Stepan Melnyk, Teresa Evans, Soheila Korourian and Reza Hakkak
The obesity epidemic in the US has continued for over two decades as the proportion of overweight and obese adults in the population continues to rise. Also, obesity has been linked with the risk of development of various diseases such as diabetes, cardiovascular disease and certain types of cancers. There are conflicting reports about the effects of obesity on serum vitamin D levels in human and animal models. We hypothesize that obesity will affect the serum levels of vitamin D metabolites. Therefore, the objective of this study was to investigate the influence of obesity on the serum concentrations of two metabolites of vitamin D [25-(OH)-D and 1,25-(OH)2-D] in rats. Sixteen 5-week-old female Zucker rats (8 obese fa/fa and 8 lean) were acclimated for one week, and at the age of 42 days, the rats were housed 2 per cage with ad libitum access to water and AIN-93G diet. Rats were weighed twice weekly. At the end of the experiment (8 weeks), all rats were sacrificed and serum was collected and stored at −20°C. Serum concentrations of 25-OH-D and 1,25-(OH)2-D were measured using HPLC-UV. Data were analyzed using Excel software and presented as mean ± SD. Obese rats had significant weight gain (P<0.001). Serum concentration of 25-(OH)-D metabolite in obese rats was significantly (P<0.05) lower compared to lean rats. At the same time, serum concentration of 1,25-(OH)2-D metabolite in obese rats was only 8% lower and did not significantly (P<0.3) change compared to the lean group. The serum ratio of 1,25-(OH)2-D:25-(OH)-D was approximately 10% higher (P<0.3) in obese rats compared to the lean group. In summary, lower serum concentration of 25-OH-D (hormonally inactive form of vitamin D) metabolite is consistent with higher body mass in rats, but obesity did not affect the serum concentration of 1,25-(OH)2-D. Our results show that the obese Zucker rat can be a good model for assessing vitamin D status.
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