Abstract

Background and objectives: Recent publications have reported alarming prevalence of hypovitaminosis D in South Asian countries including Bangladesh. But, data on vitamin D levels in different occupational groups are lacking. This study addressed the prevalence of hypovitaminosis D in different occupational groups of Bangladesh. Additionally, the study estimated parathyroid hormone, phosphate, calcium and metabolic syndrome in these groups to see the effect of hypovitaminosis D on these parameters.

Materials and method: Seven diverse occupational groups (agrarian workers, rickshaw-pullers, young cricketers and footballers, fishermen, dry fish industry workers, garment-workers and medical students) of Bangladesh were selected based on grade of physical activity and level of sun exposure. Blood was collected for the estimation of 25(OH) vitamin D, fasting glucose,lipid profiles, calcium, phosphate, magnesium and intact parathyroid (iPTH) hormone. Multiple comparisons of these variables among the 7 groups were estimated by ANOVA.

Results: A total of 785 (m / f = 359 / 426) participants volunteered. Of them, 54.2% had vitamin D deficiency. Metabolic syndrome was 5% and showed no significant association with hypovitaminosis D (x2 = 0.9, p=0.43). For biophysical characteristics, the mean (±SD) values of age, body mass index, waist to hip ratio and waist to height ratio were – 33.8±16.3y, 22.3±4.1 kg/m2, 0.87±0.06 and 0.39±0.16, respectively. The values for vitamin D (ng/ml), calcium (mg/dl), iPTH (pgm/ml) and phosphate (mg/dl) were 20.25±13.1, 9.57±1.85, 38.22±24.54 and 4.18±0.81, respectively. The comparisons of vitamin D and other related variables among the groups (ANOVA) showed vitamin D level in the garments worker was significantly (p<0.01) higher from other 6 groups. Likewise, compared with other six, rickshaw-pullers had significantly higher calcium level. Calcium, phosphate and parathyroid hormone did not show any change with decreasing vitamin D level (high to low quartile: Q4→Q1), though parathyroid hormone increased significantly at the lowest vitamin D level (Q1:<11.8ng/ml: p=0.002).

Conclusion: The prevalence of hypovitaminosis D was high irrespective of occupations, site (rural/urban), social class and sun-exposure. Overall, vitamin D level was low though varied among the groups. Despite minimum and maximum sun-exposure, the garments workers had the highest and the fishermen had the lowest vitamin D levels, respectively. Calcium level was normal in all groups. Calcium, phosphate and parathyroid hormone did not show any changes with decreasing vitamin D, though parathyroid hormone increased significantly when vitamin D decreased to the lowest quartile. The findings indicate that the specific cut off value for vitamin D deficiency needs to be determined for population of a given geographic area.

IMC J Med Sci. 2023; 17(2):001. DOI: https://doi.org/10.55010/imcjms.17.011

*Correspondence: M Abu Sayeed, Department of Community Medicine, Ibrahim Medical College, 1/A, Ibrahim Sarani, Segunbagicha, Dhaka 1000, Bangladesh. Email: [email protected]

Introduction

Vitamin D deficiency (hypovitaminosis D) has become a pandemic with the concerned implications in both skeletal and extra-skeletal health. It is common in South Asian countries and Bangladesh ranks second in the prevalence of vitamin D deficiency. Around 67% of Bangladeshi adults were reported vitamin D deficient [1,2]. In Bangladesh, vitamin D deficiency is common in all age-groups and higher in females [1]. A recent study reported vitamin D deficiency was 71% among adequately sunlight exposed coastal fisherman of Bangladesh [3]. Despite abundant sunshine, the high prevalence of vitamin D deficiency is a mystery in a subtropical country like Bangladesh. Genetic factors affecting dermal synthesis and sun avoiding behavior may be an explanation [4]. Also, all these reported level of vitamin D at which deficiency has been defined is according to the cut off values recommended by the different international bodies.

