The ongoing battle between meat lovers and veggie advocates is unlike to end anytime soon. It doesn't really help that many popular diets go to extremes of the spectrum: from the meat-heavy Atkins, phase 1 Dukan, and paleo diets1 to the veggie-praising strict vegetarian, and vegan ones,2 there is a choice of fad diet to meet your taste. But many people say they either reduced or completely stopped eating meat due to health concerns. Here at ZXplainer, we already covered the effects of dietary habits on health, and we made clear that animal products can be perfectly healthy, so we will not make you sit though all that again... Today, we will explore another question: Who - or, more to the point, What - is really behind that choice?
Meat: eating it or not, you might not be really that free to choose...
Image Credit: Bru-nO @ Pixabay.
People from Western societies are cutting back on meat. Take the UK for example: back in 2014, 58% of a total of 2,878 participants said they were considering taking less meat, or totally axing it from their diets, for health reasons.3 A similar trend was also observed in the US, where a 2015 survey on eating habits showed that 50% of the 1,112 respondents were reducing meat consumption due to health concerns.4 It is not that these folks are not justified in their consideration to eat less meat: ingesting the stuff, particularly red and processed meat, has been linked to higher risk of cardiovascular diseases,5 with one study expressly advocating against diets which are low on carbohydrates and high on animal-based proteins6 - cough, cough, fad diets.
You see, the big motivation for this crusade against processed and red meat has a culprit, which goes by the name L-carnitine5b,7 - yes, because chemistry. But there is a catch: this chemical compound, a small molecule that is found in red meat,5b does not do all the damage alone, it needs some extra help. What cruel being could possibly be involved in this almost criminal disease fest? The microbes inside your gut! And their estimated population reaches 100 trillion!8 - No, numberphiles around here, that was not a factorial... but 18!·2-6 is a factorial-like approximation.
That is right, the microscopic fellows that make an exclusive living inside our intestines - a very poor choice of scenery in my opinion - are the ones to blame here. Researchers found that particular strains of bacteria that take residence inside our guts are able to transform L-carnitine in other compounds5b (mostly TMA9a and TMAO,9b for my fellow chemists and biologists around). And it is when these so-called microbiota metabolites accumulate inside the human intestine that they are redistributed throughout the body, with express delivery guaranteed by the high blood flow around the gut, and start to wreck havoc.5b For a start, these compounds were systematically linked to the development of intestinal cancer and cardiovascular diseases, they also accelerate generalised atherosclerosis by making it harder for your body to keep your cholesterol levels in check.5b The gut microbiome, as these microorganisms are called, were also implicated in the development and progression of disorders such as irritable bowel syndrome,10a Crohn's disease,10b gastric cancer,10c obesity,10d type 1 diabetes,10e rheumatoid arthritis,10f autism,10g,h chronic fatigue syndrome,10i Alzheimer's disease,10j and cancers.8 By now you may be thinking that they have already done enough trouble for a day... I hope you fastened your seat belts, because you are in for a ride!
Microbes living inside your gut have much more control over you than you might think!
