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Regional variation and determinants of vitamin D status in sunshine-abundant Thailand
© Chailurkit; licensee BioMed Central Ltd. 2011
Received: 16 June 2011
Accepted: 10 November 2011
Published: 10 November 2011
Vitamin D insufficiency is highly prevalent. Most of the studies concerning vitamin D status were generated from countries situated at temperate latitudes. It is less clear what the extent of vitamin D insufficiency is in countries situated in the tropics and how geographical regions within country would affect vitamin D status. In the present study, we investigated vitamin D status in Thais according to geographical regions and other risk factors.
Subjects consisted of 2,641 adults, aged 15 - 98 years, randomly selected from the Thai 4th National Health Examination Survey (2008-9) cohort. Serum 25 hydroxyvitamin D were measured by liquid chromatography/tandem mass spectrometry. Data were expressed as mean ± SE.
Subjects residing in Bangkok, the capital city of Thailand, had lower 25(OH)D levels than other parts of the country (Bangkok, central, northern, northeastern and southern regions: 64.8 ± 0.7, 79.5 ± 1.1, 81.7 ± 1.2, 82.2 ± 0.8 and 78.3 ± 1.3 nmol/L, respectively; p < 0.001). Within each region, except for the northeastern part of the country, subjects living inside municipal areas had lower circulating 25(OH)D (central, 77.0 ± 20.9 nmol/L vs 85.0 ± 22.1 nmol/L, p < 0.001; north 79.3 ± 22.1 nmol/L vs 86.8 ± 21.8 nmol/L, p < 0.001; northeast 84.1 ± 23.3 nmol/L vs 87.3 ± 20.9 nmol/L, p = 0.001; south, 76.6 ± 20.5 nmol/L vs 85.2 ± 24.7 nmol/L, p < 0.001). Overall, the prevalence of vitamin D insufficiency was 64.6%, 46.7%, and 33.5% in Bangkok, municipal areas except Bangkok, and outside municipal area in other parts of the country, respectively. In addition, the prevalence of vitamin D insufficiency according to geographical regions was 43.1%, 39.1%, 34.2% and 43.8% in the central, north, northeast and south, respectively. After controlling for covariates in multiple linear regression analysis, the results showed that low serum 25(OH)D levels were associated with being female, younger age, living in urban and Bangkok.
Vitamin D insufficiency is common and varies across geographical regions in Thailand.
Vitamin D is produced endogenously when the skin is exposed to sunlight, or obtained exogenously from nutrients or supplements. The major role of vitamin D is to maintain calcium homeostasis and bone health. In addition, recent research has revealed that vitamin D may play an important role in a variety of non-skeletal health activities, such as modulation of neuromuscular and immune function, reduction of inflammation, and regulation of cell proliferation, differentiation and apoptosis. Vitamin D insufficiency is highly prevalent and is now recognized as a worldwide health problem [1–3]. It has been observed to varying degrees in many different countries, regardless of geographical location . However, most of the studies concerning vitamin D status have focused on countries situated at temperate latitudes. It is less clear what the extent of vitamin D insufficiency is in countries situated in the tropics, and how geographical regions within a country could affect vitamin D status.
Thailand, a Southeast Asian country, is located at latitudes between 5°30' N and 20°30' N. Vitamin D intake among Thais is generally low because few natural vitamin D-rich food sources are found in Thailand, and foods are not fortified with vitamin D. Up to now, there has been a lack of reliable epidemiological data concerning vitamin D status in Thais. Therefore, the purpose of this study is to investigate vitamin D status in Thais according to geographical region by assessing levels of serum 25-hydroxyvitamin D (which is the major metabolite and represents the stored form of vitamin D) by a reference method, liquid chromatography/tandem mass spectrometry (LC-MS/MS).
Subjects and methods
Serum 25-hydroxyvitamin D (25(OH)D) measurement
Serum 25(OH)D2 and 25(OH)D3 were analyzed by LC-MS/MS with an Agilent 1200 Infinity liquid chromatograph (Agilent Technologies, Waldbronn, Germany) coupled to a QTRAP® 5500 tandem mass spectrometer (AB SCIEX, Foster City, CA, USA) using a MassChrom® 25-OH-Vitamin D3/D2 diagnostics kit (Chromsystems, Munich, Germany). The summation of serum 25(OH)D2 and 25(OH)D3 was used to reflect vitamin D status. The inter-assay and intra-assay coefficients of variation of total serum 25(OH)D level were 6.3% and 5.0%, respectively.