Synthesis of vitamin D is affected by many factors including geographical location, season, environmental pollution, sunlight exposure time, exposed body surface and skin color [5]. Several factors including lack of knowledge, inadequate sunlight exposure and low intake of vitamin D rich food and disease conditions were identified as risk factors of vitamin D deficiency in Bangladeshi people [1]. Apart from these, different socio-demographic factors such as old age, female sex, low socio-economic status, urban residence and indoor occupation may be responsible for the low vitamin D level in our population [1,6,7].

The level of vitamin D at which deficiency has been defined is still a debatable issue. The optimal cut-off of vitamin D was determined by several factors including suppression of parathyroid hormone, calcium absorption, markers of bone formation and resorption, bone mineral density, osteomalacia and rickets. The Institute of Medicine (IOM) proposed 20 ng/ml is an optimal cut off level for vitamin D deficiency [8]. Evaluation of bone markers also showed similar cut-off [9]. However, several societies suggest ≤30 ng/ml as an optimal level of risk for vitamin D deficiency [10,11].The plateau of parathyroid hormone is reached at various Vit D levels. Some studies failed to find a relation between parathyroid hormone and vitamin D [9]. Similarly, two studies conducted among Bangladeshi adults found 15.2 ng/ml in female garment workers and 30.1 ng/ml in apparently healthy population, as the minimum Vitamin D level required for suppressing parathyroid hormone [12,13].

There are several studies on vitamin D with conflicting results, especially regarding the optimum level of vitamin D that is required to maintain bone health. Limited data are available regarding vitamin D status with its association with calcium and parathyroid hormone in different occupational groups of Bangladesh. Therefore, this study was undertaken to measure vitamin D, parathyroid hormone and calcium levels in different occupational groups of Bangladesh and to see the relation between them.

Materials and methods

The study was approved by the Institutional Ethical Review Board and conducted from May 2018 to July 2021.

Study design: Seven occupational groups / workers were selected. The selection was based on grading of (a) physical activities ranging from sedentary to streneous, and (b) exposure to sun from none to heavy. Thus, seven occupations cosidered were: agrarian workers and rickshaw-pullers (moderate sun-exposure with moderate to heavy physical activites, one was rural and the other was urban), garment-workers, and medical students (both sedentary and least sun-exposure and urban), young cricketers and footballers (YCF) from a training institute for athletics and sports (moderate to heavy sun-exposure and physical activities), fishermen and dry fish industry workers (DFIW) both groups had sun-exposure of 4 to 8 hours everyday with moderate to heavy physical activities.

For the agrarian workers, five villages were purposively selected in Nandail sub-district of Mymensingh district about 100 Km north-east of Dhaka city. Gament-workers and rickshaw-pullers were selected from Dhaka City. The medical students of Ibrahim medical college (IMC) in Dhaka City actively participated when they were briefed about the objectives of the protocol. The young cricketers and footballers (YCF) from Bangladesh Krira Shikkha Protistan (BKSP), an athlete and sports training institute in Dhaka volunteered. Likewise, the fishermen and dry fish industry workers agreed to volunteer when discussed with the fishermen’s (motsojibi) union of the area.

The investigations included - a) socio-economic history, b) clinical history, c) anthropometry (height, weight) d) and estimation of 25-hydroxyvitamin D [25(OH)D] and other biochemical tests namely fasting blood glucose (FBG), lipid profiles, intact parathyroid hormone (iPTH), calcium, phosphate, magnesium, alkaline phosphatase as mentioned in the Figure-1. Algorithm of the study protocol is shown in Figure-1.

Figure-1: Algorithm of the study protocol. FBG: fasting blood glucose, iPTH: intact parathyroid hormone

For each occupational group the willing participants were enlisted on the day before investigation and were informed about the objectives and procedural details of the study. They were advised to attend an investigation site at 8 AM in the next morning with an overnight fast.

