Chemosensing of Nutrients and Non-Nutrients
By: Dr. Caitlin Vonderohe
Doctors Inge Depoortere and Eugeni Roura presented “Chemosensing of nutrients and non-nutrients in the human and porcine guts” to kick off the first session of the 14th Digestive Physiology of Pigs Symposium. Dr. Depoortere, from the University of Leuven opened the talk by focusing on how the human gut uses different molecular mechanisms to “sense” contents of the gut and how these mechanisms can be used to combat obesity in humans. Dr. Roura closed the talk by demonstrating how the concepts described by Dr. Depoortere are applied to swine nutrition and physiology.
The gut “tastes” what you eat by a variety of different mechanisms. In the stomach, there are multiple cell types that are responsible for sensing food, determining passage rate from the stomach to small intestine and regulating intake. These cells have multiple receptor sites for protein, glucose and fatty acids—allowing precise regulation of digestion and intake. Ghrelin, a hormone released by the stomach during a fasted state, is responsible for the feeling of “hunger”. This is strongly regulated by carbohydrate intake, which is “sensed” by glucose transport proteins and “sweet” taste receptors on specialized cells within the stomach lining.
The epithelial layer of the small intestine is composed of multiple cell types that have stem cell, absorptive and immune functions. Enterocytes (absorptive cells) have multiple surface proteins that detect the presence of amino acids, peptides, fatty acids and carbohydrates. Neurons present throughout the small intestine also have multiple nutrient-sensing proteins. Paneth and tuft cells, commonly known to have immune function, can also detect different proteins and fats, which plays a significant role in protection against intestinal parasites.
The mucosal immune system is strongly supported by the gut’s ability to sense secretions from parasites and microbial products, among other non-nutritive substances. In fact, bitter taste receptors in the stomach and small intestine play an important role in both regulating feed intake, and protecting the body from toxic substances because the majority of plant-derived toxins are bitter. If a bitter substance is detected by the enteroendocrine system, this causes a transient increase in feed intake (via ghrelin) but a long term reduction in feed intake. “Bitter” also affects gastric emptying and smooth muscle contractility to reduce food intake and increase satiety. Because “bitter” has a strong correlation with reducing food intake, it is being studied as a potential treatment for obesity.
Dr. Roura, from the University of Queensland, specifically discussed chemosensing in the porcine gut. Pigs and humans have more similarities in their taste receptor proteins than mice and humans. However, pigs are more sensitive to sour and umami tastes than humans. The pattern of taste receptor protein expression along the gastrointestinal tract, particularly the detection of long chain fatty acids in the colon, glucose and protein in the duodenum and jejunum, and umami detection along the entire gut are also highly similar between pigs and humans. This further supports claims that the pig is an excellent model of human digestive physiology.
Interestingly, when the genomes of pigs of different breeds from different parts of the world were compared to one another, multiple mutations were found in bitter taste receptors. In fact the nature these mutations clustered based on Asian and European origin. This indicates that the ability to detect plant-based toxins with different receptors for bitter taste is an indication of adaptation to an environment. Other work is being done to determine how the body’s detection of bitter substances affects gastric emptying and passage rate through the gut.
Overall, both speakers presented evidence that the human and porcine gut has precise methods to sense contents, protect the rest of the body and regulate nutrient intake, digestion and absorption.