This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
The influence of muscular exercise upon digestion is practically exemplified by every one's personal experience. Violent exercise, even by those of robust constitution, taken immediately after the ingestion of food almost invariably retards the process if it does not produce acute dyspeptic symptoms, and even vomiting. This is due mainly to the modification in the distribution of the blood, which during active exercise passes in large amount to the periphery of the body, and in much less quantity to the abdominal organs. There are also increased products of waste matter formed during muscular activity which circulate in the blood, and it is possible, though it cannot be definitely asserted, that they may temporarily interfere with the digestive secretions. Young children between the ages of four or five and ten or twelve suffer much less from the influence upon digestion of violent exercise than do adults. It is a common experience to see children romping and playing violent games immediately after eating without necessarily provoking indigestion - a habit which would be very disastrous to adults. On the other hand, exercise has a very important relation to digestion when taken at proper times and in right amount.
This influence is to be attributed rather to the combined effect upon the circulation and respiration and general functional activity of the tissues which promotes their nutrition than to any special local action on the stomach or intestines. Exercise of a certain kind compresses or shakes the liver in such a manner as to favour the elimination of bile from it and increase its functional activity. For this reason horseback riding is unquestionably the most useful form of exercise for many varieties of dyspepsia and so-called "biliousness." Muscular fatigue following activity retards digestion very much, probably for the reason above suggested in regard to accumulation of waste matter as a result of exercise.
Moderate exercise may often be advantageously taken in the morning on rising for ten or fifteen minutes in order to get into a good perspiration before taking a cold bath. Such exercise with dumb-bells, Indian clubs, or weights with pulleys does not harm the appetite, and for some persons it is invigorating and beneficial. Stronger exercise, such as bicycle riding or taking long walks before breakfast, is not to be recommended unless the individual has been greatly overfed the night before. In the early morning hours, with an empty stomach, exposure to the influence of cold and damp, or possibly to infectious diseases, is believed to be greater than at other hours in the day. Physicians visiting cases of infectious diseases do well to go only at a time of day when they have recently taken a full meal and when they are not suffering from extreme fatigue.
Men differ greatly in the amount of exercise which they find necessary to keep them in good health and maintain a normal appetite. To keep a really vigorous man in the best bodily condition he should take daily exercise amounting to one hundred and fifty foot tons of work, or an equivalent of a walk on a level of about nine miles; but very few are able to accomplish this excepting day labourers.
The influence of food upon muscular activity has been studied by Hodge, who constructed a movable cage so arranged that any movements of the animal which it contains are communicated to the cage itself, and through it to a recording tambour and kymograph-ion. In this manner the restless activity of the hungry animal seeking for food about its cage is recorded, as well as the indolence produced by a satisfying fatty diet and the stimulating effect of nitrogenous food. For example, he demonstrated that a mouse well fed on cornmeal alone may be active but a few minutes in the day, whereas the same animal fed upon meat and cream alone will exercise for ten hours out of the twenty-four. This is no doubt due to combined effects of the influence of the feeling of satiety, exhilaration, varying functional activity of different organs of digestion, and of the circulation and nervous system. It is to be hoped that future investigations will differentiate more clearly between these factors.