On the investigation day, each participant was interviewed on socio-economic and clinical history. Height, weight and blood pressure were measured by standard procedures. Body mass index was calculated with the formula (BMI= weight in kg ÷ height in meter2). About 5 ml of blood was collected aseptically from each participant. Collected blood samples were centrifuged and sera were separated in different aliquots, which were frozen locally and transported in coldbox to biochemistry laboratory for analysis.

The measurements of plasma glucose were done by glucose oxidase- peroxidase method using Technicon M-II auto analyzer. Lipids namely triglyceride, (TG), cholesterol (Chol), high density lipid (HDL) and low density lipid (LDL) were estimated by Hitachi-704 auto-analyzer using enzymatic method. LDL-cholesterol was measured using formula: LDL-C = 0.9 TC- (0.9 TG/5)-28 [14]. Serum 25-hydroxyvitamin D [25(OH)D] was measured by enzyme linked immunosorbent assay (ELISA). Serum iPTH, calcium, albumin, phosphate and magnesium were measured by chemiluminescent enzyme-labeled immunometric assay with Immulite 2000 systems Siemens, USA analyzer. Corrected calcium was calculated from fasting calcium and albumin by using correction formula {corrected calcium (mg/dl) = measured calcium (mg/dl) + 0.8 × (4 –measured albumin in gm/ dl)}.

Diagnostic criteria: Diagnostic cut-off for hypovitaminosis D (or vitamin D deficiency) was <20 ng/ml [8,9] and metabolic syndrome was a constellation of BMI >22.3, SBP >114mmHg, FBG >5.5 mmol/l and TG >165 mg/dl [15].

Statistical analysis: The biophysical characteristics of the seven occupational groups were depicted in mean with standard deviation (SD) and 95% confidence interval (CI). The prevalence rates of vitamin D deficiency of the seven study groups by sex were given in percentages. The characteristics of participants were compared between with and without vitamin D deficiency (vitamin D<20 vs. ≥20 ng/ml) and were estimated by unpaired t-test. Multiple comparisons of variables among different groups were estimated by ANOVA with Scheffe’s Post hoc test.

Results

A total of 785 (m / f = 359 / 426) individuals were enrolled in the study (Table-1a). Of the total 785 participants, 424 (54%) had vitamin D deficiency. Compared to males, females had significantly higher prevalence of hypovitaminosis D (m / f = 43.7% / 62.8%, x2 = 28.1, p<0.001). The overall prevalence of metabolic syndrome (MetS) was 5% and not related to hypovitaminosis D (x2 = 0.9, p=0.43 NS; Table 1b). Prevalence of metabolic syndrome was highest in fishermen (12.2%) and lowest in DFI workers [0%; (Table-1c)]. Of them, 45% were non-affluent and 40% were illiterate (data not shown). Regular sun-exposure was found in 41.5%. Agrarian workers, fishermen and young cricketers/footballers had the highest rates of frequent and regular sun exposure.

Table-1a: Prevalence of hypovitaminosis D (Vitamin D <20ng/ml) by gender of the participants (N=785)

Table-1b: Prevalence of metabolic syndrome among the study population having normal (≥20 ng/ml) and deficient (<20ng/ml) vitamin D levels

Table-1c: Prevalence of metabolic syndrome by occupations (N=785)

The characteristics of the study participants are shown in Table 2a, 2b, 2c and 2d. The mean values (±SD) and 95% CI of age, BMI, waist to hip ratio (WHR), waist to height ratio (WHtR) are shown in Table-2a; systolic and diastolic blood pressure (SBP, DBP) and FBG inTable-2b, lipids in Table-2c and vitamin D, iPTH, calcium, ALP, Mg in Table-2d.The mean (±SD) of age, BMI, WHR, SBP, FBG, Chol and HDL were 33.8 (±16.3)y, 22.3 (±4.1) kg/m2, 0.87 (±0.06), 113.6 (±18.2) mmHg, 5.5 (±1.7) mmol/L, 158 (±43.8) mg/dl and 49.1(±8.5) mg/dl, respectively. The mean (±SD) of vitamin D was 20.25 (±13.1) ng/ml) and iPTH was 38.22 (±24.5)  pg/ml), calcium 9.57 (±1.85) mg/dl), phosphate 4.18 (±0.81) mg/dl and magnesium 1.82 (±0.88) mg/dl.

