Journal of Postgraduate Medicine Medknow Publications and Staff Society of Seth GS Medical College and KEM Hospital, Mumbai, India ISSN: 0022-3859 EISSN: 0972-2823 Vol. 56, Num. 2, 2010, pp. 65-70

Journal of Postgraduate Medicine, Vol. 56, No. 2, April-June, 2010, pp. 65-70

Original Article

Study of bone mineral density in resident doctors working at a teaching hospital

Multani SK, Sarathi V, Shivane V, Bandgar TR, Menon PS, Shah NS

Department of Endocrinology, KEM Hospital, Seth G. S. Medical College, Parel, Mumbai-400 012, India

Correspondence Address: Dr. Vijaya Sarathi HA, Department of Endocrinology, KEM Hospital, Seth G. S. Medical College, Parel, Mumbai-400 012, India

drvijayasarathi@gmail.com

Date of Submission: 29-Mar-2009

Date of Decision: 31-Jul-2009

Date of Acceptance: 04-Dec-2009

Code Number: jp10025

PMID: 20622382

DOI: 10.4103/0022-3859.65272

Abstract

Context : The erratic lifestyle of resident doctors may affect their serum 25-hydroxy vitamin D [25-(OH)D] levels and bone mineral density (BMD).

Aim : To study BMD and the effect of environmental factors on it in resident doctors.

Settings and Design : Prospective, cross-sectional study conducted in a tertiary healthcare centre.

Materials and Methods : BMD was obtained by dual-energy X-ray absorptiometry and was correlated with various factors including weight, height, body mass index (BMI), sun exposure, physical activity, parathyroid hormone, 25-(OH)D, dietary factors.

Statistical Analysis : SPSS software Version 10 (Unpaired t test was used to compare BMD of different groups and Pearson's correlation coefficient was used to calculate correlation).

Results : Two hundred and fourteen apparently healthy resident doctors were enrolled in the study. Based on Caucasian normative data, osteopenia was noted in 104 (59.7%) males and 27 (67.5%) females. Thirty-two (18.39%) males and five (12.5%) females had osteoporosis. The BMD values of males were 0.947±0.086, 0.911±0.129 and 1.016±0.133 at lumbar spine, femur neck and total hip while those in females were 0.981±0.092, 0.850±0.101 and 0.957±0.103 respectively. BMD of our cohort was lesser by 12.5-18.2% and 4.2-14.5% than the Caucasian and available Indian figures, respectively. BMD had significant positive correlation with weight, height, BMI, physical activity, and dietary calcium phosphorus ratio. 25-(OH)D levels were insufficient in 175 (87.5%) subjects but had no correlation with BMD.

Conclusions : Young healthy resident doctors had significantly lower BMD, contributors being lower BMI, lower height, reduced bioavailability of dietary calcium and inadequate physical activity. Deficiency of vitamin D did not contribute to low BMD.

Keywords: Bone mineral density, environmental factors, osteopenia, resident doctors

The risk of osteoporosis and hence osteoporotic fractures is related to peak bone mass (PBM) achieved. ^[1] The precise age at which PBM is achieved is still controversial. Two recent Indian studies report PBM to be achieved at an average age of 26-30 years. ^[2],[3] Several factors prevent realization of PBM. Genetic factors account for up to 85% of the variation in bone mass, while environmental factors such as calcium and vitamin D deficiency, poor physical activity and poor sunlight exposure account for the rest. ^[1],[4]

Due to hectic and erratic work schedules, resident doctors are more likely to have poor sun exposure and unhealthy dietary habits. They are also likely to have inadequate physical activity owing to their sedentary lifestyle. These factors may affect their vitamin D status and bone mineral density (BMD). Hence, we undertook a study to determine the BMD and effect of anthropometric parameters, sunlight exposure, physical activity, dietary factors including protein, calcium, and phosphorus intake, and vitamin D status on it in resident doctors at a tertiary teaching hospital in Mumbai.

