, vitamin D intake was 9 ± 4 ug/d 
and sun exposure was 70 ± 60 min/week. In all the 
subjects (100%), vitamin D levels were <50 nmol/L. 
The mean serum concentration of 25(OH)D in the 

subjects was 25nmol/L (results ranged from 12 to 
37 nmol/L) In 20 women (49%), 25(OH)D levels 
were ≥25 nmol/L. In the remaining 21 women (51%) 
the level was <25 nmol/L. All subjects had vitamin 
D deficiency which varied from mild, to moderate  
and even severe deficiency using the recommended 
cut off <50 nmol/L as a low 25(OH)D.17,18,19 

All except one subject had serum PTH, serum 
calcium and phosphate within the reference 
interval. No significant correlation between 
serum 25(OH)D levels and calcium levels or 
serum phosphate level was noted in the subjects 
investigated. The results of different biochemical 
bone markers are presented in Table 1.

The most significant correlation of the 25(OH)
D level was with sun exposure, as all the subjects 
covered their whole body except the face and hands 
(r = 0.672, P <0.001) compared to the correlation 
with food intake (r = 0.482, P <0.01). The  
correlation between 25(OH)D and age was weak 
and not statistically significant (r = 0.211, P >0.05). 
Sun screen was used by 15 subjects (36%) and was 
only applied on the face and not on regular basis 
and no correlation was found between application 
of sun screen and 25(OH)D levels (r = -0.02, P 
>0.05).

As sun exposure had the strongest correlation, 
two cut offs were defined for sun exposure time 
to assess the relation between 25(OH)D and sun 
exposure time [Table 2]. With sun exposure at <60 
min/week, the mean 25(OH)D and PTH were 27 
nmol/L and 4.14 pmol/L respectively, while with 
sun exposure ≥ 60 min/week, the mean 25(OH)
D and PTH were 29.5 nmol/L and 2.92 pmol/L 
respectively. The mean of serum calcium and 
phosphate was comparable in both subgroups 
[Table 3].

Discussion
This study reveals that 25(OH)D deficiency is likely 
to be very common among Omani adult women 
given the prevalence of 100% in the studied group. 
This may seem an unexpected result as Oman is one 
of the sunniest countries in the world where people 
would be expected to have adequate sun exposure. 
Although vitamin D deficiency is defined as serum 
25(OH)D level <50nmol/L,17,20 an epidemiological 
survey-based recommended cut-off, this may not 
necessarily reflect the desirable level among Omani 

Table 1: Biochemical bone markers in the study group 

(N = 41)

Biochemical 
markers Mean ± SD

Median 
(Range) 

Normal 
Range

Total 
calcium 
(mmol/L)

2.31 ± 0.07 2.31  (2.18-2.43) 2.1-2.6

Phosphate 
(mmol/L) 1.23 ± 0.12 

1.26  
(0.92-1.57) 0.7-1.4

ALP (IU/L) 51.59 ± 13.63 48  (30-98) 30-125

PTH 
(pmol/L) 3.81 ± 2.06

3.6  
(1.4-11.7) 1.6-9.3

25(OH)D 
(nmol/L) 27.61 ± 5.35

28  
(19-37)) 75-250

Legend: ALP = alkaline phosphatase; PTH = parathyroid hormone; 
25(OH)D = serum 25-hydroxyvitamin D.



Manal K Al-Kindi

Clinical and Basic Research | 59

women. 

The low vitamin D level may be due to several 
factors including direct and indirect avoidance 
of sunlight exposure due to cultural and social 
reasons (concern about appearance, unwillingness 
to change skin colour, the burning effect of the sun, 
and the risk of hot weather related sickness). Most 
of the women included in the study were working 
full-time so they were indoors for most of the day 
-time and, like the majority of Omanis, they anyway 
prefer only to be outdoors after sunset. Most of the 
women included in the study were asymptomatic; 
they did not suffer from bone pain, myalgia or 
tiredness and their biochemical markers were 
within the normal range.

It appears that the way people dress is not the 
only reason for the low vitamin D status. Islam et 
al. for example, showed that woman in Bangladesh, 
regardless of their lifestyle and clothing, were at risk 
of developing vitamin D deficiency and that both 
veiled and unveiled women can have vitamin D 
deficiency.11 

In this study, many women were aware about 
the need for a healthy diet containing high calcium 
intake and some were also aware of the importance 
of vitamin D for health. Hence, they were trying to 
compensate for this demand by having adequate 
calcium and vitamin D intake either by increasing 
the intake of fish and egg or using fortified food 
like margarine and milk which they know to be 
rich in these constituents. However, although 
in the study the adequate vitamin D intake was 
defined to be 5ug/day (200 IU/day) for those aged 
<50 year,17,20 surprisingly only 6 subjects (15%) had 
less then 5ug/day. The discrepancy may be due to 
coexisting vitamin D deficiency, malabsorption, 

overestimation of vitamin D content in the food 
consumed, (when  using the UK and Danish tables), 
lack of information on labels of fortified food and 
wrong estimations for home cooked foods. The 
latter could have led to inter-individual variations 
in estimating vitamin D content for home cooked 
food. Hence, there is a concern about the subjects’ 
reliability and accuracy in estimating their food 
intake.

