**1. Introduction**

Organisms need to take in nutrients from outside for biological activities and survival, and deprivation of nutrients is a heavy burden for organisms. However, various responses are induced even during eating, in order to cope with the rapid influx of nutrients. The liver plays pivotal roles in the maintenance of systemic nutritional homeostasis depending on the feeding conditions, and dynamic changes are induced during the transition between fasting and feeding, or eating. During fasting, the liver releases glucose by glycogenolysis and gluconeogenesis, and ketone bodies by fatty acid oxidation, while during feeding, it stores excessive nutrition derived from food by synthesizing glycogen and fatty acids. Conversely, dysregulation of these processes may lead to metabolic disorders, such as insulin resistance and fatty liver disease [1].

In this chapter, we are going to discuss physiological and patho-physiological aspects of stress response during eating, by reviewing the insulin signaling cascade first, then endoplasmic reticulum (ER) stress, which is becoming an emerging player in the regulation of metabolism in the liver, and finally the roles of ER stress response failure in the development of steatohepatitis comorbid with diabetes.
