A person who is not heavy enough can usually gain weight by following the general rules of hygiene, especially in the matter of increasing the fuel or energy foods. But he should not force himself to eat beyond his natural capacity to digest and assimilate the food, while overfatigue and exhausting physical exertion should be carefully avoided. [Sidenote: Diet in Middle Life] As age advances, the consumption of meat and all flesh foods should be decreased and that of fruit and vegetables, especially those of bulky character and low food value, such as lettuce, tomatoes, carrots, turnips, salsify, oyster-plant, watercress, celery, parsnips, should be increased. [Sidenote: Diet in Hot Weather] Generally the quantity of food should be slightly decreased in hot weather, when fewer calories are needed to sustain the heat of the body. In particular, less meat should be eaten in the summer, on account of what is called the "specific dynamic action of protein," that is, the special tendency of meats and like foods to produce immediate heat. Each individual must decide for himself what is the right amount of food to eat. In general, that amount is right which will maintain the most favorable condition of weight. If the weight, endurance, and general feeling of well-being are maintained, one may assume that sufficient food is taken. [Sidenote: Brainwork and Eating] It is physical, not mental work, which uses up the greater part of our food. The common impression that brain-work or expenditure of mental energy creates a special need for food is erroneous. The sedentary brain-worker often gains weight without eating very much. What he really needs is exercise, to use up the food, but if he will not take exercise, then he should reduce his food even below the small amount on which he gains weight. [Sidenote: Eating When Fatigued] Which meal in the day should be heavy and which light depends largely on one's daily program of work, the aim being to avoid heavy meals just before heavy work. When very tired it is sometimes advisable to skip a meal or to eat only lightly, as of fruits and salads. A man who eats heartily when he is very tired is likely to be troubled afterward with indigestion. (See SUPPLEMENTARY NOTES for specific directions regarding diet for underweight and overweight.) Section II--Protein Foods [Sidenote: Protein, Fat, and Carbohydrate]