THE HISTORY OF FAT IN THE BODY 893 



and tenth days of starvation, and corresponded to an average of 

 1 grm. of protein per kilo body weight daily. In order to convert 

 this amount of protein into urea, carbonic acid, sulphuric acid, 

 and water, nearly 2 grm. of oxygen would be required in the 

 twenty-four hours, i.e. about 1 c.c. per minute. Cetti's total 

 oxygen consumption was at the rate of 5 c.c. per kilo per minute, 

 so that four-fifths of the oxygen absorbed was required for the 

 oxidation of non-nitrogenous substances, and these, as we have 

 seen, could only have been fats. In animals with a large store 

 of fat the proportion of the energy obtained at the cost of the fats 

 may be still greater. In dogs Rubner and Voit reckoned that only 

 10 to 16 per cent, of the total energy was derived from proteins, the 

 rest, i.e. 84 to 90 per cent., being obtained from the oxidation of fats. 



The oxidation of fats supplies energy not only for the production 

 of heat but also for the performance of mechanical work, and it 

 seems probable that the utilisation of the fat occurs in the muscular 

 tissues themselves. Fat is found as a normal constituent of all 

 muscle fibres, and the amount of this substance is greater in propor- 

 tion to the activity of the muscles concerned. Thus the ever-active 

 heart muscle, and the red muscles of the diaphragm, contain larger 

 amounts of fat than the pale voluntary muscles which only have to 

 undertake short periods of activity. In the human heart muscle 

 15 per cent, of the solids are soluble in ether, and more than one- 

 half of the ether extract is composed of fat, and is sufficient to supply 

 the energy of the contracting heart for six or seven hours' work. 



The degree to which the muscles during contraction call upon 

 each class of food-stuffs may be judged from the respiratory quotient 

 which, as we have seen in an earlier chapter, is unaffected by muscular 

 work. If the body has previously supplied the greater part of its 

 needs at the expense of fats, it will continue to do so during muscular 

 work. This is well shown in the following Table, in which the oxygen 

 consumption and respiratory quotient are compared in a man resting 

 and working on three different diets, one principally fat, one prin- 

 cipally carbohydrate, and the other principally protein : 



