An Oxidized Fat Containing Diet Decreases Weight Gain but Increases Adiposity in Mice Fed a Low Fat Diet

AN OXIDIZED FAT CONTAINING DIET DECREASES WEIGHT GAIN BUT INCREASES ADIPOSITY IN MICE FED A LOW FAT DIET by Mary Schneider Introduction: Fast and convenience foods are abundant, relatively inexpensive, and accommodating to the fast-paced lifestyle of many Americans. One popular method of cooking used by many fast food establishments is deep-fat frying. Soybean oil is commonly used for frying and is rich in polyunsaturated fatty acids (PUFA) such as linoleic acid (LA). When soybean oil is used for deep-fat frying, LA becomes oxidized (Ox-LA). Endogenous Ox-LA has the capacity to be a ligand to peroxisome proliferatoractivated receptor gamma (PPAR-γ), a nuclear transcription factor that regulates adipocyte maturation. It is not yet known whether or not dietary Ox-LA has the same capacity with respect to PPAR-γ. Considering the fact that dietary oxidized lipids are abundant in the typical American diet, it is important to know if they regulate weight gain and especially adipose tissue mass. In this study, we investigate the effects of fresh and heated soybean oil on weight gain and adiposity in mice fed isocaloric low fat diets. Methods: Soybean oil was heated on a hot plate, under a hood, at 190oC for three hours. Fresh soybean oil served as the source of unoxidized oil (Unox-oil) and the heated oil served as the source of oxidized oil (Ox-oil). Both the Ox-oil and Unox-oil were incorporated into a low-fat (10% of calories) mouse chow by Research Diets, Inc. (New Brunswick, NJ). Sixteen C57BL/6J mice were divided into two groups and fed low fat diets with Ox-oil (low fat oxidized, LFO) or with Unox-oil (low fat unoxidized, LFU). Another group of 8 mice were pair fed to the LFO group with the Unox-oil containing chow (PLU). Mice in the LFO and LFU groups were fed ad libitum and known amounts of fresh food were added to the cages every three days. Leftover food was weighed. Body weights were measured once a week. After 16 weeks mice were euthanized and epididymal white adipose tissue (EWAT), retroperitoneal white adipose tissue (RWAT), inguinal white adipose tissue (IWAT), and intrascapular brown adipose tissue (IBAT) samples were collected, weighed and stored at -80 O C until further analysis. Fat pads were homogenized and cytosolic and nuclear proteins were extracted by standard methods. These extracts were subjected to Western blotting to determine the amount of PPAR-γ in the cytosol and nuclear compartments of the fat pads. Differences in group means were analyzed by Mann Whitney U test. Comparisons were considered statistically significant at a p-value of < 0.05. Results: Final mean body weights were significantly different when comparing the mice in the LFU group to the pair fed mice (PLU) (mean ± SD; 29.52 ± 1.09 grams (g) and 26.85 ± 1.44 g, respectively; p< 0.05). Mice fed a low fat diet consisting of Ox-oil (LFO) had a final mean body weight of 27.88 ± 2.03 g. Mice in the LFU group gained significantly more weight on average than did mice in the LFO or PLU groups (mean ± SD; 8.86 ± 1.37g, 7.10 ± 1.47 g, and 5.71 ± 1.13 g, respectively). Although mean food intakes were not significantly different between any of the three groups, the average food intake was greatest for the LFU mice in comparison to the LFO and the PLU mice (mean ± SD; 20.65 ± 0.09 g/week, 18.40 ± 0.05 g/week, and 18.38 ± 0.19 g/week, respectively). Feeding efficiency (g of weight gain/g of food consumed) was the highest in the LFU mice compared to the PLU mice (mean ± SD; 0.031 ± 0.005 g/g and0.022 ± 0.004 g/g) and this difference was statistically significant. The LFO mice gained less weight per gram of food consumed than did the LFU mice (mean ± SD; 0.028 ± 0.006 g/g). Mean weights of all fat pads in the LFO group were significantly greater than those of the LFU and PLU mice (mean ± SD; 0.329 ± 0.109g, 0.199 ± 0.055g, and 0.219 ± 0.041 for EWAT, 0.091 ± 0.039g, 0.050 ± 0.026g, and 0.051 ± 0.017 for RWAT, 0.221 ± 0.065g, 0.135 ± 0.053g, and 0.144 ± 0.038 for IWAT, and 0.079 ± 0.012g, 0.055 ± 0.013g, and 0.062 ± 0.011 for IBAT, respectively). PPARγ protein in the cytosol of EWAT fat pads was analyzed and quantified in comparison to the amount of Glyceraldehyde-3-phosphate dehydrogenase (GAPDH; loading control) present. Mean PPARγ /GAPDH ratios for LFU mice was 0.226 ± 0.082, for LFO mice was 0.264 ± 0.122, and for PLU mice was 0.234 ± 0.108. Mean PPARγ:GAPDH ratios were not significantly different between any of the groups. Conclusion: It appears that the consumption of oxidized oil caused a significant decrease in weight gain and food intake (although not significant) and a significant increase in fat pad mass in mice compared to those consuming a diet with unoxidized oil. The lack of difference in the amount of PPAR γ among the three groups of mice suggests that the changes in weight gain and fat pad mass among the oxidized oil consuming animals is not mediated through regulation of PPARγ protein. To our knowledge, ours is the first study to report that mice consuming a low fat diet inclusive of dietary oxidized lipids exhibit greater adiposity than do mice consuming a low fat diet consisting of unoxidized lipids. AN OXIDIZED FAT CONTAINING DIET DECREASES WEIGHT GAIN BUT INCREASES ADIPOSITY IN MICE FED A LOW FAT DIET