甚至在食物出现之前，大脑就在为身体的消化做准备。一想到食物，胃里就会释放出化学物质。消化的过程在口腔中继续进行，进入胃中，然后在肠子中结束。
头相
根据2008年发表在《食欲》杂志上的一篇文章，在消化的头阶段，对食物的期待为身体消化、吸收和利用食物中的营养物质做准备。The sensory aspect of food, such as the sight and smell, influences eating behaviors. For example, the smell of a favorite dish can set the stage for overeating. This occurs as a result of the activation of the medulla oblongata by food-related sensory input. The medulla oblongata controls autonomic functions like heartbeat and respiration. The feel of food in the mouth or thoughts related to food send signals to the medulla oblongata via the vagus nerve to stimulate the release of stomach chemicals, pepsin and hydrochloric acid, which play a role in the breakdown of food. The vagus nerve is the longest of the cranial nerves, or nerves originating in the brain, and extends through the abdomen.
据研究人员称，这种反应可能受到食欲肽ghrelin的影响，这种蛋白质主要存在于胃中，但也存在于大脑中。“Orexigenic”的字面意思是“刺激食欲”。Ghrelin is capable of brain signal communication. Therefore, ghrelin is what stimulates appetite. Ghrelin also influences how much of the fat from food is absorbed into the body.
胃相
当食物进入胃时，胃的消化阶段就开始了，导致胃扩张。然后胃继续释放消化化学物质盐酸和胃蛋白酶，首先在头期刺激。根据2010年发表在《生物学和药学通报》上的一篇文章，这一过程有助于改善食物摄入和消化。When food is in the stomach or mouth, the vago-vagal reflex communicates which nutrients, such as carbohydrates and protein components, are present. Based on the feedback, the required levels of digestive chemicals are released. There are taste receptors in the mouth that can detect which nutrients have been eaten.
肠期
一旦肠道消化开始，胃的化学分泌就停止了。这一作用是由十二指肠的伸展和肠泌激素的分泌引起的。根据2008年发表在《当代内分泌学》上的一篇文章，肠泌素主要在脂肪到达十二指肠时控制胃酸的分泌。The duodenum is a short piece of the small intestine that connects the stomach with the remainder of the small intestine. When this process occurs, appetite decreases in order to stop food consumption.