Data were expressed as mean ± SE. Differences between two groups were assessed by Student's t-test. Comparisons among three or more groups were analyzed by analysis of variance followed by Scheffé's test. Stepwise multiple linear regression analysis was used to examine the independent determinants of variables. A p value less than 0.05 was considered statistically significant. All analyses were performed using Stata version 10.1 (StataCorp LP, Texas, USA) and SPSS statistical software, version 16.0 (SPSS Inc., Chicago, IL, USA). All the data analyses were weighted to the probability of sampling to take into account for complex survey design.
Baseline characteristics of the population studies
39.6 ± 0.5
41.0 ± 0.4
40.3 ± 0.3
22.7 ± 0.2
24.4 ± 0.2*
23.6 ± 0.1
Duration of sunshine and mean serum vitamin D levels according to geographical region and gender
Duration of sunshine (hours/day)
4.7 - 9.1
69.0 ± 0.6
61.1 ± 1.3*
64.8 ± 0.7***
86.5 ± 1.7**
73.0 ± 1.1*, **, ***
79.5 ± 1.1**, ***
3.6 - 8.1
88.5 ± 1.7**
75.1 ± 1.8*, **
81.7 ± 1.2**
87.8 ± 1.3**
76.7 ± 1.1*, **
82.2 ± 0.8**
2.0 - 8.8
87.7 ± 2.9**
69.5 ± 0.8*, **, ***
78.3 ± 1.3**, ***
2.0 - 9.1
85.9 ± 1.1
73.0 ± 0.8*
79.3 ± 0.8
Mean serum 25(OH)D levels between gender by age, municipal area, BMI and religion
15 - 29
79.3 ± 1.3
69.3 ± 1.1
74.4 ± 0.9
30 - 44
89.1 ± 1.7*
70.7 ± 1.2 a
79.9 ± 1.1*,, a
45 - 59
86.5 ± 1.5*,
75.3 ± 1.2*,
80.6 ± 1.0*
60 - 69
90.7 ± 1.3*
80.2 ± 1.1*
85.1 ± 1.0*
70 - 79
95.0 ± 1.4*
83.8 ± 1.7*
88.6 ± 1.2* a
96.9 ± 1.5*
80.7 ± 1.7*
88.2 ± 1.4* a
88.9 ± 1.1
75.8 ± 0.8
82.3 ± 0.6
78.4 ± 1.6**
66.6 ± 1.1**
72.3 ± 1.3**
84.1 ± 1.9
73.7 ± 1.1
77.5 ± 1.1
86.5 ± 1.1
72.6 ± 0.9
80.3 ± 0.8***
81.9 ± 5.2
61.1 ± 4.2
69.0 ± 4.4
86.0 ± 10****
73.5 ± 0.7****
79.7 ± 0.7****
Prevalence of vitamin D insufficiency by geographical region and gender
Age, yrs (range)
Serum 25(OH)D levels
< 75 nmol/L
< 50 nmol/L
15 - 93
15 - 91
15 - 98
15 - 91
15 - 92
15 - 98
Independent variables for serum 25(OH)D levels by multiple regression analysis
Our study represents the first large-scale examination of vitamin D status in the Thai population. Despite the fact that Thailand is located near the equator, a sizable proportion of Thais have inadequate vitamin D status. When using a 25(OH)D threshold of 75 nmol/L, nearly half of Thais are vitamin D insufficient. When a lower threshold of 50 nmol/L was used, the prevalence of vitamin D insufficiency was found to be more than 10% in Bangkok, which is as high as the prevalence of diabetes in Thailand . Studies examining vitamin D status in the tropics are scarce, but have mostly demonstrated similarly low vitamin D status. For example, even in the sunniest areas like Saudi Arabia, the United Arab Emirates, Australia, Turkey, India and Lebanon, a high prevalence of vitamin D insufficiency has been reported in 30 to 50% of children and adults, with 25(OH)D levels under 50 nmol/L [6–10]. On the other hand, people living near the equator who are exposed to sunlight without sun protection have robust levels of 25(OH)D, well above 75 nmol/L . Taken together, this suggests that low vitamin D status is not an uncommon problem even in countries that receive abundant sunshine. Despite this, outdoor sun exposure can be limited and is likely to be the main contributing factor.
