## 3 Results  
### Changes in Body Weight and Body Weight Gain  
At the beginning of the study, 4-week-old C57 BL/6J mice (n = 76) were randomly assigned to either LFD (n = 32) or HFD (n = 44) ad libitum for 6 weeks. During this initial period, there was no significant difference in food intake between HFD and LFD mice; however, the increased caloric density of the HFD (5.24 versus 3.85 kcal/g, respectively) allowed for early induction of obesity in the HFD group.  
C57 BL/6J mice, as a polygenic developmental model of diet-induced obesity [21], allowed us to select the HFD animals differing in body weight and fat accumulation despite their inbred genetic background. Mice highly susceptible to the development of obesity that reached body weights in excess of 36 g after being fed an HFD for 6 weeks were designated as obesity-prone or super-responders (SR, n = 16). These animals were further divided into two equal groups, one of which was switched back to LFD treatment for the duration of the study (SR-LFD, n = 8). On the other hand, mice that failed to cross the 30 g body weight threshold under similar dietary conditions were designated as obesity-resistant or non-responders (NR, n = 8). Control animals fed the corresponding LFD or HFD reached average body weights of 26.2 \(\pm\) 2.3 and 33.1 \(\pm\) 3.1 g in the same timeline, respectively.  
All mice were kept on their respective diets for an additional 8 weeks. This allowed us to differentiate and further amplify the individual variations in response to the obesogenic dietary fats. Control LFD animals continued a slow, gradual increase in body weight and reached an average body weight of 30.8 \(\pm\) 3.9 g at the end of the study. The HFD controls reached 45.6 \(\pm\) 3.5 g body weights (\(p\) < 0.05). Obesity-prone SR mice rapidly gained excessive body weight and plateaued at an average of 50.6 \(\pm\) 3.0 g for the last 4 weeks of the study. Obesity-resistant NR mice failed to gain body weight in excess of 38.6 \(\pm\) 2.1 g, a \(-\)15.4% decrease relative to the HFD controls and a \(-\)23.7% decrease relative to the SR mice. Obesity-prone SR-LFD mice fed the LFD rapidly lost weight within 3 weeks of the dietary change, and their body weights remained undistinguished from the LFD controls at the end of the study (Figure 2).  
When body weight gains were determined for study weeks 4-14, SR mice gained 18.1 \(\pm\) 4.5 g, corresponding to an excess of 20.9% relative to the HFD controls (14.3 \(\pm\) 4.5 g). NR mice showed reduced body weight gain of 9.3 \(\pm\) 1.7 g, despite no statistically significant difference relative to the LFD controls (5.8 \(\pm\) 3.2 g). When compared directly, SR mice gained 48.6% more body weight than their NR counterparts, highlighting the individual differences despite the shared C57 BL/6J background and identical HFD. The SR-LFD animals that returned to the LFD for the last 8 weeks of the study showed negative body weight gain (\(-\)0.9 \(\pm\) 1.9), essentially losing additional body weight accumulated during the first part of the study (Figure 3).  
Figure 2: Body weight of lean controls (LFD), obese controls (HFD), obesity-resistant non-responders (NR), obesity-prone super-responders (SR), and super-responders an LFD during the study \(-\)14 (SR-LFD). Results are expressed as means \(\pm\) SEM (n = 8). Data were analyzed using a two-factor repeated-measures ANOVA; \({}^{*}p\) < 0.05 versus the HFD controls.