In travel, when one shifts with more or less rapidity from one temperature to another, the diet should not be altered too greatly or too suddenly; the system must be allowed time to accommodate itself to the change. The occupation must be taken into consideration. Great muscular activity requires a more liberal diet than a sedentary habit, no matter what the climate may be. Certain tribes that inhabit the tropics subsist almost entirely on meat, while many of the inhabitants of Russia and Norway live on breadstuffs almost to the exclusion of meat. * * * * * [Sidenote: Age] It is quite obvious that the food should vary according to the body needs. The needs of the adult, the child, and the infant vary. The baby may not take the food which is required by the child from the age of three to ten, and the aged, not exercising vigorously, does not need the hearty food of the growing child or the active adult. The need for food depends, however, on activity more than on years. It is more difficult to make those in middle and old age, who are not active, realize that the body no longer needs so much food, due to the fact that it is not so actively building tissue, and that an oversupply causes a serious tax on the digestive system. It brings in its train ills which might easily be avoided by simpler habits and a little study of the actual needs of the body. More food than the activity of the system demands, taken in later or middle life, causes most of the diseases which afflict this period. Obesity, arteriosclerosis, liver disease, gastro-intestinal diseases, biliousness, kidney diseases, gout, and allied conditions, can all be traced to an overtaxed digestive system, with faulty elimination and weakened organs. These show the rebellion of Nature at being compelled to work overtime. While these diseases are most frequent after forty, the condition of the system which designates age is not always measured by years. In the ordinary individual who has allowed himself to sit and become lazy in his habit of life, certain changes in the system occur and the body needs less food than is required in more active life. There are not such heavy calls on reserves for repair, either of nerve force or of material. Unless active exercises and interests have been kept up, the muscular system begins to deteriorate, the heart action is slower, and there is a lessening of nerve tone. Relaxation of the digestive and intestinal organs occurs, peristalsis is less vigorous, and the glands become less active, owing to the lessened call for energy. From this cause, unless the amount of food is reduced in proportion to the body needs, constipation and other digestive derangements may result.