The mean serum 25(OH)D levels in Thais seem relatively high when compared to those reported in various countries in the West [3, 12], Middle East  and Asia . This might be caused by a higher exposure to sunshine all year round, since the latitude of Thailand is more southerly (closer to the equator) than the countries studied. Nevertheless, differences in vitamin D status were found between regions in Thailand; subjects residing in the southern parts of the country, women in particular, generally had lower serum 25(OH)D concentrations than those residing in the northern region. This finding conflicts with the belief that vitamin D status decreases with increasing latitude. However, our results are in agreement with a European study which showed a positive relationship between serum 25(OH)D and northern latitude . This finding could be explained by the common use of cod liver oil and vitamin supplements in many northern European countries; while people in southern Europe typically have more skin pigmentation (with consequently less vitamin D production) and may prefer shade instead of sunshine. An explanation for this observation in Thailand might be regional differences in religion. Southern Thailand has a much higher percentage of Muslims, and the clothing style of Muslim women generally allows for greater body coverage. In addition, most northern Thai people are agricultural. Working in the fields and spending more time outdoors in the sunshine probably accounts for much of this difference.
Both lifestyle and environmental factors are important determinants of serum 25(OH)D concentration because of their relationship to ultraviolet exposure. In the present study, a difference in vitamin D status between populations in rural and urban areas was clearly demonstrated. Lower vitamin D levels in the urban populations were evident in almost all geographical regions of Thailand. Although a number of studies have investigated the vitamin D status of urban or rural residents, the disparity in vitamin D status between rural and urban populations has been investigated less often; but existing studies have generally shown lower vitamin D reserves among urban populations [15–17]. A number of factors may be causally related to lower vitamin D status associated with urbanization. Besides lifestyle factors, which may preclude adequate outdoor sun exposure, it is also likely that air pollution may have a contributory role. Tropospheric ozone is a common urban air pollutant and an efficient absorber of ultraviolet radiation . The phenomenon is likely to be more marked in big cities, and may partially explain why residents of Bangkok, the largest city in Thailand, had the lowest 25(OH)D concentrations.
Lower vitamin D status has been demonstrated to be more prevalent with advancing age in most studies [19–22]; this may be caused by less sun exposure and the decreased ability of the skin to produce vitamin D . In contrast, we demonstrated in the present study that vitamin D levels unexpectedly became higher with increasing age. Younger age, rather than older, was an independent risk factor for inadequate vitamin D status. The phenomenon of higher vitamin D levels with advancing age was also observed in both sexes, making it less likely that the observation was simply a chance finding. Although most studies have demonstrated lower vitamin D levels with advancing age, such findings have been predominantly generated from studies of populations residing in temperate geographical locations. There are only limited data on this issue for countries in the tropics. It was found in a study of postmenopausal women in Malaysia that vitamin D levels do not decrease with age . Likewise, 25(OH)D levels remain more or less constant from age 20 to more than age 60 in Iranian men . It is therefore conceivable that despite the decreased dermal synthesis of vitamin D in the elderly, the abundant sunlight may overcome this disadvantage, given that sun exposure is not limited. The elderly in Thailand, after retirement, may have more leisure time and spend more time in the sun. On the other hand, it is also likely that the increased use of sunblock by younger people may be partly accountable for their lower vitamin D status compared to the older population.
Vitamin D insufficiency is highly prevalent in the general adult population in Thailand. Vitamin D status is better in northern than in southern regions of the country. Low serum 25(OH)D levels were associated with being female, younger age, living in urban and Bangkok.
This study was supported by the Thailand Research Fund and the National Health Examination Survey Office, Health Systems Research Institute, Thailand.