Image Credit: qimono @ Pixabay
Numerous as these small critters are, it is not that surprising that so many diseases are influenced by them. But research is still in its early days and, except for the L-carnitine based relationships mentioned before,5b most of the available data is correlational, meaning that we still have work to do if we want to find the particular microbe and metabolites associated with disease progression.8 One thing is already clear, though, the gut microbiome is able to do a lot of stuff. We already knew that these little fellows were helping us to produce vitamins (B9, B12 and K2),11a and that they were linked to a better absorption of minerals and protection of the gut mucus, a natural sticky secretion that protects the interior walls of our intestines.11a
More recently, with the launch of two large scientific collaborations, the Human Microbiome Project (HMP) in the US and the Metagenomics project of the Human Intestinal Tract (MetaHIT) by the EU and China,11b scientists were able to make important discoveries about the interactions between the gut microbiota and their hosts (aka humans). For a start, we discovered the microbiome is able to ferment the sugars present in dietary fiber, which we are unable to digest by ourselves, producing a group of molecules called short chain fatty acids (SCFAs for short).11 These metabolites can then be used by our liver to produce cholesterol, an essential nutrient for us; SCFAs have also been implicated in controlling hypercholesterolemia (high levels of cholesterol in the blood), decreasing hunger, reducing inflammation, inhibiting cancer development, and protecting the gut mucosa.11 But it does not stop there! They benefit us by producing other chemicals such as equol, an antioxidant with anti-inflammatory properties, which also helps reduce bone loss and serve as substrate for oestrogen production - a human horomone.11a
But the great stars of this post are not in this chemical cocktail. The celebrities today are the last metabolites I will mention, I promise. They are four in total: γ-aminobutyric acid (GABA), noradrenaline, dopamine and serotonine.11,12 Now, if you ever heard about something called "neurotransmitters," those four names should ring so many bells that you could sing some good Christmas carols. And you would be completely right! Most recently, researchers have discovered a large number of gut-brain interactions that were dubbed the "gut-brain axis".12,13
And what does that mean? Microbes inside our gut are communicating directly with our brains.11-13 Well, "that is coll and all" you might say, however it does have profound implications. The gut-brain axis has been identified as a major contributor to mood and human behaviour.12,13 It gets even more scary... One particular study extracted microbes from a person with major depression and injected them into laboratory mice via fecal transplant - yes, it is as gross as it sounds - directly into the rodents' gut. The result? Mice that received these microbes quickly developed symptoms associated with major depression,12 showing a direct link between psychological health and the gut microbiome (also, see an extra case that I included down below). The gut-brain axis was also implicated in controlling hunger by either making you fill satisfied,11,13 or causing carbohydrate depletion in blood, triggering the famous sugar crazes.11 In other animals, researchers also showed that specific microbes forced their hosts to eat certain foods,14 indicating the gut microbiota is picky about what it needs and induce animals to choose what they eat.
These studies are still recent, most of them surfaced within the last decade or so, and much is yet to be discovered. For instance, despite strong evidence that the gut regulates hunger and mood, and can trigger psychological disorders,12,13 the mechanisms by which these little fellows influence us are still unknown. Microbiome diversity is known to be linked to good health,11 but since the amount of microbes inside our intestines amounts to trillions, a lot of work needs to be done if we are to pin down all important gut-brain interactions that affect human behaviour and health in general.
For now, let's encourage scientists to continue their studies and wait for what the future findings about out fellow symbiotes bring us.
By the way, the choice of whether it is you or the gut microbes that make the decision about meat is still under debate. But there is a good chance that we should trust our gut feeling and wager on the side of our little friends.
Science Extras
In another scientific study, 25 overweight children in the age range between 6 to 12 years old, together with their obese parents, were treated for 8 months with a diet that prioritised food diversity and inclusion of dietary fiber and complex carbohydrates. As a result, these children lost weight and maintained their body mass well into adulthood. How about their parents who also kept the diet for the same amount of time? They continued overweight despite loosing some weight, but never reached a healthy body mass. All these cases point to the well-known resilience of sick gut microbiota in adults to recover their original diversity.11b
Additional Resources
Do you want to learn more about the gut microbiome? You can head to the World Science Festival channel on YouTube to watch a fascinating panel of four leading scientists in the field of gut microbiome. By the way, this channel is a masterpiece of YouTube if you are into science.
Talking about amazing videos... SciShow has covered the microbiome in two episodes: one here and another piece here. Enjoy!
Bonus Facts!
- The paper reporting on the 2015 survey in the US brought us the following gem:4
"We excluded twenty-five [participants] who unrealistically completed the survey in under 4 min".
- In the early 1980s, in the dawn of the research about interactions between microbes and digestive organs, a doctor named Berry Marshall joined the hospital pathologist Dr Robin Warren to study gastric ulcers.15a After several investigations, Marshall isolated samples of bacteria called Heliobacter pyroli from autopsies that Dr Warren conducted in his ulcerous patients. Together, they discovered these microbes were present in all cases of gastric ulcers, and caused inflammation that was likely to lead to disease progression.15b
Despite extensive work, they were unable to convince the scientific community. After all, they could never take the bacteria and inject it in healthy humans, that wouldn't be ethical. It was then that Marshall drank a broth of H. pyroli, rapidly developed gastric ulcers, finally proving the link between the microbe and the disease, and was, together with Dr Warren, awarded the 2005 Nobel Prize in Physiology or Medicine for their astounding discovery.15b
Do you have any comments, questions or suggestions? Reach me out in the comments down below.
