Although one would have expected the concept of digestibility to have developed early in the history of nutrition, a view of digestion and digestibility that we would recognize today did not arise until the middle of the nineteenth century. Following Hippocrates, the common view was that although there were many kinds of foods, there was but a single aliment. The belief of a single aliment prevailed throughout many centuries because little knowledge of the chemical nature of the various foodstuffs existed. Even in the early days of scientific nutrition research, William Beaumont (famous for his studies on digestion using a subject with a fistulous opening in his stomach caused by a gunshot wound), still held to this belief despite the then-developing opinions on the role of proteins, fats, and carbohydrates as chemically present in foods (McCollum 1957). In his own words,“the ultimate principles of aliment are always the same, from whatever food they may be obtained” (Beaumont 1833: 275) and “no other fluid produces the same effect on food that gastric juice does; and it is the only solvent of aliment” (Beaumont 1833: 278).
Although the dictionary definition of digestibility -“capable of being digested” - embraces both the scientific and the popular conception of the term, scientifically the term should suggest a numerical proportion of the nutrient capable of being digested (absorbed). In contrast, popular usage of the word “digestibility” merely seems to imply that indigestion is being avoided. The concept of being absorbed applies to any nutrient, but in practice, the term digestibility is mainly employed for food energy and for protein, whereas “bioavailability” is used to describe the degree of absorption and/or utilization of other nutrients.
Digestibility considerations were recognized as important in studies published by Boussingault from the 1840s onward. He made a number of quantitative experiments to determine the rate at which different constituents of food disappeared from the digestive tract and at the same time tabulated the theoretical amounts of vegetable feeds that would produce equal effects on the growth of animal muscle. As an example, if a standard 10 pounds (lbs.) of hay were needed to produce a certain amount of muscle, only 2 lbs. of linseed oil cake and 5 lbs. of oats, respectively, were required to do the same. Or, again to produce the same growth, 52 lbs. of wheat straw and 61 lbs. of turnips were needed. These values were, in effect, comparative protein nutritional values of feeds and, as with the majority of our current biological techniques of protein evaluation, implicitly included digestibility as a component.
Boussingault (1850) was also the first to distinguish between nitrogen as ammonium salts and nitrogen as urea in urine and was instrumental in initiating nitrogen balance experiments in farm animals. His studies (see McCollum 1957) were followed at Rothamsted in England by the work of J. B. Lawes and J. H. Gilbert in 1859. They fed two pigs that were closely similar in weight, one with lentil meal containing 4 percent nitrogen and the other barley meal containing 2 percent nitrogen. The pig fed lentil meal excreted more than twice as much urea nitrogen as did the other fed barley meal. It was evident that the pigs on these two sources of protein retained very different percentages of their food protein for conversion into body protein.
Under the impetus of the naval blockades during World War I, Rubner, in Germany, took up investigations on the nutritive value of bread. His general conclusion, following a number of digestibility studies, was that flour milled at a rate above 70 percent was poorly used by humans. The nitrogen in bran was, however, an excellent food for cattle, and the high-fiber milling residue was therefore recommended for animal feed.
Although digestibility considerations were known to be important, and it was recognized that digestibility was inclusive in most biologically determined protein quality values, the concept suffered relative neglect for a long period, with amino acid composition data given far greater attention. Only recently has the relevance of digestibility as perhaps the more important component of protein quality (that is, compared to amino acid composition) again been emphasized (FAO/WHO 1991).
World History
