But important as protein is it is not everything in a diet. As we have already pointed out the food must not only repair the tissues, it must also supply fuel. It has been shown also that the fuel-value of a food can be ascertained by burning a known weight and measuring the number of units of heat or calories produced. Many samples of bread have been examined in this way in the laboratories of the American Department of Agriculture, and it appears from the figures given in their bulletins that the average fuel value of white bread is about 1·250 calories per pound, of wholemeal bread only 1·150 calories per pound. These figures are quite in accord with those which were obtained in Cambridge in 1911, in connection with the digestion experiments already described, which were also extended so as to include a determination of the proportion of the energy of the bread which the diet supplied to the body. The energy or fuel-value of the diet was determined by measuring the amount of heat given out by burning a known weight of each of the kinds of bread used in the experiment. The energy which was not utilised by the body was then determined by measuring how much heat was given out by burning the excreta corresponding to each kind of bread. The following table gives side by side the average results obtained in several such experiments in America and in Cambridge. The agreement between the two sets of figures is again on this point quite satisfactory. It is evident that a greater proportion of the total energy of white bread can be utilised by the body than is the case with any of the breads made from flours of lower commercial grades which contain more husk. In fact it appears that the more of the outer parts of the grain are left in the flour the smaller is the proportion of the total energy of the bread which can be utilised. Combining this conclusion with the fact that brown breads contain on the average less total energy than white breads, there can be no doubt that white bread is considerably better than any form of brown bread as a source of energy for the body. Kind of flour from Percentage of Amount of energy utilised which the bread the grain per 100 units in food was made contained in American Cambridge the flour experiments experiments Patents 36 96 96 Straight grade 70 92 -- Standard 80 87 95 Brown 88 -- 90 Brown 92 -- 89 Wholemeal 100 82 --