An obesity gene expressed in human adipocytes called “agouti” helps to understand the anti-obesity effect of dietary calcium. Agouti protein stimulates calcium influx7,8 and causes energy storage in human adipocytes by stimulating the expression and activity of fatty acid synthase and inhibiting lipolysis; Calcium channel agonists mimicked this action of agouti and it was inhibited by calcium channel antagonists.9,10 Moreover, using a calcium channel antagonist (ex. nifedipine) for four weeks, in transgenic mice over-expressing agouti, resulted in significant decreases in lipogenesis and in adipose tissue mass.11

Polymorphism of Vitamin D receptor gene is associated with susceptibility to obesity in patients with early onset type II diabetes 12, and circulating 1,25-(OH)2-D levels are elevated in obese individuals.13-18

It seems that obesity affects bone metabolism in different ways. Since adipocytes and osteoblasts are derived from a mutual stem cell, Obesity could lead to decrease in bone formation while increases adipogenesis.27,37 It was shown in other studies that agents that inhibit adipogenesis stimulate osteoblast differentiation 28,29 and vice versa.31 There is elevated oxidative stress and increased production of pro-inflammatory cytokines in obese people.1,30 Subsequently, more production of pro-inflammatory cytokines in obese people could stimulate osteoclast activity and then might increase serum calcium level in them32,33 Moreover, osteogenesis or bone resorption might be affected directly by more leptin secretion or a reduction in adiponectin production in obesity.34-36

Based on the results of the previous studies which have not all shown a similar outcome, we are to find the possible correlation between serum calcium, albumin, and triglyceride levels and anthropometrics of the participants in this survey.

Materials and Methods

This cross-sectional study was performed in endocrine and metabolism research center of Shiraz University of Medical Sciences located in Namazi Hospital at 2011. The target population of this study was 468 participants from Shiraz. The essential inclusion criteria were to be a resident of Shiraz city and not having any major disease (e.g. CVA, MI, endocrine diseases). In selection of participants one person from each family was chosen randomly based on 8 areas of Shiraz city considering postal codes (if the last digit was an even number). 100 families were selected from each area which means a total number of 800 people, thereafter the subject of the study was discussed to them by an expert and informed consent forms were filled by the patients. Finally, from a total of 513 cases who were present at Endocrine and Metabolism Research Center, 468 persons had completed profiles of anthropometric information, demographic characteristics, and biochemistry data and they were included as the control group.

In order to record the data of the people in the study a demographic form was designed that included age, sex and anthropometric characteristics. Weight was measured by a digital scale (100 grams accuracy) with light clothing and barefoot; height was measured by a wooden tape-meter with an accuracy of 0.1 cm. Waist circumference was measured to the nearest millimeter and taken midway between the lower limit of the rib cage and iliac crest at the end of normal expiratory phase using a non-stretchable nylon tape in a fasting setting. Hip circumference was measured by a tape-meter in the largest part with no compression. All these data were collected by an expert person. Body Mass Index (BMI) was also calculated.

A blood sample of 5cc was collected from a peripheral vein in forearm. All of the samples were collected between 8 and 9 AM in a fasting condition. Serum total calcium (using Colorimetric kit), albumin (using AssayMax Human Albumin ELISA), and triglyceride (using MaxDiscovery kit) were measured in Shiraz Endocrine and Metabolism Research Center.

Statistical analyses were carried out using SPSS software for Windows, version 19 (SPSS, Chicago, IL, USA). Data were expressed as means ± SD. Statistical significance was determined by using the t-tests and Mac Pearson correlation test. Significant difference was accepted when P-values were less than 0.05.

Results

This cross-sectional study was conducted on 468 participants, randomly selected based on 8 areas of Shiraz city. From the 468 participants 329 (70.3%) were females and 139 (29.7%) were males. Mean age of all of the participants was 46.08 years.

In this study serum albumin and calcium levels were measured and then serum level of ionized calcium was corrected according to serum albumin concentration; the mean ± SD level of the corrected calcium level was 9.88 ± 0.93 mmol. Anthropometric data of the patients and chemical profiles of the participants are shown in table 1.

Data analyses showed that serum albumin level had a direct significant correlation with serum calcium level, weight and height of the participants; while it had a negative correlation with their age and abdominal circumference (P < 0.05) (Table 2).

Serum calcium and corrected serum calcium (adding 0.8 mmol to serum calcium for each 1mg/dl that albumin level is below 4 g/dl) both had a significant direct correlation with BMI, weight, and serum triglyceride level (P < 0.05). Using the t-test in SPSS software we found no significant difference in serum calcium level or corrected calcium level in males and females.

Discussion

The results of this study revealed that serum calcium level of the participants had a significant correlation with their BMI, weight, and serum triglyceride levels. In previous studies it was shown that abnormal calcium metabolism is related to obesity . Similar to this study in some other studies a direct significant correlation between serum ionized calcium level and BMI was mentioned.20-22 Yet some other surveys showed a significant association between PTH and BMI.19,26 In most of the previous investigations obesity and serum levels of vitamin D and PTH were studied and the etiology of reduced levels of 25-hydroxy vitamin D in obese people is not known yet, but is unlikely to be due to malabsorption or inadequate intake of vitamin D. One possibility is that morbid obese patients have less exposure to sunlight than normal subjects. An alternative explanation could be that 25-hydroxy vitamin D is sequestered in fat tissue, so it has less bioavailability. It was further postulated that the increased PTH represents end-organ resistance of the bone to PTH as a consequence of increased skeletal mass. The increased level of 25-hydroxy vitamin D would be secondary to the increased PTH, which may in turn result in negative feedback on 25-hydroxylase, resulting in lower levels of 25-hydroxy vitamin D.

In this study in order to be able to have a look on all the aspects we also measured serum triglyceride level of the participants. Obesity induces an increase in pro-inflammatory cytokines1,30 which leads to osteoclasts activity and bone resorption 31,32 ; subsequently serum calcium level in obese people is higher while at the same time serum triglyceride level is higher in them and this two indicators might be not directly linked to each other.

Studying serum leptin and insulin level on serum calcium in obesity we can have a better conception of the mechanism that alters serum calcium level in obesity. In future studies we could also study the effects of vitamin D and PTH on serum calcium level in a larger group of obese people.

Conclusion

Serum calcium level is significantly higher in obese participants and probably has no relation to high level of triglyceride in obese subjects.

Acknowledgments

Authors wanted to appreciate Dr Soheil Ashkani Esfahani and the scientific writing and editing of the manuscript by scientific writing committee of Sadra-tech™ Company, Shiraz University of Medical Sciences, Shiraz, Iran, and also Ms. Keivanshokooh and Research Improvement Center of Shiraz University of Medical Sciences, Shiraz, Iran.