Normative Values of Vitamin D Among Iranian Population: A Population
There is no agreement about normal level of vitamin D and its deficiency
stages. For finding normative value of Vitamin D and evaluating the state
of vitamin D level in Iranian population we revised the data that was
collected in Iranian national Multi-center Osteoporosis Study (IMOS).
We chose 5 cities with different climates; individuals were selected by
random cluster sampling. Healthy people aged 20-69 were entered into the
study and serum vitamin D and PTH levels were measured. We stratified
subjects based on their vitamin D levels in 7 groups and compared mean
PTH levels of adjacent groups. We evaluated 5329 blood samples for vitamin
D and PTH and found three steps of PTH elevation with decreasing vitamin
D levels for women (40, 25 and 12 nmol L-1) and two (35 and
25 nmol L-1) for men. We use these values as cutoff levels
for definition of normal, mild, moderate and severe vitamin D deficiency
states. Based on these cutoffs, prevalence of all stages of vitamin D
deficiency was unexpectedly high in all cities. Vitamin D deficiency state
was seen in 75.1% of women and 72.1% of men. The high prevalence of vitamin
D deficiency in Iran is similar to the results of other studies in Middle
East area and indicates a need for a careful search for a determination
of cause and need for regular fortification program.
to cite this article:
K. Moradzadeh, B. Larijani, A.A. Keshtkar, A. Hossein-Nezhad, R. Rajabian, I. Nabipour, G.H. Omrani, A. Bahrami, M.M. Gooya and A. Delavari, 2008. Normative Values of Vitamin D Among Iranian Population: A Population
Based Study. International Journal of Osteoporosis and Metabolic Disorders, 1: 8-15.
Vitamin D is an essential factor for normal metabolism of bone and bone
minerals; it is further required in several non-bone related metabolic
processes that encompass either Vitamin D Receptor (VDR) mediated or non
VDR-mediated mechanisms (Brown et al., 1999). Vitamin D is supplied
to the human body by foods as well as through the photosynthesis of vitamin
D by Ultraviolet-B irradiation of skin (Holick et al., 1981; MacLaughlin
et al., 1982; Holick and Vitamin, 2002).
Despite the high UV exposure in tropical countries, some evidence suggests
a high prevalence of vitamin D deficiency in specific age and sex groups
in these areas (Sedrani, 1984; Sedrani et al., 1983; Dawodu
et al., 2003). Specifically, several reports from the Middle East
indicate that vitamin D deficiency is highly prevalent in this area (Sedrani
et al., 1983; Sedrani, 1984; Taha et al., 1984; Azizi et
al., 2000; Alagol et al., 2000; Goswami et al., 2000;
Dawodu et al., 2003). Therefore, elucidation of vitamin D status
at the national level is of crucial importance for making any decisions
about food fortification or screening programs.
Normal level of serum vitamin D is also a matter of debate (Lips, 2004).
There are multiple levels suggested for normal vitamin D, mild moderate
and severe vitamin D deficiency (Lips et al., 1988; Lips, 2004;
Himmelstein et al., 1990; Chapuy et al., 1997; Thomas et
al., 1998; Ooms et al., 1995; Chapuy et al., 1992).
In this study, we chose a method for a definition of normal vitamin D
and mild, moderate and severe vitamin D deficiency.
This study is a report of the Iranian national Multi-center Osteoporosis
Study (IMOS) designed for investigation of bone mineral density and influencing
factors on it in the normal Iranian population. The IMOS was performed
cross-sectional in five cities of different climates and latitudes in
Iran (i.e., Bushehr, Mashhad, Shiraz, Tabriz and Tehran).
Generally, the main aim of this study was defining normative values of
Vitamin D level among apparently healthy Iranian population and then,
based on these values finding the prevalence of Vitamin D deficiency states
in this community.
MATERIALS AND METHODS
In this study, we chose five cities in the country with different climates
and latitudes (i.e., Bushehr, Mashhad, Shiraz, Tabriz and Tehran). Individuals
were selected using random cluster sampling by dividing cities into multiple
foci based on distribution of population. Trained operators met citizens
at their home in each focus. Blood samples of eligible persons, after
filling an informed consent, were drawn and centrifuged within 30 min
in their place of residence. Serum samples were transferred to the local
participant laboratories and frozen and then sent to Endocrinology and
Metabolism Research Center (EMRC) Laboratory, Tehran by mobile freezers.
