The flow of all of the digestive secretions is dependent upon several things. Perhaps the most important is a regular time for meals. The system becomes adjusted to a regular demand for the flow of these digestive juices, and the supply corresponds to the demand. Also, proper mastication is always a signal to the secretions of the stomach and intestines to be ready for their share in the work. The sight, smell, and taste of foods have a direct effect upon the flow of the juices. Frequently one does not feel hungry until one has a whiff of food being prepared for dinner. Then the familiar gnawing feeling of hunger becomes apparent. This is a physiological fact caused by the flow of the secretions, due to the odor of food. The nervous state of the person has a direct effect on the flow of the secretions. Fatigue, worry, and anger all tend to retard digestion, so that it is far better for a very tired person to take a glass of warm milk or a cup of soup, even if it is near the regular meal hour, and rest for an hour before eating a hearty meal. Attractive service and pleasing companionship both tend to good digestion. Good humor at meal time is a valuable aid to digestion.

Large intestine

Concentrated foods such as sugar have a very irritating effect on the lining of the intestinal tract, which is one reason why candy is unwholesome between meals. The proportion in which fats, protein, and carbohydrate are eaten, affects digestion. Since fat is so largely digested in the intestines, food that is coated with fat, as are foods fried in fat, pass through the stomach without giving the juices in the stomach a chance to work upon the food material, hence, fatty foods cannot be easily digested by delicate persons or those of sedentary habits, but may be by vigorous persons doing heavy exercise who are able to utilize the energy provided by foods retained a long time in the intestines.

The part so far described of the process of the utilization of foods in the body is called digestion. The next stage of the process is called absorption.