Water is indispensable to life. About two-thirds of the total body-weight is composed of water, and water also enters largely into the composition of all foodstuffs. It is useful as a solvent, carrying nutrient material to the tissues and effete material to be eliminated by the kidneys and skin.
On an average, about 4 pints of water may be taken as the daily requirements of the tissues, of which about one-fourth is ordinarily taken in solid food. The best way of supplying water to the body is by drinking it in its pure state, when its useful solvent and eliminant properties can be fully utilised. The equivalent of the 4 pints ingested is excreted daily, 50 per cent, being voided in the urine, about 28 per cent by the skin, 20 per cent, by the lungs, and 2 per cent by the faeces. The amount of water required daily in fluid form varies with the amount of loss by the excretory organs, skin, kidneys, and lungs; and this in turn depends to an important extent on the outside temperature, on the amount of muscular activity, and on the nature of the food In hot weather there is a free diaphoresis, and more fluid is called for; whereas in cold weather the skin is less active, and less fluid is necessary. Active muscular exercise promotes diaphoresis and diuresis, hence 132 abundance of fluid is desirable. If the diet is a solid and dry one, and rich in nitrogen, more water is necessary, in order to eliminate the extra amount of urea and other nitrogenous products; whereas if vegetables and fresh fruits enter largely into the dietary, less water is required. Natural foods which are specially rich in water are milk and succulent fruits, such as grapes, oranges, melons, and plums; also vegetables, such as cucumbers, tomatoes, and salads. All vegetables contain more water than meats, and fresh fruits as a rule contain more water than vegetables.
The ingestion, even of a large amount of water, has very little effect on the tissues other than of a temporary character, this being due to the remarkable tendency on the part of the blood to maintain equilibrium as regards its composition. After the ingestion of a large amount of water the blood is very temporarily diluted, the blood-pressure slightly raised, and, in proportion as the latter is present, there is a slightly increased activity of the heart muscle. These effects quickly subside, coincid-ently with the rapid withdrawal of the water from the tissues by means of the kidneys and skin. Metabolism of proteins is not notably affected.
One of the commonest dietetic errors is taking too little water. The important influence of water on nutrition is shown by a study of the influence of water starvation. Water starvation leads very speedily to the development of thirst, a dryness of the mucous membranes, constipation, defective absorption from the gastro-intestinal tract, with resulting impaired nutrition, emaciation, muscular weakness, convulsions, coma, and death. It is important to bear in mind that many patients are willing to take water in the form of a table-water who will not readily take it in the form of ordinary drinking-water.
Drinking-waters are spoken of as hard and soft, according to the proportion of mineral matters present in the water. Hard water contains excess of lime salts, the drinking of which may induce constipation, indigestion, and disturbance of the general health. In some people the use of hard water in the cooking of food is sufficient to upset the general health. By boiling the water, its hardness, which is due to the presence of earthy carbonates, is diminished, as the carbonic acid which assists in holding the carbonates in solution is driven off. Boiling is also the most effective way of ridding the water of organic impurities. Boiled water when cooled has an insipid taste, but this may be largely got rid of by subsequent aeration in a gazogene, or by simply shaking up the water with air in a stoppered bottle.
Water, then, is an indispensable beverage. It is absorbed quickly, and as rapidly eliminated. As a general rule, it may be said that at least 11/2 to 2 pints of pure water should be taken daily. This amount, it may be added, is considerably in excess of that taken by the average man. A useful method of prescribing water is to give a full tumblerful of hot or cold water on an empty stomach -
(a) The first thing in the morning;
(b) An hour before the midday meal; and:
(c) The last thing at night.
The addition of water to the dietary is a valuable therapeutic agent in many diseases. Special value attaches to its use in fevers, where it assists in eliminating the toxins by means of the kidneys and skin. Further reference to its use in disease will be found in the sections dealing with individual diseases.
Artificial aerated waters are made by charging water with carbonic acid gas at high pressure. The chief varieties of artificial aerated waters are as follows: -
1. Aerated distilled water, e.g. Salutaris water. Here all the mineral substances have been removed by distillation prior to being charged with gas.
2. Ordinary water impregnated with carbonic acid gas.
3. Alkaline waters, such as soda, potash, or lithia water.
These contain on an average about 5 to 10 grs. of alkaline salt to the bottle.
4. Seltzer water, an imitation of the natural mineral water of Seltzer (in Nassau). This contains common salt, bicarbonate of soda, carbonate of magnesia, and hydrochloric acid.