The sampling was performed in late winter (February and March 2001).
The study protocol was approved by the research ethics committee of EMRC,
Medical ethics research center and ethics committee of the Iran ministry
of health and medical education. All participants were also invited to
have a Bone Mineral Density measurement in the local university of medicine
in the same city. Information was collected in the Iranian Multi-center
Osteoporosis Study database.
Healthy men and women aged 20-69 years were eligible for enrolment.
Exclusion criteria were chronic disease such as known hepatic or renal
disease, metabolic bone disease, malabsorption, sterility, oligomenorrhea,
type I diabetes, hypercortisolism, known malignancy, immobility for more
than one-week, pregnancy, lactation, medications influencing bone metabolism,
smoking more than 10 cigarettes per day, alcoholism; using medication
affecting vitamin D and calcium metabolism and recent vitamin D use (oral
in preceding 2 weeks and injection in preceding 6 month).
Serum concentrations of 25-hydroxy-vitamin D (25(OH) D) were measured
by Radioimmunoassay method (Biosource Europes.A, Ò). Serum intact
PTH measurement was also done using RIA (Diasorin, Ò). Intra-assay
and inter-assay coefficients of variation were 5.3 and 8.2%, respectively,
for 25-OH-D and 2 and 4.6%, respectively for PTH.
Based on the biologic relationship of vitamin D to PTH, we used a
method for determination of health based normative values of serum vitamin
D for our community. We know there are multiple adverse effects of chronically
elevated levels of PTH on general and bone health. Based on the negative
relationship between vitamin D and PTH, we assumed; in the normal population,
that sufficient levels of vitamin D are associated with serum PTH levels
within the normal range and with decreasing vitamin D, PTH level rises.
We stratified serum levels of vitamin D to seven groups (<12.5, 12.5-24.9,
25-29.9, 30-34.9, 35-39.9, 40-44.9 and >45 nmole L-1). PTH
levels were analyzed as continuous quantitative variable. For analysis
of data, we used one way ANOVA test. After assessment of normality of
PTH distribution in each group, we transformed data for reaching normal
distribution by square root extraction of PTH levels. Then by Scheffe
method, we compared mean PTH levels in adjacent groups. Adjacent groups
with no significant difference in PTH levels were pooled together and
regarded homogeneous. Then PTH levels were compared between new homogeneous
groups using linear contrast method. In decreasing fashion, the first
significant difference between mean PTH levels was regarded as cutoff
levels of mild vitamin D deficiency and second and third as cutoff levels
of moderate and severe vitamin D deficiency, respectively.
In next step, based on these cutoff levels, we evaluated prevalence of
mild, moderate and severe vitamin D deficiency in males and females of
each city. For prevention of probable effect of different age distributions
in subjects of each city, we entered weight of age groups of community
in our modeling according to the last census of Iranian Statistical Center.
Extracted data were analyzed by SPSS for windows 11.5 software and STATA
version 8/SE and significance level was 0.05 in all tests.
Finally, we evaluated 5329 blood samples for vitamin D and PTH levels.
Table 1 is demonstrating mean levels of PTH in the initial vitamin D groups
for each sex.
Figure 1-3 show mean value of PTH levels in different
vitamin D subgroups in men, women and both sexes, respectively, homogeneous
groups are presented with ellipses and cutoff levels are viewed by curved
arrows. As shown in Fig. 1-3, we have found stepwise
elevations in PTH levels with decreasing Serum vitamin D levels. There
was 2 steps for PTH levels in male subjects at Vitamin D levels of 25
and 35 nmol L-1 and three steps of PTH elevation in female
population at 12.5, 25 and 40 nmol L-1. Median and inter quartile
range of vitamin D were 26 and 24 in female and 30 and 20.1 in male population,
respectively. These values for PTH were 24 and 18 in female and 23 and
16 in male population, respectively.