Materials and Methods

This prospective, cross-sectional study was carried out between May and August 2003 after obtaining the approval of the institutional ethics committee. The research participants were enrolled after obtaining a written informed consent.

Pregnant and lactating women, those with medical disorders that are likely to affect BMD, those who had received steroids, antitubercular or antiepileptic medication within the last two years, those who had fractures within the last two years and those who were on calcium and vitamin D supplements for the last three months were excluded from the study. All subjects were asked for history of smoking (number of cigarettes per week), alcohol consumption (g/d) and history of osteoporosis or non-traumatic fractures in the family. Dietary intake of calcium, phosphorus, calories and protein from all dietary sources was evaluated by 24-h dietary recall. Calculations were based on published food tables detailing the nutritive value of Indian foods. ^[5] Desired dietary calcium to protein ratio was taken as 16 to 20:1 (mg:g) for optimal utilization of dietary calcium, ^[6] while desired dietary calcium to phosphorus ratio was taken as 1:1 for better bioavailability of calcium. ^[7]

Participation in various physical activities since the time of admission in the medical college was recorded. It was assessed using the Global Physical Activity Questionnaire (GPAQ) developed by WHO ( www.who.int/chp/steps ) and expressed as minutes/week.

Total sunlight exposure to the face and the hands (around mid-arm downwards) per day between 8.00 am and 5.00 pm in summer and between 9.00 am and 3.00 pm in winter was calculated by using a questionnaire. ^[8] Use of sunscreens was noted. Sun index was calculated as the product of hours of sun exposure per week and fraction of body surface area exposed to sunlight. Current sunlight exposure was calculated as exposure in one year prior to enrolment in study.

Blood was sampled between 8.00 am and 10.00 am in fasting state for determination of hemoglobin, serum calcium, phosphorus, alkaline phosphatase, albumin, creatinine, 25-hydroxy vitamin D (25-(OH)D) and intact parathyroid hormone (iPTH) levels. Samples were stored at -20ΊC until assayed. Serum calcium, phosphorus, albumin, creatinine, and alkaline phosphatase were measured by semiautoanalyzer. Serum 25-(OH)D was measured by radioimmunoassay (BioSource, Europe S.A., KIPI 1961, Belgium) with intra and inter-assay coefficients of variations of 3.85% and 6.49% respectively. Since there is no available normative data for Indians, subjects with 25-(OH)D concentration ≥20 ng/ml were considered as vitamin D sufficient and < 20ng/ml as insufficient, in this study. Serum iPTH was estimated by two-site binding Immunoradiometric assay (Diagnostic Systems Laboratories (DSL-8001), Webster, Texas, USA.). High iPTH values were defined as> 55 pg/ml (upper limit of normal for assay kit). The intra- and inter-assay coefficient of variations was 2% and 3.92% for iPTH respectively.

Bone mineral density was assessed at lumbar spine and proximal femur by dual-energy X-ray absorptiometry DXA using Hologic inc. USA model Delphi W 70460. The short-term in vivo precision (coefficient of variation %) in our unit was as follows; lumbar spine: 1.09%, femoral neck: 3.29% and total hip: 1.26%. Bone mineral density was expressed as g/cm ² as well as T scores and Z scores using Caucasian normative data. Osteopenia and osteoporosis were defined as per recommendations of The International Society for Clinical Densitometry. ^[9] Absolute BMD of our study population was also compared with previously available Indian data. ^[10]

Statistical analysis was done using SPSS Version 10. Continuous variables were expressed as mean±SD. Correlations between BMD and various factors were calculated by Pearson′s correlation coefficient. Comparison of study population′s BMD to that of Caucasian and available Indian data was done by using independent t test (P< 0.05 was considered to be statistically significant). Power of the overall study is 90% while that for males and females is 90% and 80% respectively.

