International Business Department           Liu Bojia           June 24, 2023

  Overweight and obesity have become a common type of metabolic disease today, and it often leads to more adverse health consequences, such as an increased risk of cardiovascular disease and diabetes. In addition to excessive energy intake, obesity is often associated with an imbalance in the body's energy balance mechanisms, which directly contributes to our inability to properly feel full and therefore make poor eating decisions.

  There are many molecules in our body that are involved in the regulation of energy intake, such as leptin, which is secreted by adipose tissue and enters the circulatory system to suppress appetite when food intake increases. The gastrointestinal tract also has a similar mechanism of manipulation. The peptide PYY, produced by L cells, the endocrine cells of the intestine, increases rapidly one to two hours after a meal, and this peptide also reduces our hunger and prompts us to stop our eating behaviour.

  PYY gets its name from its special structure, which consists of a total of only 36 amino acids with a tyrosine (Y) residue at each end, hence the name peptide tyrosine-tyrosine, or peptide YY. The primary source of PYY in our bodies is in the ileum, and one study, for example, found that a direct infusion of food into the ileum caused a spike in PYY levels. But what types of foods are most effective at triggering PYY production? A paper from Science Translational Medicine has the answer: high-fibre foods.

  According to the paper, a total of 10 volunteers took part in the trial with three types of diets, including high-fibre but unprocessed (I-HF), high-fibre and processed (D-HF), and low-fibre and processed (LF) foods. The researchers would collect samples from the ileum through a special tube with a nasal endoscope, and they found that the ileum responded quickly to regulate the amount of PYY released when shifting from fasting towards ingesting food.

  At 2 hours after eating, significantly more PYY was detected in ileal samples from volunteers who consumed I-HF and D-HF compared to those who consumed LF, and these volunteers were more likely to report that they were "full". Meanwhile, all volunteers in the high-fibre group showed a stronger appetite reduction effect, suggesting that foods high in fibre, whether or not their original food structure has been disrupted, can help to suppress the appetite, rather than being more natural, as has been previously thought.

  Under the influence of high-fibre foods, the levels of fructose and amino acids such as tyrosine, phenylalanine and aspartic acid in the ileum increase rapidly, and these conditions are conducive to the release of PYY. The authors attempted to incorporate ileal organoids in vitro with either ileal extracts from volunteers or mixtures of fructose and amino acids, both of which elevated PYY-encoding gene expression in the organoids and facilitated PYY release.

  In addition to PYY, the authors found that the microbiome in the ileum changes dynamically with feeding status, with ileal motility largely stagnant when fasting at night, and populations of specific species of bacteria, such as Proteobacteria, increased. When food is ingested, most of the bacteria in the ileum will be flushed into the colon, and with the entry of digestive enzymes and bile acids, the majority of the bacterial population plummets, and the total number of bacteria is only 1/25 of that at night, and the bacteria that predominate after eating become the Firmicutes bacteria.

  Streptococcus and Lactobacillus in the Firmicutes phylum typically favour simple carbohydrates, suggesting that fermentation in the ileum is driven by simple carbohydrates rather than complex carbohydrates, and that these changes may also be influenced by dietary structure.

  The authors suggest that dietary composition may be a safer means of weight loss than PYY injections or other medications, and that foods such as oats and legumes, which contain both dietary fibre and specific amino acids, are ideal "hunger-supporting" sources of food. If we want to control our weight, we can consider consuming these foods with meals to help control the amount of food we eat.