Table-2a: Mean (±SD and 95% CI) values of biophysical characteristics (age, BMI, WHR and WHtR) of the seven occupational groups.

Table-2b: Mean (± SD and 95% CI) values of biophysical and biochemical characteristics (SBP, DBP and FBG) of the seven occupational groups.

Table-2c: Mean (±SD and 95% CI) values of biochemical characteristics (chol, HDL, LDL and TG) of the seven occupational groups.

Table-2d: Mean (±SD and 95% CI) values of Vitamin D, serum calcium, iPTH, phosphate, ALP and magnesium level of the seven occupational groups.

The prevalence of hypovitaminosis D (<20ng/ml) according to occupational groups is shown in Table-3a. Regarding occupation, highest prevalence of hypovitaminosis D was found in DFIW (77%) followed by medical students (72.9%), fishermen (71.6%), YCF (69.9%), rickshaw-puller (42.5%) and lowest in garment workers (23.0%).

Table-3a: Prevalence of hypovitaminosis D (vitamin D <20ng/ml) according to occupational groups

Table-3b depicts prevalence of hypovitaminosis D (<20ng/ml) among the male and female of different occupational groups. Prevalence of hypovitaminosis D was significantly (p<0.05) high among the females compared to males of all occupational groups except medical students (male vs. female: 44.3% vs. 55.7%).

Table-3b: The prevalence of vitamin D deficiency (<20ng/dl) according to gender among different occupational groups

Multiple comparisons of mean (±SD) of vitamin D and calcium levels among the seven occupational groups were estimated by One-Way ANOVA and Post-Hoc tests (Table-4a and 4b). The observed estimated figures are self-explanatory. The source ‘I” denotes an occupation to which other six occupations “J” are compared. As shown in Table 4a, the agrarian workers (I) had significantly lower vitamin D level than garment workers (J), (p =0.002); whereas significantly higher than YCF (p=0.014), fishermen and DFIW (both p = 0.002).

Table-4a: Multiple comparisons of means of vitamin D levels among the seven occupational groups using One-way ANOVA: Post hoc Scheffe tests. The occupational group [‘I’] is compared with the others [‘J’]

Table-4b: Multiple Comparisons of vitamin D and serum calcium levels among the seven occupational groups using One-way ANOVA: Post hoc Scheffe tests. The occupational group [‘I’] is compared with the others [‘J’]

Figure-2 shows the comparisons of vitamin D related variables (calcium and iPTH) among the three occupational groups. Vitamin D and iPTH varied strikingly but calcium did not, rather maintained a consistent level.

Figure-2: Comparative mean (±SE) values of vitamin D (Vit D), calcium (Cal) and parathyroid hormone (iPTH) of agrarian workers (AW), rickshaw-pullers (RP) and medical students (MS).

A line graph (Figure-3) was constructed according to quartiles of vitamin D (Q1 - 4) to determine whether the mean values of iPTH, calcium, phosphate and alkaline phosphatase show any variation with increasing quartile of vitamin D levels and estimated by ANOVA . Serum calcium and phosphate showed no change with the changed vitamin D levels. Only iPTH showed significant difference between Q1 and Q4 of Vitamin D (48.0 vs. 27.7 ng/ml, p=0.002), higher being in the lowest than in the highest quartile. PTH showed significant increase when vitamin D decreased extremely (<11.8 ng/ml: p=0.002). There was a significant weak negative correlation between vitamin D and iPTH. Simple linear regression with iPTH as dependent variable showed a significant association with vitamin D (β=-0.608, p<0.001, 95% CI -0.902 to -0.295, R2=7.1). When vitamin D decreased by 1 ng/ml, iPTH increased by 0.608 pg/ml.

