Gut question!
This is your gut:
Well, your gastrointestinal tract actually. The word “gut” is clearly not a technical term… according to the Online Etymology Dictionary, it comes from the Old English “guttas” literally meaning “a channel” (related to the German word “Gosse” meaning gutter or drain! Nice.). However, it is commonly understood to mean any or all of the gastrointestinal tract from the stomach to the anus, and most commonly the intestines in particular, comprising the small (ileum) and large (colon) varieties, as labelled on a diagram gratefully replicated from Wikimedia Commons above.
Your gut is far from a simple channel or gutter, however. I love this quote from Michael Mosley’s book “The Clever Guts Diet” (see Useful Resources page):
…buried in our intestines, deep inside its tissue, is a very thin layer of brain.
Whilst the gut is there to channel our food, take out the goodness, and get rid of the waste, it also plays a pivotal role in the body’s immune system. As a number of scientists have pointed out in their fascinating articles about the complexities of the gut and its functions, the gastrointestinal tract is the largest part of the body’s surface that extends internally.1,2 It is because of this that it is constantly exposed to an enormous array of diverse environmental stimuli, ranging from your beans on toast and cup of tea to a myriad of microbes, and even inorganic materials such as trace elements from cooking utensils and food packaging (another controversial area of interest), heavy metals found in predatory fish, medicines and, in the case of my 1 year old daughter, wax crayons.1,2 Two of these scientists, Elena Tomasello and Sammy Bedoui, state that:
the intestine is therefore challenged with the daunting task of segregating the underlying tissues from potentially noxious determinants of the environment.1
So, in other words, our gut has to form a barrier between all those potentially harmful substances that we eat and drink (both purposefully and inadvertently – I mean, I don’t think most people mean to ingest the bugs that can cause a nasty bout of “the runs” but the poor gut has to deal with it!), whilst also remaining penetrable enough to allow fluids and nutrients through (the “goodness” we are trying to extract from our food).
Therefore, in addition to the simple physical barrier:
the intestinal tract is also equipped with a highly complex, intrinsic immune system.1
Which Tomasello and Bedoui’s editorial then goes on to describe in details far too complex for the scope of this blog, but do feel free to go and swot up on it if you’re interested and want some good bedtime reading! (see References below)
But I take you back to the concept of a “second brain” hidden within our gut. This actually has a name – the Enteric Nervous System. Sounds fancy. And it is! For a brilliant description, with diagrams, have a look at a free educational resource available from the British Medical Journal’s online journals site, originally having been published in Gut, the British Society of Gastroenterology’s official journal.3 It is a rather old article (17 years old), but I think it still holds up pretty well, or certainly to the degree that we need it here anyway.
The important take home message from it as far as I’m concerned is that we have known for some time that this “second brain” helps to control multiple functions, both within and distant to the gut, including those directly involved in the breaking down and digesting of our food and uptake of nutrients, and those which modulate a multitude of immune (defensive) and endocrine (body control) responses, many of which we still don’t clearly understand. A whole host of gut reactions in fact!
So, we know what the gut is and, in very basic terms, what it does.
Or do we….?
Now, this is where the microbiome bit comes in.
Glossary and Useful links (also see Useful Resources page)
Immune system: the body’s natural defence system
Microbes: microscopic organisms; living things that are so small we are unable to see them with the naked eye; these include bacteria, fungi and viruses (although there is some debate as to whether viruses are truly “living” as they require a living host cell to be able to survive, grow and reproduce); often colloquially referred to as “bugs” or “germs”
Inorganic: materials which are not made up of, or derived from, living things
Enteric Nervous System: a complex of nerves transmitting messages to and from, and within, the gut, also connected to the brain and spinal cord
Endocrine: this word is a little tricky to explain but it is basically to do with a system in the body that releases chemicals which act on other parts of the body to make things work… these chemicals are usually called hormones and they have a whole repertoire of different functions working across every system in the body (cardiovascular, respiratory, gastrointestinal, genitourinary/reproductive, etc.).
Microbiome: see What is a microbiome? post
References
- Tomasello E and Bedoui S. “Intestinal innate immune cells in gut homeostasis and immunosurveillance.” Immunology and Cell Biology (2013) 91, 201–203; doi:10.1038/icb.2012.85 https://www.nature.com/icb/journal/v91/n3/full/icb201285a.html
- Hooper LV, Littman DR, Macpherson AJ. “Interactions between the microbiota and the immune system.” Science 2012; 336: 1268–1273 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4420145/
- Costa, M, Brookes, SJH, Hennig, GW. (2000) “Anatomy and physiology of the enteric nervous system.” Gut. 47: iv15-iv19. PMC 1766806 . PMID 11076898. doi:10.1136/gut.47.suppl_4.iv15.http://gut.bmj.com/content/47/suppl_4/iv15