Although a significant correlation was observed 
between 25(OH)D and sun exposure, none of 
the subjects had enough sun exposure. The 
amount vitamin D produced by the human skin is 
proportional to the surface area of skin exposure  
to sunlight. All the subjects were known to be 
covered (except the face and hands), but at least 15–
20% of body surface needs to be exposed to sun to 
provide the minimal erythemal dose of ultraviolet 
light that is sufficient for the first step in vitamin 
D synthesis (conversion of cholcalciferol from it 
precursors).17

The amount of sunlight exposure that is 
needed also varies depending on skin type, time 
of day, altitude and season. Since, for religious and 
cultural reasons, it is hard for Omani women  to 
increase the body surface area that is exposed to 
sun, they have to increase the time of exposure to 
sunlight or/and to take vitamin D supplements to 
avoid the consequences associated with  vitamin 
D deficiency. Special consideration is required for 
pregnant and lactating women in order avoid the 
perinatal and post-natal complications associated 
with low maternal vitamin D.  

Conclusion
A high prevalence of subclinical vitamin D 

Table 2. Biochemical bone markers according to sun exposure

Biochemical bone 
markers

Sun exposure P value

          ≥60 min/week         <60min/week

Mean±SD Median (range) Mean Median (range)

Total calcium 
(mmol/L)

2.32 ± 0.007 2.33 (2.19–2.42) 2.30 ± 0.06 2.30 (2.18−2.43) <0.01

Phosphate (mmol/L) 1.20 ± 0.10 1.23 (1.01−1.31) 1.25 ± 0.13 1.26 (0.92−1.57) <0.01

ALP (IU/L) 39.18 ± 9.11 48 (38−69) 52.47 ± 14.99 47.5 (30−98) <0.01

PTH (pmol/L) 2.92 ± 1.05 2.60 (1.7−4.8) 4.14 ± 2.25 3.7 (1.4−11.7) <0.01

25(OH)D (nmol/L) 28.7 ± 5.33 29.50 (20−37) 27 ± 5.41 27.50 (19−36) <0.01
Legend: ALP = alkaline phosphatase; PTH = parathyroid hormone; 25(OH)D = serum 25-hydroxyvitamin D.



Vitamin D Status in Healthy Omani Women of Childbearing Age 
Study of female staff at the Royal Hospital, Muscat, Oman

60 | SQU Medical Journal, February 2011, Volume 11, Issue 1

deficiency amongst Omani females of childbearing 
age has been observed in this population. Sunlight 
is major contributor to 25(OH)D levels, but their 
exposure still appears to be insufficient. If the 
subjects can not improve their sunlight exposure, 
they should take oral vitamin D supplements. 
Further studies may be recommended to determine 
the incidence of osteomalacia and osteoporosis in 
the Omani population and assess vitamin D status 
in patients with chronic diseases.

a c k n o w l e d g e m e n t s 
The author would like to thank all women who 
volunteered to take part in this study. She would 
also like to thank Dr Ken Sikaris (Melbourne 
Pathology, Australia), Dr Waad-Allah Mula-Abed 
(Royal Hospital, Oman), and the staff of the clinical 
biochemistry laboratories at both Melbourne 
Pathology, Australia, and the Royal Hospital, Oman.

c o n f l i c t o f i n t e r e s t
The author reported no conflict of interest.

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Table 3. Biochemical bone markers according to 25 (OH) D levels. Data are presented as mean ± standard deviation 
(median)

Biochemical bone markers 25(OH)D P value

<25 nmol/L ≥ 25 nmol/L

Total calcium (mmol/L) 2.31 ± 0.06 (2.30) 2.31 ± 0.06 (2.32) <0.01

Phosphate (mmol/L) 1.22 ± 0.11 (1.22) 1.24  ± 0.15 (1.28)
(0.92−1.57)

<0.01

ALP (IU/L) 48.2 ± 12.75 (45) 54.15 ± 12.73 (51) <0.01

PTH (pmol/L) 4.38 ± 2.52 (4.30) 3.68 ± 1.83 (3.10) <0.01

25(OH)D (nmol/L) 19.47 ± 1.77 (19) 30.35 ± 3.48 (30) <0.01

Legend: ALP= alkaline phosphatase; PTH= parathyroid hormone. 



Manal K Al-Kindi

Clinical and Basic Research | 61

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