No. 3792, JULY 4, 1942 NATURE 21 LETTERS TO THE EDITORS hydrate, so ventilation (largely controlled by carbon dioxide) and therefore evaporation from the lungs 'J.'he Editors do not hola tketMelvu responsible may actually sometimes be greater when carbo• for opinions expressed by their correspondenta. hydrate is being used. This does not invalidate the No notice is taken of anonymous communications. conclusions given above concerning the amounts of produced for each calorie when different food• Metabolic Water and Desiccation stuffs are burned, and without further work it is THE utilization by the body of ingested food sub• very doubtful whether such differences in the oxygen stances and of tissue reserves yields among other content of the venous blood of normal resting in• things quantities of metabolic water. As the com• dividuals are to be found. In general then, it is clear plete combustion of 100 gm. of produces about that fat cannot in a given time give appreciably more ll0 gm. of metabolic water, whereas 100 gm. of metabolic water than , and most of the carbohydrate yields only 55 gm. of water, fat reserves evidence suggests that it gives considerably less. and fatty foods are believed to be particularly valu• The only advantage of fat over carbohydrate under able as a protection against desiccation. This con• conditions where desiccation is likely (in deserts or tention would appear to be supported by the fact lifeboats at sea) is that a smaller weight will have a that many animals which exist in deserts have large higher calorific value and will give more water of meta• reserves of fat. bolism than the same weight of carbohydrate. How• I believe that the idea that fat is primarily a pro• ever, carbohydrate foods are generally less bulky, tection against desiccation is based on fallacies. more easily stored and non-perishable; furthermore, Terrestrial animals, which have evolved eventually very fatty diets are likely to give nausea and acidosis. from aquatic forms of life, are always apt to suffer Desert animals store fat in their bodies because from desiccation, and many well-known mechanisms fat is the only food substance which animals can for water conservation exist. Respiration always store in any quantities. But they store the fat as a presents a difficulty. and a serious loss of water often food and not as a water reserve, for in deserts starva• takes place from the surface of respiratory organs. tion is as great a danger as desiccation. In fact, many The more a terrestrial animal takes in oxygen, during animals avoid desiccation by living in burrows where respiration, for its , the more water there an almost saturated micro-climate is found even will be lost in the (practically saturated) expired air ; although the outside air may be extremely dry. this is equally true of insects and . In man, KENNETH MELLANBY. the water lost from the lungs is generally greater Sorby Research Institute, than the total production of metabolic water. When Sheffield. desiccation is the limiting factor (and animals usually June 13. die from lack of water long before their food reserves are exhausted) the significant figure is not the amount of metabolic water produced for unit consumption of any reserve, but the amount of metabolic water Biology of Young Salmon (S. solar, Linn.) produced for unit oxygen uptake. When burning IN the period immediately preceding 'the outbreak fat, to produce 100 gm. of metabolic water, 255 gm. of the present War a close study had been made of the of oxygen must be absorbed; to produce 100 gm. biology of young salmon, that is, salmon parr (S. salar), of metabolic water from carbohydrate only 213 gm. in various localities of Great Britain and Irelan::! 1-•. of oxygen need be used. I believe therefore that an It seems advisable in the existing conditions of war animal living on fat may actually be worse off as to review these investigations and particularly those regards the production of metabolic water than one carried out on the Welsh Dee. The accompanying living on carbohydrate. figure correlates in one picture much of the work The same conclusion may be arrived at in a different done on this river ; the results obtained in other way. Fat has a much higher calorific value than carbo• localities are not very different. hydrate, but while for every 1,000 calories produced It would appear from all these investigations that by burning fat only 120 gm. of water are produced rapid growth is related to feeding only at or above as a by-product, when carbohydrate is burnt 130 gm. a particular temperature-level and may, moreover, of water are manufa.ctured for the same calorific yield. be checked in the male by the later stages of gonad Thus if an animal performs the same sort of activities growth and spawning. Gonad development in the on a predominantly fatty diet, it will actually produce male is correlated with an accumulation of fat less water than on a predominantly carbohydrate reserves following rich feeding and apparently also diet. with a particular temperature-level. Spawning in the There is one way in which the conclusion given male is correlated with a rapid fall in temperature above may need to be modified. Possibly when an and a falling off in the amount of food taken (and animal is metabolizing fat it may obtain the extra available). Fat reserves are used up in the young oxygen, not entirely by increased ventilation, but male in the production of gonad but not in the female. partly by an increased utilization of the oxygen in An increase in the amount of food taken is not fol• the blood, with a consequent reduction of the venous lowed at once by an accumulation of mesenterial fat blood to a level lower than that found when carbo• reserves, nor apparently an improvement in 'con• 3 hydrate is burned. We do not know that this actua• dition', (K = W/L ), hut the laying down of mesen• ally happens, for there appears to be little information terial fat is soon followed by testis development in concerning the composition of venous blood when the males. Smolt migration begins during the early different food substances are used. If it does, how• phase of acceleration of growth in body-length, and ever, carbon dioxide production in relation to the appears t o receive an early impulse about the time control of ventilation must be considered. 244 gm. of the rise in river temperature from the minimum. of carbon dioxide are produced for each 100 gm. of A close relation exists between river and sea tempera• metabolic water manufactured from fat, and 293 gm. ture at the time of migration. There is no information of carbon dioxide for 100 gm. of water from carbo- available yet on metabolic state at the time of

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