NHESIV was conducted by the National Health Examination Survey Office, Health Systems Research Institute, Thailand. The NHESIV study group includes: National Health Examination Survey Office: Wichai Aekplakorn, Rungkarn Inthawong, Jiraluck Nonthaluck, Supornsak Tipsukum, Yawarat Porrapakkham; Northern region : Suwat Chariyalertsak, Kanittha Thaikla (Chiang Mai University), Wongsa Laohasiriwong, Wanlop Jaidee, Sutthinan Srathonghon, Ratana Phanphanit, Jiraporn Suwanteerangkul, Kriangkai Srithanaviboonchai; Northeastern region: Pattapong Kessomboon, Somdej Pinitsoontorn, Piyathida Kuhirunyaratn, Sauwanan Bumrerraj, Amornrat Rattanasiri, Suchada Paileeklee, Bangornsri Jindawong, Napaporn Krusun, Weerapong Seeuppalat (Khon Kaen University); Southern region: Virasakdi Chongsuvivatwong, Rassamee Sangthong, Mafausis Dueravee (Prince of Songkla University); Central region: Surasak Taneepanichskul, Somrat Lertmaharit, Vilai Chinveschakitvanich, Onuma Zongram, Nuchanad Hounnaklang, Sukarin Wimuktayon(Chulalongkorn University); Bangkok region: Panwadee Putwatana, Chalermsri Nuntawan, Karn Chaladthanyagid (Mahidol University). The Thai National Health Examination Survey IV was supported financially by the Health System Research Institute; Bureau of Policy and Strategy, Ministry of Public Health; Thai Health Promotion Foundation; National Health Security Office, Thailand.
- Holick MF: Vitamin D deficiency. N Engl J Med. 2007, 357: 266-281. 10.1056/NEJMra070553.View ArticlePubMedGoogle Scholar
- Holick MF: High prevalence of vitamin D inadequacy and implications for health. Mayo Clin Proc. 2006, 81: 353-373. 10.4065/81.3.353.View ArticlePubMedGoogle Scholar
- Chapuy MC, Preziosi P, Maamer M, Arnaud S, Galan P, Hercberg S, Meunier PJ: Prevalence of vitamin D insufficiency in an adult normal population. Osteoporos Int. 1997, 7: 439-443. 10.1007/s001980050030.View ArticlePubMedGoogle Scholar
- van der Wielen RP, Lowik MR, van den Berg H, de Groot LC, Haller J, Moreiras O, van Staveren WA: Serum vitamin D concentrations among elderly people in Europe. Lancet. 1995, 346: 207-210. 10.1016/S0140-6736(95)91266-5.View ArticlePubMedGoogle Scholar
- Aekplakorn W, Stolk RP, Neal B, Suriyawongpaisal P, Chongsuvivatwong V, Cheepudomwit S, Woodward M: The prevalence and management of diabetes in Thai adults: the international collaborative study of cardiovascular disease in Asia. Diabetes Care. 2003, 26: 2758-2763. 10.2337/diacare.26.10.2758.View ArticlePubMedGoogle Scholar
- Sedrani SH: Low 25-hydroxyvitamin D and normal serum calcium concentrations in Saudi Arabia: Riyadh region. Ann Nutr Metab. 1984, 28: 181-185. 10.1159/000176801.View ArticlePubMedGoogle Scholar
- Marwaha RK, Tandon N, Reddy DR, Aggarwal R, Singh R, Sawhney RC, Saluja B, Ganie MA, Singh S: Vitamin D and bone mineral density status of healthy schoolchildren in northern India. Am J Clin Nutr. 2005, 82: 477-482.PubMedGoogle Scholar
- El-Hajj Fuleihan G, Nabulsi M, Choucair M, Salamoun M, Hajj Shahine C, Kizirian A, Tannous R: Hypovitaminosis D in healthy schoolchildren. Pediatrics. 2001, 107: E53-10.1542/peds.107.4.e53.View ArticlePubMedGoogle Scholar
- McGrath JJ, Kimlin MG, Saha S, Eyles DW, Parisi AV: Vitamin D insufficiency in south-east Queensland. Med J Australia. 2001, 174: 150-151.PubMedGoogle Scholar
