Amajor side effect that stress can cause on the gastrointestinal system is a. diminished levels of gastrin

elevated level of cortisol causing overeating

Explanation:

The stress affects the gastrointestinal system negatively in a number of ways and thus imbalance the normal functions. The low oxygen and flow of blood, cramps can be caused by stress. The stress response allows the signal to adrenal gland by nerves and hormone. The adrenal gland secretes cortisol and adrenaline.  

The elevated level of cortisol allows the body to demand for food and fats. Thus, this increases the craving for eating food. So, a major side effect that stress can cause on the gastrointestinal system is elevated level of cortisol causing overeating.

the abdomen (less formally called the belly, stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. the region occupied by the abdomen is termed the abdominal cavity. the stomach secretes acid and enzymes that digest food. ridges of muscle tissue called rugae line the stomach. the stomach muscles contract periodically, churning food to enhance digestion. the pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.

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