Hypovitaminosis d: are medical students at risk?

BACKGROUND: Vitamin D deficiency is a pandemic problem mostly diagnosed in elderly. Few studies are available exclusively done on the topic among young adults. Specific professions such as medical students may have higher risk for developing hypovitaminosis D. We aimed to assess the vitamin D status in medical students of Iran University of Medical Sciences; and to define a cut-off point for 25-hydroxyvitamin-D (25(OH)D) level based on secondary hyperparathyroidism.
METHODS: This was a cross-sectional study on 100 medical students conducted during October 2012. Serum 25(OH)D, intact parathyroid hormone (iPTH), and calcium were measured. Age, sex, body mass index, daily dietary fish and egg consumption, sun exposure, and sunscreen usage were recorded. The association between serum 25(OH)D and iPTH was assessed. Receiver operating characteristics curve analysis was performed.
RESULTS: 25-hydroxyvitamin-D level was <30 ng/ml in 99% of all participants, and <20 ng/ml in 77%. Mean serum 25(OH)D level was 16.8 ± 4.7 ng/ml. iPTH level in the group with 25(OH)D level of <10 ng/ml was significantly higher than in those with serum 25(OH)D level of 10 to <20 ng/ml and 20 to <30 ng/ml (109 ± 47 pg/ml, 47 ± 27 pg/ml and 46 ± 19 pg/ml, respectively; P = 0.0001). There was a significant linear inverse correlation between serum iPTH and 25(OH)D (r = -0.36, P = 0.0001). 25(OH)D level of 15.4 ng/ml was determined as the optimal cut-off point in detecting possible secondary hyperparathyroidism.
CONCLUSIONS: To improve the community vitamin D status, in addition to population-based food fortification programs, educational programs seem essential; not only for general population, but also for the more educated groups.

INTRODUCTION

Vitamin D deficiency is a common and important worldwide medical problem, which causes rickets in children and may precipitate or exacerbate musculoskeletal pain, fibromyalgia, osteopenia, osteoporosis, and fractures in adults. It has been associated with increased risk of common cancers, autoimmune diseases, hypertension, infectious diseases, and even depression.[123456] Besides inadequate exposure to sun light as the major cause, wearing sunscreen and using natural vitamin D depleted foods are other important factors of the deficiency. Even vitamin D food-fortifying programs may be inadequate to satisfy vitamin D requirements and based on recent studies the prevalence of the problem is high and increasing in sunny countries including Saudi Arabia, United Arab Emirates, India, and Australia.[78910] Fortunately this important health problem may be preventable and if occurred, simply treated.[1112]

Definition of desirable vitamin D level is still a matter of debate. Using serum 25-hydroxyvitamin-D (25(OH)D) level, measured by a reliable assay, is the standard test for vitamin D status. Many experts currently classify serum 25(OH)D as three different categories: Deficiency, a 25(OH)D of <20 ng/ml; insufficiency, a 25(OH)D of 20-29 ng/ml; and sufficiency, a 25(OH)D of 30-100 ng/ml;[1113] still the Institute of Medicine insists on a 25(OH)D level of >20 ng/ml as sufficiency, mainly based upon evidence related to bone health.[14] There are different ways to set cut-off points for vitamin D deficiency. Optimal vitamin D stores are necessary for achieving peak bone mass, which is usually attained at about third decade of life. It is also well-known that serum parathyroid hormone (PTH) and intestinal and urinary calcium absorption play an important role in getting optimal bone mass. Considering the inverse relation between 25(OH)D and serum intact PTH (iPTH), at least from the bone health point of view, it seems logical to accept the level of serum 25(OH)D that would be enough for maintaining PTH at normal level, as a probable suitable cut-off value for vitamin D deficiency.[15]

Vitamin D insufficiency is mostly diagnosed in elderly, in residents of nursing homes, and in hospitalized individuals.[16] Young adults especially in countries with cultural background that preclude adequate sun exposure, like veil wearing, may be even at more risk for vitamin D deficiency.[1718] In contrast to many studies existing on vitamin D deficiency in adults aged over 65, there are few studies published on the vitamin D situation among young adults who are still at critical age for developing peak bone. Considering presence of multiple risk factors, specific professions may have higher risk for developing hypovitaminosis D. Based on some recent reports medical professionals seem to be such a subgroup.[192021222324]

This study was designed to determine the vitamin D status in young population of medical students. It also examined the relation between their serum iPTH and 25(OH)D, and aimed to define a cut-off level of vitamin D deficiency based on secondary hyperparathyroidism.