Results

Two hundred and fourteen (174 males and 40 females, M:F=4.35:1) resident doctors were enrolled in the study. The baseline characteristics of the study population are shown in [Table - 1]. When compared to Caucasian normative database (Manufacturer′s database), only 38 male and eight female resident doctors were found to have normal BMD at all sites. While rest had osteopenia or osteoporosis [Table - 2]. Absolute BMD (g/cm ² ) at various sites is compared with Caucasian and previously available Indian data in [Table - 3] and [Table - 4]. The absolute BMD of resident doctors was lower than that of Caucasians and previously available Indian data at all measured sites. It was lesser by 12.5-18.2% in males and 13.5-17.3% in females than that of Caucasians and by 4.2-12.2% in males and 8.35-13.5% in females than that of available Indian data.

Bone mineral density of study population is correlated with various factors separately in male [Table - 5] and female [Table - 6] resident doctors. Weight and body mass index (BMI) had significant positive correlation with BMD of all measured sites in both groups. Height had significant positive correlation with BMD of all measured sites in males but not with BMD of any site in females. Age had no significant positive correlation with BMD of any site in either sex.

Sunlight exposure showed a decreasing trend over the years after entry into medical profession as seen in [Figure - 1]. Current sunlight exposure/week was lower in both males and females as shown in [Table - 1]. Current sunlight exposure had significant positive correlation with serum 25-(OH)D in males (r=0.18, P=0.02) as well as females (r=0.38, P=0.01) but not with BMD of any measured site in either group.

Physical activity showed decreasing trend over the years after entry into medical profession as seen in [Figure - 2]. In males, both current and average physical activity over previous eight years had significant positive correlation with BMD of femur intertrochanter and lumbar spine. However, no significant correlation was found between physical activity and BMD at any site in females.

As shown in [Table - 1], mean daily calcium, caloric and protein intake of study population was adequate in both males and females. Dietary calcium and phosphorous intake had no significant correlation with BMD of any site. Average dietary calcium to protein and calcium to phosphorus ratio were abnormal in resident doctors. We found positive correlation of calcium to phosphorus ratio with BMD of all measured sites in males and with all except lumbar spine in females. No significant positive correlation was found between calcium to protein ratio and BMD of any site in either sex. Twenty-two and 60 males were occasional smokers and alcoholics respectively and no correlation was found between smoking and alcohol intake with BMD at any site in either group.

Twenty-nine of 214 subjects (13.5%) had family history of osteoporosis (based on DXA scan report or history of osteoporotic fracture) but presence of family history had no significant correlation with BMD of any site in either sex.

As shown in [Table - 1], mean serum calcium (corrected), phosphorus, albumin, and creatinine were normal while mean serum 25-(OH)D was low in both males and females. Vitamin D status of study population is shown in [Figure - 3]. Vitamin D insufficiency was found in 175 (87.5%) subjects when 20 ng/ml was used as the cutoff for defining vitamin D insufficiency. Among males 137 (85%) were vitamin D insufficient while 38 (97.5%) females had vitamin D insufficiency. No correlation was observed between vitamin D status and BMD at any site in either group.

Elevated mean serum iPTH was found in 91(45%) subjects [70 (43.7%) males and 21 (53.84%) females], but no correlation was observed between serum iPTH and BMD of any site. Significant negative correlation was found between serum 25-(OH)D and iPTH levels in males (r=-0.22, P=0.003) but not in females (r=-0.19, P=0.29). As shown in [Figure - 4], serum iPTH started rising at serum 25-(OH)D below 35 ng/ml and there was a steep rise at 25-(OH)D levels < 20 ng/ml.

Discussion

Bone mineral density of our study population was 12.5-18.2% lower than that of the Caucasian population with significantly higher prevalence of osteopenia and osteoporosis which is similar to an earlier study from India (50% osteopenia in healthy Indian men). ^[10] Assessment of BMD in the Indian population by comparing with a reference Caucasian population may have limited validity due to difference in genetic factors and skeletal size. ^[11] In addition, widely prevalent calcium and vitamin D deficiencies may contribute to this difference. ^[4] However, despite high prevalence of both vitamin D insufficiency and low BMD, there was no correlation between them in our study. A recent meta-analysis has shown inconsistent association between vitamin D status and BMD except in old age. ^[12] Low availability of calcium from diet and lesser physical activity of our resident doctors might have contributed to low BMD in them. However, it is difficult to explain high prevalence of low BMD on the basis of its known determinants. Moreover nutritional status, sunlight exposure and physical activity of the study population during childhood and adolescence that may significantly affect BMD, were not assessed in this study.