||Mean PTH levels in different vitamin D levels among Iranian men
Ellipses: Homogeneous groups, Arrows: Significant differences, 1:
Contrast value = 0.33, p<0.0001 (cutoff for mild vitamin D deficiency)
2: Contrast value = 0.22, p = 0.05 (cutoff for moderate or severe
vitamin D deficiency)
||Mean PTH levels in different vitamin D levels among Iranian women,
Ellipses: Homogeneous groups, Arrows: Significant differences, 1:
Contrast value = 0.17, p<0.05 (cutoff for mild vitamin D deficiency)
2: Contrast value = 0.26, p = 0.001 (cutoff for moderate vitamin D
deficiency) 3: Contrast value = 0.50, p = 0.001 (cutoff for severe
vitamin D deficiency)
||Mean PTH levels in different vitamin D levels among all subjects,
Ellipses: Homogeneous groups, Arrows: Significant differences, 1:
Contrast value = 0.23, p<0.0001 (cutoff for mild vitamin D deficiency)
2: Contrast value = 0.31, p<0.0001 (cutoff for moderate vitamin
D deficiency) 3: Contrast value = 0.30, p = 0.007 (cutoff for severe
vitamin D deficiency)
||Mean and standard deviation of serum PTH levels in various vitamin
|*: Distributions of PTH levels were not normal, so data
were transformed (by extraction of square root of values); mean and
standard deviation of PTH levels after retransformation is presented
in this table, :
Numbers inside parentheses are the standard deviations
Using values of stepwise elevation of PTH as cutoff values for definition
of vitamin D deficiency states (normal, mild, moderate and severe vitamin
D deficiency) the prevalence of vitamin D deficiency was 75.1% in female
population (mild: 27.2%, moderate: 42.8% and severe:5.1%) and 72.1 in males
(mild: 41.1%, moderate and severe: 35.8%). Prevalence of Vitamin D deficiency
states are shown in Table 2
prevalence of vitamin D deficiency was compared between two sexes in every
city. There was no significant deference in prevalence of mild vitamin D
deficiency between two sexes in Tehran and Bushehr [(p = 0.212) and (p =
0.15), respectively]. In other cities the prevalence of mild vitamin D deficiency
was significantly higher in men, (Tabriz: p = 0.0212 Mashhad: p = 0.007
and Shiraz: p<0.0001). Moderate or severe vitamin D deficiency was significantly
more prevalent in female sex in Tehran, Tabriz, Shiraz and Bushehr (p =
0.041, p<0.0001, p<0.0001 and p<0.0001, respectively). In Mashhad,
moderate or severe vitamin D deficiency had a trend to higher prevalence
in male sex (p = 0.92).
||Prevalence of vitamin D deficiency in females in each city
|Â¶: The prevalence rates are age-adjusted
||Prevalence of vitamin D deficiency in males in each city
|Â¶: The prevalence rates are age-adjusted
There was no significant difference between mean PTH levels of pre and post
menopausal females in each vitamin D group and PTH rise with vitamin D decrease
was uniform in pre and postmenopausal subjects.
There were several methods suggested for determination of normal levels
of vitamin D. These methods were historically based on antirrhachitic
levels of vitamin D but gradually they have become more physiologic so
at present, normal levels of vitamin D are based on the negative relationship
between vitamin D and PTH, positive effect of vitamin D supplementation
on 1,25(OH)2D, or effect of vitamin D on BMD or fractures (Lips,
Because this study was part of a large scale, cross sectional study,
we consider the negative correlation between PTH and vitamin D for the
basis of our study. In this method PTH after transformation showed stepwise
elevations, two steps in men and three steps in women were found. So,
we defined mild vitamin D deficiency in women, between 25 and 39.9 nmol
L-1, moderate 12.5-24.9 and severe deficiency below 12.5 nmol
L-1. Mild vitamin D deficiency in men was defined between 25
and 34.9 nmol L-1 and moderate/severe deficiency state below
25 nmol L-1. The different results might be due to the effect
of different levels of sun exposure, different levels of daily calcium
consumption which diminishes the need for elevated levels of vitamin D
for suppression of PTH, or possible innate difference in calcium and vitamin
D regulatory mechanism in two sexes. Presence of two steps in men despite
three steps in women also might be because of the effect of very low prevalence
of vitamin D deficiency states which can induce a third rise of PTH among
men (severe vitamin D deficiency). Although the presence of two or three
steps in PTH rise may only means the exponential elevation of PTH with
decreasing vitamin D levels, in clinical viewpoint we can accept these
cutoffs because chronically elevated PTH can cause adverse effects.
