The implications of cooking foods and methods used

Part Three

Bioavailability of Food-Stuffs

The quantity of each food-stuff used in the body can be determined by weighing the quantity of food eaten, and then estimating the waste cast off from the bowel. The difference represents the amount of food retained in the body for use. Two foods may contain the same amount of nutritive materials, but if one of them yields more waste, it is of less nutritive value; it is, that is to say, less digestible. This is found mostly with foods of plant origin. But even milk yields this residue, which is helped in children by the quicker action and passage through the bowels. The results of numerous experiments are summarized in the following table. In each case the calculation is made for 100 parts of the food-stuff, and in one column is shown the proportion of that retained in the body, and in the other the proportion expelled as waste.

Thus, many of the foods we eat contain very little waste material and are valuable. In the case of bread it is interesting to note that the quantity of nutriment extracted depends on the fineness of the flour. Thus rye bread is far behind white bread, and the coarse black bread still further. We have here an illustration of the fact that though the bran in the wheat-grain contains more tissue-forming material than the white kernel, its addition to the flour to any extent makes the bread less valuable, both because the coarser particles are less digestible and because they stimulate the bowel to hurry the material faster along the alimentary canal that less useful material is absorbed.

Milk. As regards milk, it is very interesting to observe that there is less waste on an exclusively milk diet in the case of the infant than in the case of the adult. Thus in the case of a child, observed for 11 days, 6.35% of the dry solids of the milk were expelled daily from the bowel, while in the case of the adult the amount varied from 7.8% to 10.2%. The experiments made with milk and cheese were performed because cheese could not be made an exclusive diet.

It may seem remarkable that milk with a small proportion of cheese should be more completely made use of than milk alone. The reason seems to be that milk alone forms large masses of curd in the stomach, which the gastric juice cannot quickly attack, while, if cheese has been eaten, the small particles prevent the curd forming in such masses, and permit of the gastric juice attacking a large number of smaller portions of curd at the same time. If the proportion of cheese rises unduly, then it is not so completely digested.

It is also noteworthy how much more digestible the animal foods are than those vegetable foods which are rich in proteins or tissue-forming materials, such as peas, beans, and so on. If animal food is to be entirely replaced by vegetable food, it must partly be by such food as peas, beans, and lentils, and nuts, for only these contain a rich supply of the needful protein in moderate bulk.

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