Cephalic phase digestive responses, the salivary, gastric acid, and enzyme secretions that begin before food even reaches the stomach, are triggered by seeing, smelling, and thinking about food. Eating while distracted, eating rapidly, and eating in a stressed state all reduce the efficiency of cephalic phase responses and impair the initial stages of digestion. These are not trivial effects. They affect nutrient absorption, gastric emptying rate, and the release of satiety hormones throughout the meal.
Eating Speed and Calorie Intake
Satiety hormones including GLP-1, PYY, and CCK are secreted in the small intestine in response to incoming nutrients. This process takes approximately 20 minutes from the start of eating. People who eat rapidly consume significantly more calories before satiety signals have time to reach the brain and reduce appetite. A study of 30 adults found that eating the same meal over 30 minutes rather than 5 minutes produced 50 percent higher satiety hormone levels and significantly lower reported hunger 60 minutes after the meal.
Chewing and Nutrient Bioavailability
Thorough chewing reduces particle size and increases the surface area available to digestive enzymes. For plant foods, adequate chewing disrupts cell walls to release nutrients that would otherwise pass through the gut undigested. Almonds chewed to a fine paste are absorbed far more completely than almonds swallowed in large pieces: the physical particle size determines both the caloric availability and the micronutrient absorption from the same food.