METHODS

Participants

This cross-sectional study was conducted on 100 medical students of Iran University of Medical Sciences (IUMS), Firouzgar General Hospital, Tehran, Iran, during October 2012, and finally analyzed in January 2013. Eligible participants were all medical students aged 20-30 years; but were excluded if they had history of liver or renal diseases, metabolic bone diseases, hypercortisolism, active malignancy, clinically apparent malabsorption syndrome, using drugs known to affect vitamin D metabolism (anticonvulsants, glucocorticoids) or receiving any form of supplements containing calcium or vitamin D in the last 3 months before the study. Pregnant and lactating students were also excluded. Volunteers were fully informed about the study, and written consent was obtained. The study was approved by the IUMS Research Committee.

Methodology

Clinical assessment

Age, sex, height (cm), weight (kg), and body mass index (BMI) (kg/m2) were recorded for all participants. A checklist was used to assess daily dietary fish and egg consumption, sun exposure, and also sunscreen usage.

Biochemical assessments

Fasting serum calcium, albumin, iPTH, and 25(OH)D levels were assessed. Serum samples were stored at −20°C until final analysis. All of the analyses were carried out at the laboratory of Firouzgar hospital by trained technicians. Serum 25(OH)D was measured using a commercial radioimmunoassay kit (Immune diagnostic systems, Boldon, UK) according to international instructions. Specificity and sensitivity for 25(OH)D measurements were 100% and 2 ng/ml, respectively. Inter- and intra-assay coefficients of variation were 6.4 and 5.6% at 70 ng/ml, respectively. iPTH was measured by ELISA assay (BIOMERICA, Inc., Hannover, Germany), with sensitivity of 1.6 pg/ml. Inter- and intra-assay coefficients of variation were 3.6 and 3.7%, respectively. Normal range for serum iPTH and albumin adjusted calcium were 10-66.5 pg/ml and 8.2-10.3 mg/dl, respectively. Secondary hyperparathyroidism was defined as serum iPTH level >65 pg/ml. In this study, we categorized participants based on their serum 25(OH)D level as: Group 1 if it was 20 to <30 ng/ml, Group 2 in case of 10 to <20 ng/ml, and Group 3 if it was <10 ng/ml.

Statistical analysis

The data were analyzed using SPSS for windows, version 18 (SPSS Inc., Chicago, IL, USA). Numeric variables were presented as means ± standard deviation. Nominal and categorical variables were illustrated in tables using percentages. The independent sample t-test was used to compare the means across sex variable. Chi-square test was used to compare categorical variables across sex variable. Analysis of variance (ANOVA) was used to test the mean of iPTH across three groups of 25(OH)D level (Groups 1-3). To adjust the effect of serum calcium level on iPTH, Analysis of covariance (ANCOVA) was used to compare mean level of iPTH across three groups of 25(OH)D. The association between 25(OH)D and iPTH was tested by Pearson correlation. The optimal cut-off value of serum 25(OH)D to detect iPTH >65 pg/ml as indicator of secondary hyperparathyroidism was estimated from receiver operating characteristics (ROC) curve. The sensitivity, specificity and Youden index were calculated for the determination of the diagnostic value of each cut-off level.[25] All statistical tests were two-tail, with a significance level of 0.05.

RESULTS

Based on the study criteria 100 healthy students aged 20-30 years old, out of total of 120, were involved in the study (83% response rate). The main reasons of exclusion were nonwilling for participation or taking drugs inconsistent with the study protocol. Basic characteristics were shown in Table 1.

Table 1

Basal characteristics of the 100 participants

Serum values

Mean serum 25(OH)D level was 16.8 ± 4.7 ng/ml (range: 8.8-36 ng/ml) which was significantly higher in males than females (18.6 ± 4.2 vs. 15.6 ± 4.6 ng/ml, respectively; P = 0.001). Mean corrected serum calcium level was 9.7 ± 0.5 mg/dl (range: 8-10.3 mg/dl), which was higher in males. Mean serum iPTH was 51.9 ± 31.4 pg/ml (range: 11-192 pg/ml), with no gender difference. Only 26% of students had serum iPTH level >65 pg/ml [Table 1].

25-hydroxyvitamin-D level was <30 ng/ml in 99% of all participants. The frequency of Groups 1-3 was 20%, 72%, and 7%, respectively [Table 2]. The three groups were similar regarding age, gender, BMI, serum calcium, sun exposure, sunscreen usage, and vitamin D rich dietary use.