- Gannage-Yared MH, Chemali R, Yaacoub N, Halaby G: Hypovitaminosis D in a sunny country: relation to lifestyle and bone markers. J Bone Miner Res. 2000, 15: 1856-1862. 10.1359/jbmr.2000.15.9.1856.View ArticlePubMedGoogle Scholar
- Vieth R: Why the optimal requirement for Vitamin D3 is probably much higher than what is officially recommended for adults. J Steroid Biochem. 2004, 89-90: 575-579.View ArticleGoogle Scholar
- McKenna MJ: Differences in vitamin D status between countries in young adults and the elderly. Am J Med. 1992, 93: 69-77. 10.1016/0002-9343(92)90682-2.View ArticlePubMedGoogle Scholar
- Fuleihan GE, Deeb M: Hypovitaminosis D in a sunny country. N Engl J Med. 1999, 340: 1840-1841. 10.1056/NEJM199906103402316.View ArticlePubMedGoogle Scholar
- Lips P, Duong T, Oleksik A, Black D, Cummings S, Cox D, Nickelsen T: A global study of vitamin D status and parathyroid function in postmenopausal women with osteoporosis: baseline data from the multiple outcomes of raloxifene evaluation clinical trial. J Clin Endocrinol Metab. 2001, 86: 1212-1221. 10.1210/jc.86.3.1212.View ArticlePubMedGoogle Scholar
- Heere C, Skeaff CM, Waqatakirewa L, Vatucawaqa P, Khan AN, Green TJ: Serum 25-hydroxyvitamin D concentration of Indigenous-Fijian and Fijian-Indian women. Asia Pac J Clin Nutr. 2010, 19: 43-48.PubMedGoogle Scholar
- Soontrapa S, Chailurkit LO: Hypovitaminosis D in Thailand. J Med Assoc Thailand. 2009, 92 (Suppl 5): S26-S29.Google Scholar
- Manicourt DH, Devogelaer JP: Urban tropospheric ozone increases the prevalence of vitamin D deficiency among Belgian postmenopausal women with outdoor activities during summer. J Clin Endocrinol Metab. 2008, 93: 3893-3899. 10.1210/jc.2007-2663.View ArticlePubMedGoogle Scholar
- Holick MF: Environmental factors that influence the cutaneous production of vitamin D. Am J Clin Nutr. 1995, 61: 638S-645S.PubMedGoogle Scholar
- Dubbelman R, Jonxis JH, Muskiet FA, Saleh AE: Age-dependent vitamin D status and vertebral condition of white women living in Curacao (The Netherlands Antilles) as compared with their counterparts in The Netherlands. Am J Clin Nutr. 1993, 58: 106-109.PubMedGoogle Scholar
- Jacques PF, Felson DT, Tucker KL, Mahnken B, Wilson PW, Rosenberg IH, Rush D: Plasma 25-hydroxyvitamin D and its determinants in an elderly population sample. Am J Clin Nutr. 1997, 66: 929-936.PubMedGoogle Scholar
- Burnand B, Sloutskis D, Gianoli F, Cornuz J, Rickenbach M, Paccaud F, Burckhardt P: Serum 25-hydroxyvitamin D: distribution and determinants in the Swiss population. Am J Clin Nutr. 1992, 56: 537-542.PubMedGoogle Scholar
- Baker MR, Peacock M, Nordin BE: The decline in vitamin D status with age. Age Ageing. 1980, 9: 249-252. 10.1093/ageing/9.4.249.View ArticlePubMedGoogle Scholar
- MacLaughlin J, Holick MF: Aging decreases the capacity of human skin to produce vitamin D3. J Clin Invest. 1985, 76: 1536-1538. 10.1172/JCI112134.View ArticlePubMedPubMed CentralGoogle Scholar
- Rahman SA, Chee WS, Yassin Z, Chan SP: Vitamin D status among postmenopausal Malaysian women. Asia Pac J Clin Nutr. 2004, 13: 255-260.PubMedGoogle Scholar
- Masoompour SM, Sadegholvaad A, Larijani B, Ranjbar-Omrani G: Effects of age and renal function on vitamin D status in men. Arch Iran Med. 2008, 11: 377-381.PubMedGoogle Scholar
- The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2458/11/853/prepub
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