With the influence of the Western Diet, obesity has become a rising problem in the country today. Western Diet is characterized by the overconsumption of processed food that is low in nutritional values and high in saturated fats. Study showed that every two out of three adults in the United States are either overweight or obese. Being obese increase the risk of many other disease such as diabetes, cardiovascular disease and insulin resistance. Besides being a great health concern, obesity is also cause a great financial burden. Many efforts have been made to understand the defense against obesity and weight loss. The goal of this study was to understand the characterization of food intake and weight gain responses when imposed on a high-fat diet (HFD) using rats. It was predicted that weight gain would be dependent on energy intake and it would have a significant effect on adiposity compared to energy intake. Data showed that energy intake had high significance with adiposity whereas weight gain showed no significance. Also for the rats that were on HFD, the obesity-prone (OP) rats exhibited a great amount of weight gain and energy intake while the obesity-resistance (OR) rats showed a similar weight gain to the controlled group on low-fat diet (LFD) despite being hyperphagic. This suggests that OR is characterized by equal weight gain despite hyperphagia but this alone cannot explain the boy defense against obesity. More research is needed with a larger sample size to understand weight gain responses in order to fight against the epidemic of obesity.