Table 2

Characteristics of participants according to their serum 25(OH)D level

There was no significant difference in serum iPTH level between the Group 1 and the other added Groups 2 and 3 (P = 0.36). Using ANOVA, there was significant difference in mean serum iPTH between the three Groups of 1, 2, and 3, which still remained significant after adjustment for the possible effect of serum calcium using ANCOVA (P = 0.03). The highest iPTH level was found in the 3rd group with 25(OH)D level of <10 ng/ml (P = 0.0001); post-hoc analysis using Sheffe test defined that the difference was significant when comparing the 3rd group (109.4 ± 47 pg/ml) with each of the 1st or the 2nd groups (P = 0.0001 and P = 0.0001, respectively). There was no significant difference among mean serum iPTH between Groups 1 and 2 (46 ± 19 vs. 47.1 ± 27 pg/ml, respectively; P = 0.52). Mean corrected serum calcium was similar between the three groups [Table 2].

Correlation between serum intact parathyroid hormone and 25-hydroxyvitamin-D

The relation between serum 25(OH)D and iPTH was assessed using a linear regression analysis. There was a significant linear inverse correlation between serum iPTH and 25(OH)D (r = −0.36, P = 0.0001) [Figure 1]. There was also a significant inverse relation between serum calcium and iPTH level (P = 0.02); so ANCOVA analysis was used to assess the possible confounding effect of serum calcium on the relation between serum iPTH and 25(OH)D (P = 0.03).

Figure 1

The relation between serum 25-hydroxyvitamin-D (25(OH)D) and intact parathyroid hormone (iPTH) concentrations in medical students. Serum levels of 25(OH) D were inversely correlated with iPTH levels (r = −0.36, P = 0.0001)

Cut-off point of serum 25-hydroxyvitamin-D concentration

According to the analysis using the ROC curve and Youden index, the optimal cut-off value of serum 25(OH)D to detect subjects with serum iPTH concentration of >65 pg/ml as indicator of secondary hyperparathyroidism was determined to be 15.4 ng/ml that showed the highest sensitivity, specificity, and Youden index (74%, 69%, and 0.44, respectively) [Table 3]. The area under the curve was 0.77 (confidence interval95% 0.65-0.89, P = 0.0001) [Figure 2]. Applying this cut-off point, 25(OH)D level was >15.4 ng/ml in 73 participants while it was lower than that in the other 37 students.

Table 3

Sensitivity, specificity and Youden index at some values of serum 25 (OH)D

Figure 2

Receiver operating characteristics curve analysis of serum 25-hydroxyvitamin-D for detection of intact parathyroid hormone >65 pgl/ml. Area under the curve was 0.77, P = 0.0001

DISCUSSION

This study confirmed the presence of 25(OH)D level of <30 ng/ml in nearly all 100 healthy medical students of IUMS (Tehran/Iran). Only one student showed the level of >30 ng/ml, while 77% of the study populations had 25(OH)D level of <20 ng/ml. Although the males demonstrated higher mean serum 25(OH)D level, the severity of hypovitaminosis D was equally distributed between both sexes. Independent to calcium effect, a significant linear inverse correlation between serum iPTH and 25(OH)D level was also shown. Noticeable rise in serum iPTH was found only in case of 25(OH)D level of <10 ng/ml in comparison to those with a higher 25(OH)D level. Using ROC curve analysis, serum 25(OH)D level of 15.4 ng/ml was the optimal cut-off value to detect secondary hyperparathyroidism (iPTH >65 pg/ml).