Bone mineral density of resident doctors was 4.2-13.5% lower than that of previously available Indian data (members of paramilitary forces). ^[10] This difference may be due to lesser physical activity of resident doctors compared to members of paramilitary forces, low availability of calcium from diet and genetic constitution (our study population was predominantly from western and southern India).

Bone mineral density was higher in men than women at all measured sites. However, low BMD was more prevalent in men than women in our study. A previous study involving young, healthy Indians has also shown similar results. ^[10] This paradoxical finding may be due to the fact that younger subjects changing over to adult reference databases may end up with a lower Z-score despite a real increase in BMD. ^[13]

There are no studies on BMD-fracture relationship in Indians (fracture threshold). It has been suggested that if Indians fracture at the same level of BMD as Caucasians, there would be no reason to have separate normative data for Indians. ^[4] However, it appears that low BMD in Indians may get translated into increased risk of fractures in old age because of higher prevalence of osteoporotic fractures in Indians than Caucasians. Moreover, it has been suggested that osteoporotic fractures occur 10-20 years earlier in Indians (peak age: 50-60 years) than Caucasians (70-80 years). ^[14],[15]

Genetic factors contribute up to 85-90% of bone mass; however, no significant correlation was found in our study between family history of osteoporosis and BMD. It is probably due to small number of subjects having positive proven family history of fractures and lack of screening of family members. Hence, low prevalence of family history of osteoporosis or fractures may not rule out genetic contribution to BMD. Contribution of genetic factors to BMD is important since it is a non-modifiable factor despite its highest contribution.

Weight and BMI had highly significant positive correlation with BMD at all sites in either group, making them the most powerful determinants of BMD. Lack of positive correlation of BMD with height in females was most probably due to small sample size.

There was a decreasing trend in physical activity from first year of medical training. Average physical activity was inadequate in resident doctors probably due to lack of time and erratic and hectic work schedule contributing to overall low BMD. Lack of positive correlation of BMD with physical activity in females was likely due to small sample size.

Duration of sunlight exposure is positively correlated with vitamin D status. Most of our study subjects had vitamin D deficiency which may be due to decreased current sunlight exposure resulting from erratic lifestyle during residency period and lack of vitamin D fortified food in India. Increased skin pigmentation, crowded housing with limited sunlight exposure, smog, and the angle of ultraviolet rays may also contribute to vitamin D deficiency.

In our study, total calories, calcium and protein intake did not have significant influence on BMD at any site. Although both dietary calcium to protein and calcium to phosphorous ratio were abnormal in most of our subjects, only the latter had positive correlation with BMD.

Resident doctors may not be representative of the general population. They may have some unknown risk factors that are unique to them. Hence generalization of these findings is difficult. Other limitations of the study include younger population that might have not reached peak BMD and small sample size in individual groups especially in women.

Long-term prospective studies are required to prove whether our findings of low BMD in our study population will directly translate into higher risk of future fracture or not. Further studies are also necessary to elucidate the possible unknown determinants of BMD that might have led to high prevalence of low BMD in resident doctors.

To conclude, young healthy resident doctors in a tertiary referral centre in Mumbai had significantly lower BMD than that of Caucasians and earlier studied healthy Indian population. Besides genetic factors, lower body weight, lower height, lower BMI, decreased bioavailability of calcium from diet, and inadequate physical activity contribute to low BMD in resident doctors but not vitamin D. Vitamin D insufficiency was highly prevalent in resident doctors. It is necessary to draw attention of our resident doctors towards the ill effects of their lifestyle on their BMD and routine supplementation of resident doctors with recommended dietary allowances of vitamin D may thus be recommended.

References

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