Figure-3: The mean values of iPTH, calcium, phosphate and alkaline phosphatase are shown according to quartiles of vitamin D (Q1<11.8, Q2 11.8 – 17.9, Q3 18 – 24.9 and Q4>25.0 ng/ml), estimated by ANOVA.

Discussion

This study was unique considering the inclusions of several occupations that are distinctively different from one another, each with their own entity and characteristics. For example, at one hand there was the non-affluent rickshaw-pullers who were urban dwellers and heavily exposed to the sun doing strenuous physical activities (BMI=19.0); and on the other hand there was the affluent medical students who were also urban, but rarely exposed to the sun and doing minimum physical activities (BMI=25.5). Thus, each group differed from the other with respect to site (urban/ rural), social class (affluent/non-affluent), grading of sun-exposure (maximum/moderate/minimum) and physical activity (strenuous/moderate/sedentary).The observed biophysical characteristics of different groups (Table 2a-2d) also proved such differences.

There was a high rate of vitamin D deficiency in the study population, with no association with metabolic syndrome. Surprisingly, the highest rate of vitamin D deficiency was seen in occupations with maximum sun exposure. Despite the low levels of vitamin D, iPTH, calcium and phosphate were in the normal range. There was a weak inverse relation between vitamin D and iPTH, which became more apparent below a vitamin D level of 11 ng/ml. Several studies opined in favor of “association between hypovitaminosis D and the metabolic syndrome, its component factors, cardiovascular disease (CVD) and mortality” [16,17]. However, we found no association between vitamin D deficiency and metabolic syndrome (Table-1b), nor was there any correlation with component factors (correlation matrix not shown).

Overall, more than half of the participants had hypovitaminosis D, which is consistent with other South Asian studies [1,-3,7,12,18]. Some unexpected findings were encountered. It is expected that individuals with maximum sun-exposed occupations should have the lowest prevalence of vitamin D deficiency. On the contrary, the garment workers who had minimum sun-exposure had the lowest prevalence (23%) of vitamin D deficiency compared to maximum sun exposed (≥8h/d) occupations – DFIW (77%) and fishermen (71%) (Table3a). Furthermore, the least sun-exposed (garments workers) had the highest vitamin D level, which differed significantly from other groups (Table -4a).Vitamin D level depends on genetic, epigenetic and environmental factors [4]. As the population belonged to low socioeconomic class, poor nutrition may have contributed to the low levels. Bangladesh is a tropical country with abundant sunlight (23.6850° N, 90.3563° E). Lowest level of vitamin D (13.7±7.8ng/ml) was observed in fishermen despite abundant sun exposure. A study in Hawaii also showed that 51% of the population with mean sun exposure of 28.9 hours/week had vitamin D deficiency [18]. Sun exposure does not increase vitamin D above 60 ng/ml. Authors believe that the skin may restrict production of vitamin D in response to excess sun exposure [19]. Possible mechanisms include decreased production, enhanced breakdown, decreased transport of vitamin D in the skin and increased melanin production [18]. In addition to environmental factors, there is an influence of genetic polymorphism on serum 25(OH)D and 1,25(OH) vitamin D levels. Several steps of vitamin D metabolism are under genetic control [4]. Although the genetic influence of vitamin D is still poorly understood, family studies in different populations have found that genetic factors contribute to 70% of the variation in serum vitamin D level [20]. Genetic polymorphisms arising from evolutionary responses to the environment may explain different levels of vitamin D in different populations.

