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The present study was planned to elucidate the effects imposed by the alteration in serum vitamin D level on adipose tissue metabolism in obese rats.
The study was performed on 32 albino adult female rats, allocated into 4 groups:
group I (Control, C) (n=8):
Rats fed with standard rat diet according to the AIN-93 Purified Diets for Laboratory Rodents (3.766 Kcal/g); containing standard vitamin D dose (1000 IU/kg diet) for 12 weeks.
group II (High Caloric Diet with standard vitamin D, HCD+SVD) (n=8):
Rats fed with high caloric diet (4.771 Kcal/g); containing standard vitamin D dose (1000 IU/kg diet) for 12 weeks.
group III (High Caloric Diet with Low Vitamin D, HCD+LVD) (n=8):
Rats fed with high caloric diet (4.771 Kcal/g); containing low vitamin D dose (25 IU/kg diet) for 12 weeks.
group IV: (High Caloric Diet with High Vitamin D, HCD+HVD) (n= 8):
Rats fed with high caloric diet (4.771 Kcal/g); containing high vitamin D dose (5169 IU/kg diet) for 12 weeks.
All rats were subjected to the following:
1- Measuring body weight (initial, final & weekly)
2- Calculation of body mass index and Lee index
3- Collection of blood samples for determination of: serum 25(OH)D, TNF-α, triglycerides, total cholesterol, HDL-c, LDL-c, VLDL-c and fasting glucose
4- Estimation of visceral fat weight
5- Measuring visceral adipose tissue UCP-1 levels
6- Histopathological study of rat’s visceral adipose tissue
The encountered results showed that, in all HCD groups, there were a significant increase in body weight, BMI, Lee index, weight of different fat depots and visceral adipocytes’ size compared to the control group. However, HCD+HVD group showed a significant increase in all these parameters compared to all other groups, despite equal food intake.
Serum 25(OH)D was significantly lower in all HCD groups, including the high vitamin D supplemented group, compared to control group, which could be explained by vitamin D sequestration or dilution in the excess adipose tissue.
To demonstrate the effect of different doses of vitamin D on metabolic parameters, serum fasting glucose and lipid profile were measured and it was found that, compared to the HCD+SVD group, HCD+HVD group showed significant decrease in serum total cholesterol and LDL-c levels, while it showed a non-significant change in serum glucose. This may indicate that vitamin D can exert a protective role against the increase in serum cholesterol. Moreover, serum glucose level was also improved compared to the HCD+LVD group despite the significant body weight difference. Thus, higher vitamin D supplementation could improve glucose tolerance and insulin sensitivity. However, the 12-week high-caloric diet did not lead to significant changes in the serum levels of triglycerides, VLDL-c and HDL-c in the different groups.
A positive association between obesity and inflammation has been detected based on the elevated serum level TNF-α in all HCD groups compared to the control group. However, the difference among these groups was insignificant.
To investigate the precise mechanism by which vitamin D affect the progression of obesity, UCP-1 level was measured in visceral adipose tissue. All HCD groups encountered higher UCP-1, which may be an adaptation against the excess energy intake. UCP-1 level was significantly higher in both standard and low vitamin D groups, while it was insignificant in the high vitamin D group compared to the control group although this group showed the highest body weight and fat depots. The HCD+LVD group showed the highest visceral adipose tissue UCP-1 compared to all groups, while the lowest value was detected in the HCD+HVD group. In addition, serum 25(OH)D level was significantly negatively correlated to UCP-1 in visceral adipose tissue, pointing to the direct effect of vitamin D on the level of UCP-1 and thus energy expenditure.
It could, thus, be concluded that, there is an inverse relationship between vitamin D and UCP-1, where vitamin D promoted adiposity by decreasing the UCP-1 level in visceral adipose tissue, however, it improved the associated metabolic derangements, including total cholesterol and serum glucose. Further studies are indicated to throw more light on the interplay between vitamin D, adiposity and UCP-1.