Review: Effects of fibre, grain starch digestion rate and the ileal brake on voluntary feed intake in pigs

Grains rich in starch constitute the primary source of energy for both pigs and humans, but there is incomplete understanding of physiological mechanisms that determine the extent of digestion of grain starch in monogastric animals including pigs and humans. Slow digestion of starch to produce glucose in the small intestine (SI) leads to undigested starch escaping to the large intestine where it is fermented to produce short-chain fatty acids. Glucose generated from starch provides more energy than short-chain fatty acids for normal metabolism and growth in monogastrics. While incomplete digestion of starch leads to underutilised feed in pigs and economic losses, it is desirable in human nutrition to maintain consistent body weight in adults. Undigested nutrients reaching the ileum may trigger the ileal brake, and fermentation of undigested nutrients or fibre in the large intestine triggers the colonic brake. These intestinal brakes reduce the passage rate in an attempt to maximise nutrient utilisation, and lead to increased satiety that may reduce feed intake. The three physiological mechanisms that control grain digestion and feed intake are: (1) gastric emptying rate; (2) interplay of grain digestion and passage rate in the SI controlling the activation of the ileal brake; and (3) fermentation of undigested nutrients or fibre in the large intestine activating the colonic brake. Fibre plays an important role in influencing these mechanisms and the extent of their effects. In this review, an account of the physiological mechanisms controlling the passage rate, feed intake and enzymatic digestion of grains is presented: (1) to evaluate the merits of recently developed methods of grain/starch digestion for application purposes; and (2) to identify opportunities for future research to advance our understanding of how the combination of controlled grain digestion and fibre content can be manipulated to physiologically influence satiety and food intake

Variability in cereal grain composition and nutritional value: the importance of fibre

Large variation in the nutritional value of cereal grains, within and between animal types, is due predominantly to differences in digestive tract anatomy, site and extent of microbial fermentation, and amount and type of dietary fibre. Energy available from digestion is reduced by grain fibre in pigs, broilers and ruminants. The effects are greatest for broilers and least for ruminants, reflecting large differences in intestinal microbial populations. Individual grain samples are often better suited for digestion by one animal type than another. In pigs, the extent of starch digestion in the small intestine scales with the rate of amylase diffusion into grain particles, grain particle size and the time particles have for digestion, determined by residence time of digesta to the terminal ileum. Amylase diffusion rate is influenced by endosperm tissue and cell wall integrity, solubility of grain protein matrix, composition of starch and processing methods. Rate of passage of digesta to the terminal ileum increases as fibre content of the diet increases to similar to 15% neutral detergent fibre (NDF), then declines with additional fibre causing distention of the stomach. Diet hydration capacity, which is influenced by soluble fibre content, slows digesta passage rate once it exceeds similar to 1.2 g water/g dry matter. Undigested starch reaching the colon causes a decrease in passage rate through activation of the ileal/colonic brakes. Voluntary feed intake in monogastric animals is related to rate of passage to the terminal ileum for nutritionally balanced grain-based diets. Approximately 15% of energy in starch fermented in the large intestine is lost to the pig as heat, methane and microbial products excreted in faeces. Algorithms predicting effects of diet fibre, water-holding capacity and amylase diffusion rate can be used to predict threshold grain particle size, where all starch is digested by the terminal ileum, and the likely loss of energy through fermentation