The other uncommon finding – despite low vitamin D level, serum iPTH, calcium and phosphate levels were in the normal range. Usually, iPTH maintains inverse association with vitamin D. In this study, inverse relation was found only at extremely low vitamin D level (<11.8ng/ml), when iPTH increased significantly [Figure-3]., We found iPTH did not increase with decreasing vitamin D till it reached <11.8ng/ml, after which iPTH increased significantly. Possibly, this rise was inevitable to maintain dynamic calcium-phosphate homeostasis. There are many studies looking at the relationship between vitamin D and parathyroid hormone. However, they had controversial results. Not all studies found a definite level of vitamin D at which iPTH level increased [9]. However, some of them demonstrated that iPTH reached a plateau below a vitamin D of 30 ng/ml [5,21]. The threshold of vitamin D below which markers of bone resorption and formation start to increase was only 18 ng/dl [9]. A study on 200 young Bangladeshi female garments workers found that iPTH level increased below a vitamin D cut off of 15.22 ng/ml [12]. Another similar study in 130 healthy Bangladeshi adults with a mean age of 37 years showed a cut off of 27.55 ng/ml [13]. Reciprocal association between vitamin D and iPTH may not be simple. Some unexplored determinants might influence calcium-phosphate-magnesium homeostasis. Phosphate homeostasis is under direct influence of calcitriol, iPTH, and phosphatonins, including fibroblast growth factor 23 (FGF-23). Receptors of vitamin D, FGF-23, iPTH, and calcium-sensing receptor (CaSR) also play an important role in phosphate homeostasis [17,22]. It is now clear that there is interplay of FGF-23, Klotho and parathyroid hormone on the calcium and phosphate homeostasis [23,24].

Regarding limitations of the study, we could not ascertain drug history (anyone taking vitamin D or other micronutrients), dietary habits (fat-deficient), steatorrhea and presence of inflammatory bowel syndrome (IBS). Had there been dual energy X-ray absorptiometry (DEXA scan) we could have shown association between vitamin D deficiency and osteopenia. We could not also investigate iPTH, phosphate, and magnesium for all participants.

Conclusions

We conclude that the prevalence of hypovitaminosis D in the study population was high and was not related to metabolic syndrome (obesity, hyperglycemia, hypertension, dyslipidemia). It was also revealed that sun-exposure had insignificant effect on vitamin D level. Calcium and phosphate showed no association with vitamin D. Also, parathyroid hormone and vitamin D levels showed no significant association except at the lowest quartile of the vitamin D level. Despite very low vitamin D level, the participants were found physically active and mentally healthy with respect to their occupations. We may assume ‘hypovitaminosis D’ is not the only player in maintaining electrolytes and health. So, our findings reasonably demand a careful evaluation of the existing cut-offs values for hypovitaminosis D based on and including other regulatory substances or secretions namely FGF-23, Klotho, osteocalcin and phosphatonins.

Acknowledgments

We are grateful to all those who volunteered the study. We are indebted to village union council members for giving the venue of investigation of agrarian workers at the local union council office. We are indebted to “Dressmen” Garments authority and the volunteers for their active participation. Motsojibi (fishermen) union leaders helped organizing the investigation at their local office. The owners of the dry-fish industries assisted to get the study done with their labors’ cooperation. We are also thankful to the students of Ibrahim Medical College. The Rickshaw-pullers union leaders kindly provided us the rickshaw-garage and making all arrangements to accomplish the study. We appreciate all out support and active cooperation including laboratory facilities of the Ibrahim Medical College authority.

Fund

We shall remain obliged to the Government of the People’s Republic of Bangladesh, Ministry Of Education and Bangladesh Bureau of Educational Information & Statistics (BANBEIS) for funding the project.

Author’s contributions

TH: designing of the study, wrote the manuscript and analyzed the data; MSM: wrote the introduction of the manuscript; NT, MM, HM, AB, MMHC, KNH, MMT: involved in designing of the study, data collection, organization, computing, editing, and assisting reviewing literatures and laboratory assay; MAS: involved in designing of the study, data collection, data analysis and editing manuscript,

Competing interest

The authors declare no conflict of interest.

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Cite this article as:

 HaqT, TomalikaN, MohsenaM, MomtazH, BanuA, ChowdhuryMMH, HashemKN, TagarMM, MorshedMS, SayeedMA. Vitamin D levels in seven non-identical occupational groups entail redefining of existing vitamin D deficiency diagnostic cut off level for native Bangladeshi population. IMC J Med Sci. 2023; 17(2): 001. DOI: https://doi.org/10.55010/imcjms.17.011