FACTORS INFLUENCING GASTRIC ACTIVITY
Motor and secretory activities of the stomach are modified, usually simultaneously and in the same direction, by a number of factors, both local (gastric) factors and external or systemic factors. Chief among them are the following:
LOCAL (GASTRIC) FACTORS INFLUENCING GASTRIC ACTIVITY
1. Character of the Food. A meal that is sufficiently high in fat to yield an intragastric fat content in excess of about 10% empties much more slowly and stimulates considerably less acid secretion than does a meal predominantly of protein. The inhibitory effect of fat on gastric secretion is not a local one but a result of the hormone, which is released systemically when fat enters the duodenum. This enterogastric inhibitory action of fat and cholecystokinin is much more effectively achieved by the ingestion of 15 to 30 mL of a vegetable oil before meals.
A meal exclusively or mainly of starch tends to empty more rapidly, though stimulating less secretion, than does a protein meal. Thus, other factors being equal, a person may expect to be hungry sooner after a breakfast of fruit juice, cereal, toast, and tea than after one of bacon, eggs, and milk. The amount of gastric secretion and gastric acidity is highest with the ingestion of proteins. This relationship of quantity and rate of secretion and its acid or pepsin concentration is subject to great individual variations as well as variations in a single individual under different conditions.
2. Consistency of the Food. Liquids, whether ingested separately or with solid food, empty from the stomach more rapidly than do semisolids or solids. This does not apply to liquids such as milk, from which solid material is precipitated on contact with gastric juice; milk also releases CCK from its fat content. In the case of foods requiring mastication, the consistency of the material reaching the stomach should normally be semisolid, thereby facilitating gastric secretion, digestion, and evacuation. Important exceptions to the general rule that liquids are weak stimulants of gastric secretion are (1) the broth of meat or fish, by virtue of their high secretagog content, and (2) coffee, which derives its secretory potency from its content both of caffeine and of the secretagogs formed in the roasting process.
SYSTEMIC (EXTERNAL) FACTORS INFLUENCING GASTRIC ACTIVITY
1. Hunger. A meal eaten at a time of intense hunger tends to be evacuated more rapidly than normally, apparently as the consequence of the heightened gastric tonus. Because hunger results from the depletion of body nutrient stores, it is understandable on teleologic grounds that in the hunger state, the body should have some mechanism for hastening the delivery of ingested nutrients into the intestine. Levels of the hormone ghrelin are higher when the stomach is empty and one becomes hungry, possibly signaling the time to consume a meal. Ghrelin also has prokinetic effects that accelerate gastric emptying.
2. Exercise. Mild to moderate exercise such as walking, particularly just after eating, increases gastric emptying of the meal compared with resting conditions. With strenuous exercise, gastric contractions are temporarily inhibited and gastric emptying decreases. Secretory activity does not appear to be materially influenced by exercise.
3. Position. Gastric emptying is facilitated in certain individuals when lying on the right side when the position of the body is such that the pylorus and duodenum are in a dependent position. In the supine position, gastric emptying is delayed because the gastric content pools in the dependent fundic portion. No evidence is avail- able that secretion is affected by position. For patients with gastrointestinal reflux symptoms, sleeping in the right recumbent position may reduce nocturnal symptoms, because delayed gastric emptying can cause reflux symptoms.
4. Emotion. The retarding effect of emotional states on gastric motility and secretion has been documented by clinical and experimental observations. In healthy humans, anger, fear, labyrinthine stimulation, painful stimuli, preoperative anxiety, and intense exercise slow gastric emptying. The influence of emotions on gastric activity may be augmentative or inhibitory, depending on whether the emotional experience is of an aggressive (hostility, resentment) or depressive (sorrow, fear) type, respectively.
5. Pain. Severe or sustained pain in any part of the body (as from a kidney or gallbladder stone, migraine, or sciatic neuritis) inhibits gastric motility and evacuation by nervous reflex pathways.