Digestibility
There is strong popular conviction but not very much basic knowledge about the "digestibility" of specific foods and their effect on the physiology of the gastrointestinal tract. Foods are said to be "hard to digest" or, conversely, "easy to digest." They are classed as "irritating" or "gasforming." Or they possess a quality described as ''bland.'' Most of these terms may be traced to reported experiences of individuals who have gastrointestinal disorders, to incorrectly interpreted results of early studies of gastrointestinal function, and perhaps, most frequently, to long usage.
"Digestibility" of a food has been equated with the rate at which it leaves the stomach. Because fat remains longest in the stomach, it is thought to be more "difficult to digest" than protein and carbohydrate foods. Fried foods are said to be "irritating" to the gastrointestinal tract, although there is little evidence to support this statement.
When foods are ingested together in a meal the picture becomes even more complex. Our knowledge about the effects of individual foods on the physiology of the gastrointestinal tract when consumed in mixed diets is very scarce. It is obvious also that the factors mentioned above are not related to the true digestibility of foods. In professional usage the term digestibility refers to the proportion of the food that becomes available to the body as absorbed nutrients; the indigestible portion is excreted in the feces.
Under normal conditions the bulk of the indigestible residue consists of cellulose, pectins, and other complex carbohydrates found in foods of plant origin, which cannot be degraded by the digestive enzymes in humans. The amount of bulk in the diet has received a lot of attention in recent years, as it may have a role in the development of some disorders of the colon which are common today in the Western world.
Some of the protein, fat, and carbohydrate offoods is also passed in the feces. In normal, healthy people the average proportions of the major dietary nutrients digested (and absorbed) are 98% for carbohydrate, 95% for fat, and 92% for protein. It is in this connection that the word digestibility is most meaningful and \videly used by professionals. It is often express~d as a coif.ficient of digestibility, which would be 0.98, 0.95, and 0.92 for carbohydrate, fat, and protein, respectively. The coefficients of digestibility for nutrients vary \vith each person and \vith the food source, especially \vith protein. For example, the proteins of egg, milk, and meat have a coefficient of digestibility of 0.97, whereas those for plant products range from 0.89 for flour to 0.65 for most vegetables.
The amounts of nutrients lost the feces can be greatly increased in disease states. For example, a deficiency in intestinal lactase activity results in fecal loss of milk sugar lactose and is responsible for milk intolerance in some individuals. Other disease conditions may be less specific and cause general malabsorption of nutrients. The various gastrointestinal disorders that influence digestion and absorption.
Some foods are also said to cause "indigestion and heartburn." Although sensitivity to specific foods has been demonstrated in some individuals with symptomatic esophagitis, the same foods have had no effect in others. The mechanism causing diet-induced heartburn remains unknown. Although several recent studies have suggested that heartburn is associated with acid reflux caused by lowering ofthe pressure ofthe lower esophageal sphincter (LES) by the offending food, other investigations have not supported this conclusion. Of the foods and beverages commonly associated \vith heartburn, fat, chocolate, and alcohol have been shown to reduce LES pressure, but orange juice and tomato products showed no sustained effect.The results \vith coffee have been inconsistent both a reduction 7 and an increases in LES pressure have been reported. Price and co-workers found that sensitivity to orange juice, tomato juice, and coffee was not related to their acidity because it remained even after the foods were neutralized. There was also variability among susceptible people about the specific foods to which they were sensitive. Therefore, it is not possible to attribute any common characteristics to the foods that cause heartburn in some people.