In this population-based study which was done in all rural districts of Bushehr province, it was shown that a half of the rural participants had vitamin D deficiency, more than one to five of the participants had vitamin D insufficiency, and one to five of total population had sufficient vitamin D levels. Unfortunately, there are limited studies on rural population in Iran regarding vitamin D levels, and most of the studies have been done on the capital cities; therefore, it is impossible to compare vitamin D serum levels of rural and urban populations. The only study that compared the vitamin D serum levels was done on Guilan province in the northern part of Iran which compared 750 postmenopausal women in rural and urban areas and demonstrated that vitamin D deficiency was more common in urban than rural subjects [16].This difference among rural and urban subjects was also observed in other countries [17]. In a systematic review and meta-analysis study in Africa it was shown that the mean of serum vitamin D levels in urban places was less than the rural areas [18]. This difference could be explained with different lifestyles, jobs, and habitual conditions because the rural inhabitants live more outdoor and expose to more sun-light with resulting with more absorption of vitamin D by their skin; in another aspect, the urban lifestyle patterns cause less amount of vitamin D absorption via sun-light exposure or less amount of dietary vitamin D intake through food habits [16, 18].
Although, in the current study, we could not compare the prevalence of vitamin D deficiency in urban and rural regions, we found that the prevalence of vitamin D deficiency in rural areas was at least similar to those prevalence rates that has been reported from Bushehr city ( the capital of Bushehr province ), the middle east and south east of Asia ,and China [10, 19-21]. In two systematic reviews and meta-analyses from Iran, it was reported that more than a half of the Iranian population, especially those who live in capital cities, had vitamin D deficiency [14, 15]. The observed high prevalence of vitamin D deficiency in rural areas in the current study may indicates that the lifestyle of these villagers has been changed due to industrialization, and also their nutritional habitus has converted to the urban styles. These changes of life style among villagers of Bushehr province may have induced their mean vitamin D serum levels to approach to their urban counterparts.
In our study, we found a linear relationship between serum vitamin D levels and sun-light exposure. In the European population, it has been revealed that sun exposure of 18 percent of body surface area for 15 minutes per-day, 2 to 3 times per week is sufficient for absorbing vitamin D [22]. However, in the current study, only 29 percent of participant had fulfilled the above criteria. This low level of sun-exposure in the rural areas may be due to the mentioned changes of lifestyle towards the urban patterns. The cultural factors may also have an effect on the amount of villager’s sun-light exposure because of the types of their clothing which cover their arms and legs for all seasons.
The effect of body coverage on circulating vitamin D is so important that in sunny countries such as Saudi Arabia and the United Arab Emirates, a high prevalence of vitamin D deficiency could be found; likewise, in Iranian sunny cities such as Zahedan and Isfahan, a high vitamin D deficiency have been reported. [23, 24] [25, 26].
In another aspect, the effect of sun exposure on vitamin D serum levels could be ascribed by its seasonal patterns of sun-light exposure; in winter, we can expect to obtain less amount of vitamin D by decreasing in sun-light exposure [26]. In the current study, all the serum samples were obtained during winter; therefore, the effect of seasonal patterns could not be evaluated. However, it seems that there is a complex interaction between the effect of season and bio-cultural factors.
Surprising, the mean serum vitamin D was highest in winter and lowest in the summer in a sunny country like the United Arab Emirates [23]. The climate of the United Arab Emirate is very similar to Bushehr province in the northern parts of the Persian Gulf. It could be postulated that the high temperature of these places induces people live most of their times indoor to escape the hot condition during summer times. Hence, they receive less amounts of sun-light exposure, leading to the lowest range of serum vitamin D levels during summer. In order to elucidate the complex interaction of cultural, and bio-environmental factors on circulate vitamin D levels, more studies during different seasons are warranted.
