Digestion in the Small Intestine
By: Dr. Caitlin Vonderohe
Professor Mike Gidley from the University of Queensland presented “Rate and extent of macronutrient digestion in the small intestine: multiple mechanisms and consequences” on Thursday, August 23, 2018 at the 14th International Digestive Physiology of the Pig Symposium in Brisbane, Australia. The major non-infectious disease challenges that humans face are primarily related to nutrition such as, obesity, diabetes, cardiovascular disease. The foods that people choose to eat have significant impacts on their health. Dietary guidelines that are found in most first world countries have started placing an emphasis on balance and whole foods, though presented within different cultural contexts. In non-ruminant production animals, the goal of the nutritionist is to balance diets for maximal protein and energy absorption in the small intestine, control feed cost, and manage feed intake.
Nutritionists tend to place a lot of research emphasis on understanding rate and site of nutrient digestion within the gastrointestinal tract. The gut has multiple, nutrient-specific feedback mechanisms to regulate rate and location of digestion. Digesta moves extremely rapidly through the beginning of the small intestine, but significantly slows as it exits the small intestine and moves into the hindgut. This means that there are significant time limitations for nutrient digestion in the small intestine.
When soluble fiber was included in the diet, passage rate speeds up through the small intestine. Protein digestion in the small intestine decreased, resulting in twice the amount of protein reaching the hindgut, compared to a diet that does not include soluble fiber. Investigators expected this dramatic increase in protein in the hindgut to encourage overgrowth of pathogenic bacteria. However, because this protein was in the presence of fiber, it encouraged the proliferation of beneficial bacterial species. However, if excess protein is fed, or excess protein escapes to the hindgut, it may result in the overgrowth of pathogenic bacteria and protein fermentation can produce toxic metabolites that will negatively affect the animal.
When animals are adapted to high fiber diets, lipid digestion and absorption is also significantly affected, particularly bile acids. High levels of circulating bile acids are associated with negative metabolic and health outcomes in human medicine. Adaptation to a high fiber diet results in reductions in bile acid secretion, which is a significant outcome for human medicine and nutrition.
The degree of starch digestion in the small intestine is dependent on particle size. In fact, the degree of starch digestion of a particular feed source is so dependent on particle size, that it can be predicted by a mathematical equation. Starch digestion is controlled by enzyme diffusion into particles. Although the rate of diffusion is grain-specific, large particles, regardless of grain type will be incompletely digested in the small intestine. However, the answer to this finding is not to radically reduce particle size; a great deal of research has shown that pigs will get gastric ulcers when particle size is too small. It is far more important to ensure that particle size is consistent in the diet.
Professor Gidley also presented some interesting work about the mucus layer of the gastrointestinal tract. Mucus is composed of lipid and proteins produced by goblet cells. Each goblet cell may produce a unique contribution to mucous layer. These individual types of mucus may be affected by the environment of the small intestine, and represents an interesting and up-coming area of research. Within the context of digestion and absorption of the small intestine, some fiber types adhere to the mucus layer, while others do not. This affects the digestion and absorption of other nutrients associated with those fiber types.
Digestion in the small intestine is dependent on the access of small intestine enzymes to their substrates (particle size) and movement of nutrients from the intestinal lumen into the body, both of which are affected by the passage rate of digesta through the small intestine. Although soluble fiber increases small intestinal passage rate, and therefore decreases protein digestion in the small intestine, there are benefits to using fiber to improve markers of gut health. Food choice and feed ingredient inclusion are significant aspects of digestion and health, therefore it is important to understand how they affect and are affected by the gut.