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1 National Health Service UK - NHS, Top diets review, https://www.nhs.uk/live-well/healthy-weight/top-diets-review/, retrieved on 10/04/2019.
2 National Health Service UK - NHS, Vegetarian and vegan diets Q&A, https://www.nhs.uk/live-well/eat-well/vegetarian-and-vegan-diets-q-and-a, retrieved 10/04/2019.
3 Lee, L.; Simpson, I., Are we eating less meat? A British Social Attitudes Report, London: NatCen Social Researh, 2016, 31p.
4 Neff, R. A.; Edwards, D.; Palmer, A.; Ramsing, R.; Righter, A.; Wolfson, J., Public Health Nutr. 2018, 21, 1835.
5 a) Key, T. J. et al., Public Health Nutr. 1999, 1, 33; b) Koeth, R. A. et al., Nat. Med. 2013, 19, 576; c) Song, M. et al., JAMA Intern. Med. 2016, 176, 1453;
6 Seidelmann, S. B. et al., Lancet Public Health 2018, 3, 419.
7 Wikipedia - The Free Encyclopedia, Carnitine, https://en.wikipedia.org/wiki/Carnitine, retrieved 10/04/2019.
8 Paul, B.; Barnes, S.; Demark-Wahnefried, W.; Morrow, C.; Salvador, C.; Skibola, C.; Tollefsbol, T. O., Clin. Epigenetics 2015, 7, 112.
9 a) Wikipedia - The Free Encyclopedia, Trimethylamine, https://en.wikipedia.org/wiki/Trimethylamine, retrieved 10/04/2019; b) Wikipedia - The Free Encyclopedia, Trimethylamine N-oxide, https://en.wikipedia.org/wiki/Trimethylamine_N-oxide, retrieved 10/04/2019.
10 a) Rajilić-Stojanović, M. et al., Gastroenterology 2011, 141, 1792; b) Martinez-Medina, M. et al., Inflamm. Bowel Dis. 2006, 12, 1136; c) Ley, R. E. et al., Proc. Nat. Acad. Sci. 2005, 102, 11070; d) Kuitunen, M. et al., Autoimmunity 2002, 35, 365; e) Loscalzo, J., Circ. Res. 2011, 109, 127; f) Vaahtovuo, J. et al., J. Rheumatol. 2008, 35, 1500; g) Bolte, E., Med. Hypotheses 1998, 51, 133; h) Parracho, H. M. et al., J. Med. Microbiol. 2005, 54, 987; i) Logan, A. C.; Rao, A. V.; Irani, D., Med. Hypotheses 2003, 60, 915; j) Samsel, A.; Seneff, S., Entropy 2013, 15, 1416.
11 a) Vlieg, J. E. T. H.; Veiga, P.; Zhang, C.; Derrien, M.; Zhao, L., Current Opin. Biotech. 2011, 22, 211; b) Heiman, M. L.; Greenway, F. L., Mol. Metab. 2016, 5, 317.
12 Sharon, G.; Sampson, T. R.; Geschwind, D. H.; Mazmanian, S. K., Cell 2016, 167, 915.
13 Heijtz, R. D. et al., Proc. Nat. Acad. Sci. 2011, 108, 3047.
14 Leitão-Gonçalves, R. et al., PLoS Biol. 2017, 15, e2000862.
15 a) Weintraub, P., The Doctor Who Drank Infectious Broth, Gave Himself an Ulcer, and Solved a Medical Mistery, Discovery Magazine, published 08/04/2010, http://discovermagazine.com/2010/mar/07-dr-drank-broth-gave-ulcer-solved-medical-mystery, retrieved 11/04/2019; b) NobelPrize.org, The Nobel Prize in Physiology or Medicine 2005, Nobel Media AB, published 03/10/2015, https://www.nobelprize.org/prizes/medicine/2005/press-release/, retrieved 11/04/2019.
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