CHAPTER XV. The Digestibility of Foods

THE DIGESTIBILITY OF FOODS

225. Digestibility, How Determined.--The term "digestibility," as applied to foods, is used in two ways: (1) meaning the thoroughness of the process, or the completeness with which the nutrients of the food are absorbed and used by the body, and (2) meaning the ease or comfort with which digestion is accomplished. Cheese is popularly termed indigestible, and rice digestible, when in reality the nutrients of cheese are more completely although more slowly digested than those of rice. In this work, unless otherwise stated, digestibility is applied to the completeness of the digestion process.

The digestibility of a food is ascertained by means of digestion experiments, in which all of the food consumed for a certain period, usually two to four days, is weighed and analyzed, and from the weight and composition is determined the amount, in pounds or grams, of each nutrient consumed. In like manner the nutrients in the indigestible portion, or feces, are determined from the weight and composition of the feces. The indigestible nutrients in the feces are deducted from the total nutrients of the food, the difference being the amount digested, or oxidized in the body. When the food is digested, the various nutrients undergo complete or partial oxidation, with the formation of carbon dioxid gas, water, urea (CH{4}N{2}O), and other compounds. The feces consist mainly of the compounds which have escaped digestion. The various groups of compounds of foods do not all have the same digestibility; for example, the starch of potatoes is 92 per cent digestible, while the protein is only 72 per cent. The percentage amount of a nutrient that is digested is called the digestion coefficient.

In the following way the digestibility of a two-days ration of bread and milk was determined: 773.5 grams of bread and 2000 grams of milk were consumed by the subject. The dried feces weighed 38.2 grams. The foods and feces when analyzed were found to have the following composition:

===================================================================== COMPOSITION | BREAD | MILK | FECES --------------------------------------------------------------------- Water | 44.13 | 86.52 | -- Crude protein | 7.75 | 3.15 | 25.88 Ether extract | 0.90 | 4.63 | 18.23 Ash | 0.32 | 0.70 | 26.35 Carbohydrates | 46.90 | 5.00 | 29.54 Calories per gram | 2.450 | 0.79 | 5.083 =====================================================================

STATEMENT OF RESULTS OF A DIGESTION EXPERIMENT

============================================================================= FOOD CONSUMED | WEIGHT | PROTEIN | ETHER | CARBO- | | HEAT OF | OF | N x 6.25 | EXTRACT | HY- | ASH | COMBUS- | MATERIAL | | | DRATES | | TION ------------------+----------+----------+---------+--------+-------+--------- | Grams | Grams | Grams | Grams | Grams | Calories Bread | 773.5 | 60.0 | 6.9 | 362.8 | 2.5 | 1895 Milk | 2000.0 | 63.0 | 92.6 | 100.0 | 14.0 | 1585 | | ------- + ------- + -------+-------+--------- Total | 38.2 | 123.0 | 99.5 | 462.8 | 16.5 | 3480 Feces | | 9.9 | 7.0 | 11.3 | 10.1 | 194 | | ------- + ------- + -------+-------+--------- Total amount | | 113.1 | 92.5 | 451.5 | 6.4 | 3286 digested | | | | | | Per cent digested | | | | | | or coefficients | | | | | | of digestibility| | 92.0 | 93.0 | 97.5 | 38.8 | 94.4 | | | | | | Available energy | | -- | -- | -- | -- | 90.0 =============================================================================

In this experiment 92 per cent of the crude protein, 93 per cent of the ether extract, and 97.5 per cent of the carbohydrates of the bread and milk ration were digested and absorbed by the body. In calculating the available energy, correction is made for the unoxidized residue, as urea and allied forms. It is estimated that for each gram of protein in the ration there was an indigestible residue yielding 1.25 calories.

226. Available Nutrients.--A food may contain a comparatively large amount of a compound, and yet, on account of its low digestibility, fail to supply much of it to the body in an available form. Hence it is that the value of a food is dependent not alone on its composition, but also on its digestibility. The digestible or available nutrients of a food are determined by multiplying the per cent of each nutrient which the food contains by its digestion coefficient. For example, a sample of wheat flour contains 12 per cent protein, 88 per cent of which is digestible, making 10.56 per cent of available or digestible protein (12 x 0.88-10.56). Graham flour made from similar wheat contains 13 per cent total protein, and only 75 per cent of the protein is digestible, making 9.75 per cent available (13 x 0.75 = 9.75). Thus one food may contain a larger total but a smaller available amount of a nutrient than another.

227. Available Energy.--The available energy of a food or a ration is expressed in calories. A ration for a laborer at active out-of-door work should yield about 3200 calories. The calory is the unit of heat, and represents the heat required to raise the temperature of a kilogram of water 1 deg. C., or four pounds of water 1 deg. F. The caloric value of foods is determined by the calorimeter, an apparatus which measures heat with great accuracy. A pound of starch, or allied carbohydrates, yields 1860 calories, and a pound of fat 4225 (see Section 13). While a gram of protein completely burned produces 7.8 calories, digested it yields only about 4.2 calories, because, as explained in the preceding section, not all of the carbon and oxygen are oxidized. The caloric value or available energy of a ration can be calculated from the digestible nutrients by multiplying the pounds of digestible protein and carbohydrates by 1860, the digestible fat by 4225, and adding the results. For determination of the available energy of foods under different experimental conditions, and where great accuracy is desired, a specially constructed respiration calorimeter has been devised, which is built upon the same principle as an ordinary calorimeter, except it is large enough to admit a person, and is provided with appliances for measuring and analyzing the intake and outlet of air. The heat produced by the combustion of the food in the body warms the water surrounding the calorimeter chamber, and this increase in temperature is determined by thermometers reading to 0.005 of a degree or less.

228. Normal Digestion and Health.--While the process of digestion has been extensively studied, it is not perfectly understood. Between the initial compounds of foods and their final oxidation products a large number of intermediate substances are formed, and when digestion fails to take place in a normal way, toxic or poisonous compounds are produced and various diseases result. It is probable that more diseases are due to imperfect or malnutrition than to any other cause. There is a very close relationship between health and normal digestion of the food.

The cells in the different parts of the digestive tract secrete fluids containing substances known as soluble ferments, or enzymes, which act upon the various compounds of foods, changing them chemically and physically so that they can be absorbed and utilized by the body. (See