Vitamin D deficiency in Kazakhstan: Cross-Sectional study
Introduction
Globally, Vitamin D deficiency (VDD) remains one of the unresolved public health issues affecting at least one billion of world population [1]. Prevalence of vitamin D deficiency in the northern hemisphere gets even higher in winter, with decreased insolation [2] and remains unresolved even with daily intake of vitamin complexes, drinking of a glass of milk and eating salmon [3]. Although people residing near the equator and not protecting their skin from sunlight have adequate levels of Vitamin D [4], others living even in the sunniest world’s countries and shielding most of their skin, are affected by VDD [5].
In fact, Vitamin D presents a family of fat-soluble vitamins, of which 25-hydroxyvitamin D3 (cholecalciferol) and 25-hydroxyvitamin D2 (ergocalciferol) are the most important for humans. Vitamin D is responsible for a range of physiological effects, including absorption of calcium, magnesium, and phosphorous in small intestine [6]. Vitamin D could be obtained both through a direct synthesis in human skin following exposure to a sunlight (wavelength 290–315 nm) or through a diet containing Vitamin D-rich products (primarily dairy products and oily fish). Vitamin D status could be evaluated by means of 25-hydroxyvitamin D (25(OH)D), which is synthetized in liver and is metabolized in kidneys to 1.25-dihydroxyvitamin D [7].
However, to judge about the prevalence of VDD properly, the issue of definition is of utmost importance. Although there is no unified classification related to Vitamin D status, the serum level of 25(OH)D below 10 ng/mL (25 nmol/L) is considered to be deficient by a number of international clinical guidelines [2,8]. Nevertheless, the Institute of Medicine [9] states that serum levels of 25(OH)D should not drop below 20 ng/mL (50 nmol/L), while according to the Endocrine Society [2], to achieve optimal skeletal health and muscle strength, the serum levels of 25(OH)D should be at least 30 ng/mL (75 nmol/L). Still, it has to be noted that the Endocrine Society guidance relates to patient populations while for a healthy adult population the threshold of 30 ng/mL is not applicable and an individual threshold of 20 ng/mL (50 nmol/L) should be used instead. The cut-off of 30 ng/mL was selected based on the findings that serum PTH begins to plateau and maximum calcium absorption occurs when 25(OH)D reaches the level of 30–40 ng/mL [10], and that this level reduces the risk for falls [11]. In their Practice Guidelines on Vitamin D, the Endocrine Society went further and defined VDD as the serum 25(OH)D < 20 ng/mL, and insufficiency as 21–29 ng/mL [2].
Over the past decade of years numerous studies evaluating the vitamin D status in countries across the world were performed. Still, to the best of our knowledge, none of them described the situation in Kazakhstan – a Central-Asian state located at longitude from 46.490 to 87.310 E and at latitude from 40.670 to 54.905 N. The country has a markedly continental climate with approximately 200 sunny days a year for most of its territory [12]. The population of Kazakhstan is around 18.3 million people and being the ninth largest world country, Kazakhstan has one of the lowest population densities (6 people per square kilometer). The country is composed of 14 regions that could be geographically divided into eastern provinces (East Kazakhstan and Pavlodar regions), central provinces (Karaganda and Akmola regions), northern provinces (North Kazakhstan and Kostanay regions), western provinces (West Kazakhstan, Aktobe, Atyrau and Mangystau regions), and southern provinces (South Kazakhstan, Almaty, Zhambyl and Kyzylorda regions). Besides, Kazakhstan has three so-called “cities of republican significance” – Nur-Sultan, Almaty and Shymkent that administratively are considered to be equal to a region [13]. The majority of population (56 %) resides in urban areas and the proportion of women makes up 51.7 %. Kazakhstan is one of the most industrialized countries in the world possessing ample natural resources [14]. The total fertility rates vary between the country’s regions being the highest (>3.2) in more religious southern provinces [15].
The aim of this study was to evaluate Vitamin D status in adult population of both sexes residing in different geographical areas of Kazakhstan and to elucidate the possible contributing factors related to VDD.
Section snippets
Materials and methods
This cross-sectional study enrolled 1347 healthy adults (out of them 819 were females), residing in 6 regions of Kazakhstan with a mean age of 44 ± 14 years, who attended a single-consultation outpatient clinic for routine check-up. The systematic random sampling was applied to select 10–15 patients per day from each region during summer months (from May 2018 to August 2018). General characteristics of the study participants are presented in Table 1.
The exclusion criteria were: (1) presence of
Results
The median serum 25(OH)D concentrations in all studied regions of Kazakhstan were below the reference threshold (20 ng/mL). In two southern provinces – Almaty and South Kazakhstan regions – the median serum 25(OH)D levels were 18.0 (12.00; 24.00) ng/mL and 16.0 (11.00; 22.00) ng/mL, respectively. Meanwhile, the central provinces of Kazakhstan had median rates of 25(OH)D equal to 18 (11.0; 26.0) ng/mL in Karaganda region and to 11 (8.00;18.00) ng/mL in Akmola region. The population of western
Discussion
This study aimed at evaluation of VDD rates amongst adults residing in different regions of the Republic of Kazakhstan representing different geographical areas and at identification of the possible contributing factors related to VDD. The major finding of this study was a high rate of VDD as two thirds of study participants had serum 25(OH)D concentrations below 20 ng/mL, while not more than 10 % of them had optimal levels of serum 25(OH)D defined as 30 ng/mL (75 nmol/L). Female gender,
Conclusion
High prevalence of VDD remains an important problem in countries of northern hemisphere especially in winter months. The most predisposed population groups are elderlies, pregnant and postmenopausal women, obese children, people with increased skin pigmentation, and individuals avoiding direct exposure to sunlight. According to the study findings, there was a remarkably high rate of VDD as two thirds of study participants had serum 25(OH)D concentrations below 20 ng/mL, which is internationally
Contributors
All authors have contributed to manuscript writing and review, and have approved the final version.
Funding
The part of funding for the study (measurement of 25-hydroxy vitamin D(25-OHD) on the base of “IN VITRO” laboratories) was provided by Pharmaceutical company “Unipharm INC”, Almaty, Kazakhstan.
CRediT authorship contribution statement
Olga Gromova: Conceptualization, Methodology. Aikerm Doschanova: Validation, Conceptualization, Resources, Data curation. Vyacheslav Lokshin: Validation, Conceptualization, Resources. Ainur Tuletova: Investigation, Resources. Galina Grebennikova: Methodology, Investigation, Data curation. Laura Daniyarova: Methodology, Investigation, Data curation, Resources. Gulnaz Kaishibayeva: Validation, Conceptualization, Resources. Tair Nurpeissov: Validation, Conceptualization, Resources. Viktoriya Khan:
Declaration of Competing Interest
No conflict of interest for any of the authors.
Acknowledgments
We thank all the study participants who give us own time for survey and assistants Ph.D. student Andreyeva O. and Sayapina Ye.
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