Dietary fiber and starch, digestive physiology, and metabolic health

Abstract Text:

In monogastric nutrition, analyses of fiber and starch have focused on assessing quantity. However, both have a wide range of functional properties. Fibers ranging from low to high viscous affect digesta flow and from slowly to rapidly fermentable alter production of volatile fatty acid (VFA) serving as energy for the gut or the whole body. Likewise, starches ranging from low to high amylose change from rapidly digestible in the upper gut to poorly digestible but fermentable in the lower gut, thereby changing from glucose source into VFA source. Poorly digestible or resistant starch thus basically acts as dietary fiber. Functionality of these carbohydrates for nutrition or health was studied in lab and pig models. Our hypothesis is that total extent, kinetics, and site of digestion or fermentation of starch and fiber are important for whole body energy utilization and gut health. To elucidate their effects, we developed in vitro, lab-based methodologies to describe kinetics of digestion and fermentation and linked these with in vivo models including: 1) ileal cannulation to collect digesta, 2) portal-vein catheterization to sequentially sample blood, 3) slaughter method to collect site-specific intestine tissue and digesta, and 4) indirect calorimetry. Using these models, kinetics of absorption of glucose was associated with insulin and incretin release into the portal vein, intestinal microbiota, and gene expression in intestinal tissue and microbiota. These studies confirmed that slowly digestible starch is partially degraded in the large intestine and fermented into VFA including butyrate (10-fold increase in net portal appearance), reducing insulin responses by 60% and reducing whole body energy utilization. Starch entering the distal intestine altered mRNA abundance of nutrient transporters, increased portal release of the incretin glucagon-like peptide-1 (GLP-1), and was bifidogenic in the large intestine. Extreme viscous purified fiber dampened glycemic responses and reduced digesta passage rate by 50%, thereby increasing small intestine digestion of dietary nutrients, whereas increased fiber in feed grains reduced nutrient digestibility. Fermentable fiber increased butyrate and insulin production. In whole grains with ranging content of amylose and fermentable fiber, effects of similar direction but less extreme were observed. In summary, fiber and starch characteristics influence digestive physiology and thereby gut health, metabolic health, and whole body nutrient utilization. Functional characteristics of fiber and starch should also be considered is diet formulation.

Keywords: Digestive physiology, Fiber, Starch