Vitamin D deficiency is rather widespread and common in different parts of the world. Although multiple studies have been published on its prevalence, but the majority have only assessed elderly and housebound individuals;[16] still few reports are available on its epidemiology in young and healthy people, especially from different geographic parts of the world. Skin exposure to ultraviolet B radiation is the most important way in vitamin D synthesis and food should provide the rest of its daily need.[26] Based on these facts, it had been theoretically postulated that the tropical zone (latitudes between 23.5°N and 23.5°S) should be rather free of hypovitaminosis D, and also that the economically developed countries might have better situation regarding vitamin D deficiency compared to the developing ones. Although it is still accepted that the latitude and economy may play a general role in epidemiology of hypovitaminosis D, there are increasing evidence on high prevalence of the deficiency even in the tropical zone or in the developed countries, and also on similarity of the temperate (23.5°-66.5°) to the tropics regarding the problem.[27] Reports from Saudi Arabia and Australia are some examples.[710] Besides the mentioned traditional risk factors; age, obesity, skin pigmentation, and different styles in clothing are being more recognized as the other possible important modulators in vitamin D status.[28293031] However, the diverse results obtained from studies on prevalence of hypovitaminosis D may be at least in part explained by some other minor risk factors. Special professions should be more evaluated in this regard. We aimed to study vitamin D status in medical students not only because of their homogenous young age, but also because of some of their specific professional life situations; they should usually study for long hours in libraries or stay in medical wards with no sun exposure, have long working shifts in closed environments, have unpredictable meal times and may use more high-caloric food with poor nutritional value. The high frequency of hypovitaminosis D found in the current study was in consistence with those of few other studies published before in this specific group. Using 30 ng/ml as the cut-off value, the frequency of hypovitaminosis D in three studies done on medical students or residents from Saudi Arabia, Boston (USA), and Spain were reported as 100, 69, and 60%, respectively;[242023] three other studies including medical residents from India, Brazil, and Portland (USA) used cut-off level of 20 ng/ml and reached 88, 58, and 50% hypovitaminosis D, respectively.[212219] Adding our results to the other available six studies it may be concluded that the geographic region and not the latitude alone, is a minor risk factor for developing hypovitaminosis D. Although it is a worldwide matter, the problem is more prevalent in Eastern countries than in Europe or North and South America. As discussed above, economy did not have a major distinctive role in the frequency of vitamin D deficiency in the mentioned studies. Based on our knowledge, this is the first study done exclusively on young adult Iranian population. High prevalence of vitamin D deficiency has been shown before in a multicenter study in different parts of Iran.[32] Although it had mainly studied middle-aged and older population, Hashemipour et al. found >80% vitamin D deficiency in a heterogeneous age-group, with a significantly more frequency in the younger populations.[33]

In order to define vitamin D normality it is essential to consider all the possible effects of it on the human's health that may be classified as skeletal and nonskeletal actions. Vitamin D plays an inevitable role in attaining and maintaining proper bone architecture so considering the established effect of iPTH in bone physiology, it is rational to consider the serum 25(OH)D level aimed at preventing secondary hyperparathyroidism as one of the criteria of vitamin D normality. During the last decade, many studies have been published on the relation between 25(OH)D and iPTH;[34] although many of them showed an inverse correlation between the two factors, but some few others were unable to find any meaningful association.[35] As discussed earlier the majority of those studies have mainly assessed middle-aged or elderly. The significant inverse correlation found in the current study, r = −0.36, was consistent with those prior positive findings. Four studies have previously assessed the relation between 25(OH)D and iPTH in medical students; the Pearson correlation coefficient (r) was –0.33 and –0.58 in the studies from Brazil and Portland area of USA, respectively.[2219] In the other Indian study, the relation was only significant in males not females, with r = –0.22 and the Saudi Arabian study did not show any significance.[2124] Although 25(OH)D level was <30 ng/ml in 99% of participants, only 26% of our students had secondary hyperparathyroidism; this number was 40% and 45% in Brazilian and Indian studies, respectively.[2221] Using Youden index based on sensitivity and specificity analysis, this study found 25(OH)D level of 15.4 ng/ml as the optimal cut point of possible secondary hyperparathyroidism. None of the other above-mentioned studies done on medical staffs has tried to set the 25(OH)D cut-off point. We, as well, could not find any clear evidence on that cut-off point individually in young adults to compare with ours, but our finding was in agreement with Sai et al., who assessed 70 studies to define a threshold value of 25(OH)D regarding bone health. They concluded that considering all inconsistencies in the literature the cut-off point would be within the wide range of 10-50 ng/ml.[34]

This study carries some limitations. In addition to small sample size and insufficient information on the participant's calcium intake, serum magnesium was not measured. A checklist method was used to collect the data on consumption of seafood and exposure to sunlight rather than using a validated questionnaire. Besides these limitations we tried to give more strength to our study by doing it in a uniform young population, which still has not been fully assessed in the literature.

CONCLUSIONS

Near all of medical students in this study revealed 25(OH)D level of <30 ng/ml. To prevent iPTH rise, serum 25(OH)D level of 15.4 ng/ml was determined as the optimal threshold. Added to few available results on vitamin D status in medical students worldwide, this study more clarified a pitfall. In fact, the future doctors who should provide medical care to the general population in the near future are not really aware of the common, but latent health matter they suffer from, themselves. Further investigations in other professional subgroups may be also advisable to reach a comprehensive insight regarding hypovitaminosis D. To improve the community vitamin D status, in addition to population based food fortification programs, more educational activity seems to be essential to increase awareness about the need for vitamin D supplementation.