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John Goodfellow has shown that of very coarse wholemeal bread quite 14 per cent. was undigested, whilst bread made from ordinary grade wholemeal showed 12.5 per cent. Such a method of analysis was adopted as it was believed would exclude other than the food waste. The experiments were made on a person who was eating nothing but the bread. The difference between the proportions digested of the other food constituents was much less. Although there is here a theoretical advantage in favour of animal food, there are other considerations of far more importance than a little undigestible waste. The main question is one of health. In some dietary experiments of a girl aged 7, living upon a fruit diet, of whom we have given some particulars elsewhere, Professor Jaffa gives the following particulars. During the ten days trial the percentages absorbed were proteids 82.5, fat 86.9, nitrogen free extract 96, crude fibre 80, ash 5.7, heat of combustion in calories 86.7. He says,
"generally speaking, the food was quite thoroughly assimilated, the coefficients of digestibility being about the same as are found in an ordinary mixed diet. It is interesting to note that 80 per cent. of the crude fibre appeared to be digested. The results of a number of foreign experiments on the digestibility of crude fibre by man are from 30 to 91.4 per cent., the former value being from mixed wheat and rye, and the latter in a diet made of rice, vegetables and meat."
There is considerable variation in the same kind of food, according to the variety of seed and conditions of growth &c. Whenever it has been possible the average of the analyses of many samples have been given. This is the case in the analysis of the mushrooms.
Excerpt from: The Chemistry of Food and Nutrition by by A. W. Duncan