When all the facts are taken into account it appears that the contention of the food reformers, that the various breads which contain those constituents of the grain which lie near the husk are capable of supplying more protein for the needs of the body than white breads, cannot be upheld. From statistics collected by the Board of Trade some few years ago as to the dietary of the working classes it appears that the diet of workers both in urban and in rural districts contains about 97 grams of total protein per head per day. This is rather under than over the commonly accepted standard of 100 grams of protein which is supposed to be required daily by a healthy man at moderate work. Consequently a change in his diet which increased the amount of protein might be expected to be a good change. But the suggested change of brown bread for white, though it appears to increase the total protein, turns out on careful examination to fail in its object, for it does not increase the amount of protein which can be digested.
From the same statistics it appears that the diet of a working man includes on the average about 1¼ lb. of bread per day. This amount of bread contains about 60 grams of protein, or two-thirds of the total protein of the diet. Now it was pointed out in the last chapter that the protein of wheat was very rich in glutaminic acid, a constituent of which animals require comparatively small amounts. It is also correspondingly poor in certain constituents which are necessary to animals. Apparently therefore it would be better to increase the diet in such cases by adding some constituent not made from wheat than by changing the kind of bread. From the protein point of view, however we look at it, there appears to be no real reason for substituting one or other of the various kinds of brown bread for the white bread which seems to meet the taste of the present day public.
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 which bread was made | Percentage of the grain contained in the flour | Amount of protein digested per 100 parts eaten | |
|---|---|---|---|
| American experiments | Cambridge experiments | ||
| Patents | 36 | 96 | 96 |
| Straight grade | 70 | 92 | — |
| Standard | 80 | 87 | 95 |
| Brown | 88 | — | 90 |
| Brown | 92 | — | 89 |
| Wholemeal | 100 | 82 | — |
There is one more important substance in respect of which great superiority is claimed for brown breads, namely phosphoric acid. From the table on page 122 there can be no doubt that flours containing more of the outer parts of the grain are very much richer in phosphoric acid than white flours, and the disparity is so great that after allowing for the larger proportion of water in brown breads they must contain far more of this substance than do white breads. In the Cambridge digestibility experiments quoted above the proportion of the phosphoric acid digested from the different breads was determined. It was found that for every 100 parts of phosphoric acid in white bread only 52 parts were digested, and that in the case of the brown breads this proportion fell to 41 parts out of 100. Again, as in the case of protein and energy, the phosphoric acid in white bread is more readily available to the body than that of brown bread, but in this case the difference in digestibility is not nearly enough to counterbalance the much larger proportion of phosphoric acid in the brown bread. There is no doubt that the body gets more phosphoric acid from brown bread than from the same quantity of white bread. But before coming to any practical conclusion it is necessary to know two things, how much phosphoric acid does a healthy man require per day, and does his ordinary diet supply enough?
From the Board of Trade statistics already quoted it appears that, on the assumption that the average worker eats white bread only, his average diet contains 2·4 grams of phosphoric acid per day, which would be raised to 3·2 grams if the white bread were replaced by bread made from 80 per cent. flour containing ·35 per cent. of phosphoric acid. Information as to the amount of phosphoric acid required per day by a healthy man is somewhat scanty, and indicates that the amount is very variable, but averages about 2½ grams per day. If this is so, the ordinary diet with white bread provides on the average enough phosphoric acid. Exceptional individuals may, however, be benefited by the substitution of brown bread for white, but it would probably be better even in such cases, for the reasons stated when discussing the protein question, to raise the phosphorus content of their diet by the addition of some substance rich in phosphorus but not made from wheat.
Finally comes the question of the variation in the composition of bread due to the presence or absence of the germ. The first point in this connection is to decide whether germ is present in appreciable proportions in any flour except wholemeal. The germ is a soft moist substance which flattens much more readily than it grinds. Consequently it is removed from flour by almost any kind of separation, even when very coarse sieves are employed. If this contention is correct no flour except wholemeal should contain any appreciable quantity of germ, and it is certainly very difficult to demonstrate the presence of actual germ particles even in 80 per cent. flour. Indirect evidence of the presence of germ may, however, be obtained as already explained by estimating by chemical analysis the proportion of fat present in various flours. The figures for such estimations are given by Dr Hamill in the report of the Local Government Board already referred to. They show that the percentages of fat in different grades of flours made from the same blends of wheat are on the average of seven experiments as follows: patents flour ·96: household flours 1·25: 80 per cent. or standard flour 1·42. These figures show that the coarser flours containing more of the whole grain do contain more germ than the flours of commercially higher grade, in spite of the fact that it is difficult to demonstrate its presence under the microscope.
Remembering, however, that the whole of the germ only amounts to about 1½ per cent. of the grain, it is clear that the presence or absence of more or less germ cannot appreciably affect the food-value as measured by protein content or energy-value. It is still open to contention that the germ may contain some unknown constituent possessing a peculiar effect on nutrition. Such a state of things can well be imagined in the light of certain experimental results which have been obtained during the last few years.
It has been shown for instance by Dr Hopkins in Cambridge, and his results have been confirmed at the Carnegie Institute in America, that young rats fail to thrive on a diet composed of suitable amounts of purified protein, fat, starch, and ash, but that they thrive and grow normally on such a diet if there is added a trace of milk or other fresh animal or vegetable substance far too small to influence either the protein content or the energy-value. Another case in point is the discovery that the disease known as beri beri, which is caused by a diet consisting almost exclusively of rice from which the husk has been removed, can be cured almost at once by the administration of very small doses of a constituent existing in minute quantities in rice husk. The suggestion is that high grade flours, like polished rice, may fail to provide some substance which is necessary for healthy growth, a substance which is removed in the germ or husk when such flours are purified, and which is present in flours which have not been submitted to excessive purification.