Original Full Length ArticleVitamin D deficiency in northern Vietnam: Prevalence, risk factors and associations with bone mineral density
Highlights
► Vitamin D deficiency has been linked to osteoporosis and to the risk of cancer, autoimmune and cardiovascular diseases. ► The prevalence of vitamin D deficiency is high in the Vietnamese population, reaching 30% in women and 16% in men. ► In women, urban residency and age less than 30 years were significant predictors, whereas contraceptive pills use was protective. ► In men, winter season was the only significant predictor of vitamin D deficiency. ► In multiple linear regression analysis, serum levels of 25(OH)D were positively associated with BMD in both women and men.
Introduction
Osteoporosis and its consequence of fracture represent a global public health problem, because fracture is associated with increased mortality, concomitant morbidity, and reduced quality of life [1]. We have recently reported a high prevalence of osteoporosis in Vietnam [2]. In spite of a high consumption of soy and other phytoestrogens and a sub-tropical climate with high sun exposure, the prevalence of osteoporosis in Vietnam was found similar to that of many Western populations [2], [3], [4]. We hypothesize that part of this high prevalence could be explained by vitamin D deficiency in the Vietnamese population.
Vitamin D plays a classically important role in the regulation of calcium and bone metabolism [5], [6], [7]. Lack of 25-hydroxy-vitamin D [25(OH)D] is a cause of rickets due to abnormality in bone remodelling, and supplementation of 25(OH)D reverses the abnormal bone formation [8]. Recent studies have also demonstrated the presence of specific receptors in a wide variety of tissues [7], [9], [10], and indicated many important effects of vitamin D besides bone health [6], [7], [9]. Indeed, vitamin D deficiency has been associated with certain forms of cancer [9], [11], [12], [13], type II diabetes [14], [15], hypertension and other types of cardiovascular disease [16], [17], [18], [19], autoimmune and infectious disease [20], [21], [22], [23]. A recent meta-analysis found a marked increase in the risk of mortality among vitamin D deficient individuals as compared to those with normal 25(OH)D levels [24].
Although there is no consensus of a definition of vitamin D deficiency, it has been generally agreed that measurement of 25(OH)D should be used as an indicator of an individual's vitamin D status [8], [25]. Serum 25(OH)D levels below 20 ng/mL are considered as “deficiency”. Using this criterion, studies on vitamin D status in different populations have shown considerable variation in the prevalence of vitamin D deficiency [26]. The common trend in all studies is that populations in temperate regions have higher prevalence than populations in tropical regions [25], [26], which indicates the effect of sunlight exposure on vitamin D variation.
Approximately 90% of vitamin D is synthesized in the skin after sunlight exposure [8], [27]. Melanin is extremely efficient in absorbing UVB radiation and thus, increased skin pigmentation markedly reduces vitamin D synthesis [8], [28], [29]. A small amount of vitamin D can be absorbed through food intake [30]. It is therefore assumed that people living in countries with high amounts of sunlight may have a lower risk of vitamin D deficiency. However, recent studies in tropical countries have indicated that the prevalence of vitamin D deficiency still could be as high as that observed in Western populations [26]. Reports from Hong Kong [31], Malaysia and Singapore [32] have indicated that between 60% and 100% of the population have vitamin D levels below 30 ng/mL, the level that is considered “insufficient”. However, these studies were conducted on urban residents who may have a lower level of sunlight exposure than rural residents. Moreover, 25(OH)D production is known to be affected by seasonal variation [33], [34] which was not taken into account in these studies.
North Vietnam is perhaps one of the ideal settings for evaluating the effect of urbanization and seasonality on the risk of 25(OH)D deficiency. North Vietnam is located in the Southeast Asia where there is a tropical monsoon climate with different seasons. Due to recent economic development, Vietnam has undergone rapid urbanization, in which there is a clear separation between urban and rural areas. The present study was designed to assess the prevalence of, and risk factors for, vitamin D deficiency in a representative sample of urban and rural women and men in North Vietnam.
Section snippets
Study design
The study was designed as a cross-sectional investigation with a multistage sampling scheme. Within the setting of northern Vietnam (latitude 21oN), two districts (Dong Da in Hanoi and Kim Bang in Hanam) were selected to represent urban and rural areas, respectively. From each of these districts, 4 communes were randomly selected, and a full list of all inhabitants was obtained from the local government authority which served as the sampling frame. The lists of inhabitants were then sorted by
Results
The study involved 222 men and 269 women, aged between 13 and 83 years (Table 1). There were no differences between men and women in terms of age and BMI. As expected, men had greater height and weight than women. The prevalence of smoking, alcohol and coffee consumption was several times higher in men than in women.
Serum levels of 25(OH)D and PTH were significantly higher in men than in women (Table 1). The prevalence of vitamin D deficiency, as defined as 25(OH)D < 20 ng/mL, was twice as large in
Discussion
Due to lack of data, it is commonly believed that vitamin D deficiency is more prevalent in Caucasian populations than in tropical populations [26]. However, in the present study, we found that more than 30% of the women and 16% of the men in a Vietnamese population had serum levels of 25(OH)D below 20 ng/mL (50 nmol/L). Moreover, in this population, we found that being woman, of younger age, living in city and winter season were independent predictors of vitamin D deficiency.
There is no general
Acknowledgments
The authors wish to thank Sida for research funds; FrieslandCampina Vietnam for partially financial support for study; the Swedish Research Council (20324 ALH); Hanoi Medical University; Bach Mai Hospital; Karolinska Institutet; and Garvan Institute. We are grateful to all members of the FSH group for the assistance in data collection. Especially thanks to Dr. Nguyen Thu Hoai for the technical assistance in the analysis of vitamin D and PTH; Dr. Hoang Hoa Son, Dr Tran Thi To Chau. Dr.Le Tuan
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