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 question as to how far gain or loss in the body weight may be taken as an indication of the appropriateness of diet is important. In prescribing systems of diet for many diseases, such as dyspepsia, obesity, diabetes, phthisis, etc., it is very desirable to have the patient weighed systematically at least once a week, and good scales are very useful in the physician's office. Care should, of course, be observed that changes in clothing are not allowed to interfere with the accuracy of the observations. Loss of weight, under some conditions, may be much more rapid than gain ever is. The latter at best does not usually proceed at a rate of more than two or three pounds in a week, even during convalescence from such an illness as typhoid fever; but in diseases like cholera, for example, in which emaciation is extremely rapid, the loss of weight may amount to two or three pounds a day, or possibly more.
In some diseases a fallacy arises in comparing food consumption with body weight. In fevers there is usually loss of weight, but this is not always the case. Sometimes in aseptic fevers the weight may remain uniform; and in phthisis there is sometimes uninterrupted gain during a moderate hectic. Leyden has found that in fever, while the albuminous ingredients of the body are diminishing, there may be more water stored in the tissues, and in cases of anasarca and dropsical accumulations of rapid development it is quite possible for the weight to increase, while in reality the body is rapidly wasting away, and it would be a very grave mistake to deduce any conclusions for feeding the patient from the weight alone. Another important consideration is the rapidity of loss of waste food products from the body.
The normal weight of faeces which should be voided each day is five or six ounces - about one fourth of which is solid matter, the rest water. Many persons do not pass over half this quantity for days together, and thus go on accumulating waste material, which adds to their weight. The same disproportion between food ingestion and the elimination of urea and different salts in the urine may exist. Moreover, the loss of weight by excessive perspiration may reach a pound or two a day. Athletes subjected to violent physical strain and stokers working in great heat - sometimes of 1200 F. - may lose two pounds in an hour in this manner. The night sweats of phthisis cause considerable actual loss in weight, besides the exhaustion which they occasion. The loss of weight through the aqueous vapour and carbon dioxide in the lungs may vary somewhat, although it is less changeable than might be supposed.
For all these reasons it is necessary to be somewhat cautious in accepting conclusions from the scales for application in ordering more or less food, or altering its character.
The weighing of infants immediately before and after nursing at the breast is an excellent and fairly accurate method of determining how much food they are getting, and infants whose nutrition gives any cause for anxiety should be frequently weighed, for in them, for obvious reasons, the record of slight variations in weight can be much more accurately utilised in regulating feeding.