Our cutoff levels for vitamin D sufficiency state are similar to levels
suggested in other studies. There are two vitamin D supplementation studies
for determination of normal vitamin D level based on the positive effect
of vitamin D on serum level of 1,25(OH)2D in the Netherlands
(Lips et al., 1988) and the United states (Himmelstein et al.,
1990) in these two studies. The serum level of 1,25(OH)2D did
not change significantly after supplementation of persons with baseline
25(OH)D levels more than 30 and 40 nmol L-1, respectively.
In French, postmenopausal women who were followed in the Suvimax study,
a negative relationship between serum PTH and serum 25(OH)D below 78 nmol
L-1 was shown (Chapuy et al., 1997). Another study performed
on inpatients in the United States revealed similar results to the Suvimax
study (Thomas et al., 1998). In Amsterdam Vitamin D Study, at baseline
a negative correlation was detected between serum PTH and serum 25(OH)D
only when serum 25(OH)D was lower than 25 nmol L-1 (Ooms et
al., 1995). Analysis of vitamin D status in the MORE study (a study
for assessment of raloxifen on osteoporosis) revealed levels of more than
50, 50-25, below 25, normal vitamin D level, mild and severe vitamin D
deficiency, respectively (Chapuy et al., 1992).
Studies for assessment of prevalence of vitamin D deficiency were mostly
limited to a small sample size or specific age and sex group. In countries
with regular vitamin D fortification of foods (USA and some European countries),
prevalence of vitamin D deficiency is between 1.6-14.8% in different age
groups (Omdahl et al., 1982; Kinyamu et al., 1997; Burnard
et al., 1990). In European countries with no regular vitamin D
fortification program, the deficiency state is more prevalent (Vander
et al., 1995).
Studies which assessed middle-aged and elderly people showed 14 to 59.6%
vitamin D deficiency (Chapuy et al., 1997; Burnard et al.,
1990). Vitamin D deficiency prevalence is much higher in Asian countries
(Du et al., 2001). Studies in Middle East revealed vitamin D deficiency
even in countries with high sun light (Sedrani et al., 1983; Sedrani,
1984; Taha et al., 1984; Azizi et al., 2000; Alagol et
al., 2000; Goswami et al., 2000; Dawodu et al., 2003).
Results of the present study indicate a high prevalence of vitamin D
deficiency in various cities of the country even in sea side and lower
latitude. These results are similar to other studies in nearby countries
and may indicate a need to food fortification by vitamin D in national
High prevalence of vitamin D deficiencies in all assessed cities may
be due to different causes such as low exposure of skin to UVB despite
abundant sun shine in these areas (life style factor), higher skin pigmentation
of residents compared to western ethnic groups and low oral intake of
vitamin D. Other factors such as VDR polymorphism in Iranian populations
and low daily calcium intake which increase vitamin D levels needed for
PTH suppression may contribute to this high prevalence of increased PTH.
Low calcium intake also can increase vitamin D catabolism. There is evidence
of increased activity of vitamin D 24-hydroxylase in Indian immigrants
to United States compared to other Americans (Awumey et al., 1998).
It is possible that Iranian population have similar genetic characteristics.
In this study, we did not assess daily calcium and vitamin D consumption
in all cities, but in Tehran (in a part of present study) there was no
significant difference between daily calcium intake of persons with normal
and low vitamin D levels (Hashemipour et al., 2004). Present study
was performed in urban area but a decision about the need for a national
fortification program should also include data from rural areas.
The authors would like to thank Dr. Gooya and Ministry of Health for
funding support and coordination of the study. Also we wish to acknowledge
Fatemeh Bandarian MD for kindly assistance and editing the manuscript.
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