In this study, an association between vitamin D serum levels and age was found. It was interesting that the minimum serum levels of vitamin D was observed in 30 to 39 years old age group; and surprisingly the highest levels of vitamin D were found among rural subjects who had more than 80 years old. In an Iranian meta-analysis the highest prevalence for vitamin D deficiency was found among the 20 to 50 years old age group [15]. In contrast, in a previous study, children and elder people had the highest prevalence of vitamin D deficiency [27]. In Iran, due to not including older people in the previous population based studies about the prevalence of vitamin D deficiency, a comparison would not be possible [14].
The consumption of multivitamins and vitamin D pills by the elderly may lead to a lower prevalence of vitamin D in this age group in comparison to the younger participant in our study. Another contributing factor for this difference may be the changes of life styles of younger people that induce them to choose living and working indoor places with resulting in less sun-light exposure. In consistent with our finding, a positive correlation between age and vitamin D serum levels was found in Zahedan city (the capital of Sistan , and Balochestan province in the south east of Iran) [14, 15]. Likewise, the younger age group had a higher prevalence of vitamin D deficiency than older age group in Isfahan city ( the capital of Isfahan province in the center of Iran) [26]. Taken together, the change of lifestyle patterns and the trend of younger people to stay in indoor places, and living in apartments may explain this difference for prevalence of vitamin D deficiency among different age groups.
In our study, the prevalence of vitamin D deficiency was more common in men than women. The results of two meta-analyses studies from Iran showed that the prevalence of vitamin D deficiency, like other Asian countries, was more common in women than men [14, 15]. In a trend prevalence study of vitamin D deviancy during 1990-2010 in Iran, it was also reported that women gained a higher vitamin D deficiency than men year-over-year[28].
There are contradictory results about vitamin D deficiency in relation to sex in the world. In contrast to a previous study in America, no difference could be found between sex groups in relation to vitamin D deficiency in NHANES 2001-2004 [29]; but, in a later study in America , women had a higher vitamin D levels than men [30]. In Africa, the prevalence of vitamin D deficiency was higher in women than men [18]. In the United Arab Emirates (UAE), the prevalence of vitamin D deficiency was similar in both sexes [23]. It has been suggested that cultural and religious factors might be the causative factors for the observed higher prevalence of vitamin D among Muslims women; for instance, in Lebanon, the Muslim women had lower vitamin D serum levels than the Christian women [31]. These contradictory results indicate that other contributing factors beyond veiling should be considered to explain this difference.
Obesity is another condition that has a connection both to the patterns of life style and vitamin D serum levels [32]. The body fat content has a reverse correlation with vitamin D concentration. This inverse correlation may be due to the decrease bio-availability of vitamin D3 from dietary sources and skin because of the deposition of vitamin D in body fat compartments [33]; even the elder subjects with high body fat and higher body mass index have lower levels of 25-hydroxy vitamin D levels [34]. Other factors contribute to the inverse relationship between body mass index and vitamin D levels, such as decreased mobility in obese people, which reduces their exposure to sunlight [32].
In our study, men with vitamin D deficiency had a higher anthropometric index (BMI) than men with sufficient vitamin D levels, while no difference was observed between the two groups in women; therefore, other contributing factors beyond obesity should be considered that modify the effect of obesity, such as intake of vitamin D supplements and the number of pregnancies.
Although the current study is one of the largest studies that investigated vitamin D deficiency in the Iranian rural subjects, it has some limitations. One of these limitations is non-repetition of measurements for vitamin D levels during different seasons. The physical activity, and their smoking status of the participants were not also assessed. However, the evaluation of nutritional status as well as sun exposure which were addressed in this study could be ascribed as one of its strengths.
Another strength of the study is its design so that the effect of the latitude (mountainous, plain and coastal places) of living areas on the level of vitamin D could be evaluated.
In this study, the northern half part of the rural mountainous area of Bushehr province had a higher mean of vitamin D level than the plains and coastal areas. There was no significant difference in daily consumption of dairy, and milk products between mountainous area, and other parts this province.