The Gut-Brain Axis
There is a two-way communication between our gut and brain, named the gut-brain axis, which has several effects on our physiological and psychological health. One way this communication works is through our gut hormones and gut microbiome (1). Research in this area has been growing, what is presented in this blog is up-to-date as of publication, but more is needed to understand this relationship fully.
One way gut hormones influence appetite and satiety is by modulating “appetite centres” within the central nervous system, such as the hypothalamus and brain stem.
· Hormones that decrease our appetite include the so-called “satiety hormone” leptin, as well as Peptide YY (PYY), Glucagon-Like Peptide-1 (GLP-1) and Cholecystokinin (CCK) (1,2).
· Hormones that increase our appetite include the so-called “hunger hormone” ghrelin, as well as neuropeptide and agouti-related protein (1,2).
The Gut Microbiome
All healthy human adults have a gut microbiome, a collection of bacteria, viruses, archaea, and eukaryote along the gastrointestinal tract totalling more than 1014 microorganisms, belonging to more than 1000 species of bacteria (3,4). The bacteria are classified into four dominant phyla, Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. Then sub-classified into class, followed by order, family, genus, and species (5).
The gut microbiome has evolved with us to form a symbiotic relationship, with systematic beneficial effects on the hosts' health, including regulating appetite (5,6). The bacteria in our gut can affect our food preferences. Currently, the mechanisms behind this are not fully understood, but it has been hypothesised that the gut microbiome may influence host cravings for food which gives them a competitive advantage. For example, the phylum Bacteroidetes prefer fats, the genus Bifidobacteria prefer dietary fibre, and the genus Prevotella prefer carbohydrates (8,9).
Gut Hormones, Gut Microbiome and Appetite Regulation
Several studies have shown the central role of gut hormones and the gut microbiome in regulating our appetite (1).
Certain cells along our gastrointestinal tract, called enteroendocrine cells, release gut hormones. These cells are regulated by the composition of our gut microbiome (10). This has led researchers to suggest that interactions between our gut microbiome and enteroendocrine cells communicate with the brain as part of appetite (11). Furthermore, our gut microbiome produces intermediates called short chain fatty acids, such as acetate, which has been shown to also play a role in reducing appetite by increasing the release of satiety hormones, GLP-1 and PYY (1, 12).
Leptin is made by adipose cells and is a long-term mediator of energy balance by suppressing food intake. This is because once released by adipose cells into the bloodstream, it crosses the blood-brain barrier and tells us information about our body’s energy stores (1, 13). Ghrelin is a fast-acting hormone released by the stomach, increasing our appetite, and playing a role in the initiation of hunger. We often seen rises and falls of leptin and ghrelin throughout the day, corresponding with mealtimes. For example, ghrelin levels in our blood have been found to be nearly twofold increased immediately before meals and falling within 1 hour after eating (11,14).
The role of exercise
Currently, the research looking at the role of exercise on the gut and appetite is limited. However, it has been found that ghrelin is suppressed in response to aerobic exercise, which may be due to an effect on the systematic nervous system mediated by the hormones adrenaline and noradrenaline. Similarly, there is conflicting research into leptin, finding the effects of exercise to have either no effect or decrease the level of this hormone. Overall, more research is needed in this area to understand the role of exercise, and different forms of exercise, on our gut and appetite (15).
Hunger and satiety cues
Becoming aligned with our hunger and satiety cues is at the heart of intuitive eating. There are 10 principles to intuitive eating but let us introduce a few concepts included in those principles (16,17).
1. If you have foods you do not allow yourself, try and challenge this. Foods are neither “good” nor “bad”. Allowing ourselves all foods that we enjoy can help take us away from a diet mentality and get in tune with our hunger and satiety signals. We can start to do this by shutting down the “food police”, that inner critic telling us what we are “allowed” or “not allowed”. This is about turning our backs on diet culture, learning how unique and amazing each of our bodies is, and allowing ourselves to enjoy all the foods we enjoy (16,17).
2. Try to eat mindfully, encouraging your mind to slow down, being present, removing distractions, appreciating, and enjoying your food and how you feel can play a powerful role in building a positive relationship with our diet, body and internal signals of hunger and satiety. Research shows that may, at least partly, be attributed to paying attention to our food, enhances our memory that we have eaten, helping our brain to make decisions later about hunger and satiety (16,17,18).
3. Try and include some intuitive movement that you enjoy, shifting the focus to how it feels to move your body in a way that makes you feel good, makes you smile and feel energised. A few ideas may be walking, yoga, and mobility, stretching, hiking, running, weight training… what works in your routine and what you enjoy (16,17)!
4. A little gentle nutrition… including a range of plant foods that are nutrient-dense and rich in fibre, have been shown to support a symbiotic gut microbiome and keep us fuller for longer, for example, including wholegrains, nuts, seeds, fruits, and vegetables. Having a protein source in each of our meals can support our feelings of satiety. Also, remembering all foods play their role in a sustainable, balanced diet for us, foods have nutritional properties, it is important not to underestimate their role in being kind to ourselves (16,17).
Remember, it is about progress, not perfection! For more on the 10 principles of Intuitive Eating, you can see them here (19).
· Laura Thomas (Registered Nutritionist, RNutr) book “Just Eat It” (17).
· Megan Rossi (Registered Dietitian) website “The Gut Health Doctor” (20).
· Sophie Bertrand (Registered Nutritionist, RNutr) blog post “Intuitive Eating: Where Do I Start?” (21)
This blog post was written by Emilia Fish, a Food Science and Nutrition graduate MSc Clinical and Public Health Nutrition student at UCL. She has interned as part of the Nutrition Rocks team, has experience in Food Science labs and enjoys sharing simple, evidence-based nutrition on @nutritionnourishment. Emilia has recently launched a second series of her podcast, The Nutrition Nourishment Podcast: Sharing Our Journeys.
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