Articles on this blog have addressed the all-encompassing hype for the microbiome we have seen over the last few years. This is great for drawing attention to the importance of gut health, however we’ve also seen some over-hyped breakthroughs, and a hope that the microbiome is a cure for all ills. The truth is, we are only just starting to understand the effects of the microbiome and how it is composed. In light of the growing popularity of diet plans to ‘heal your gut’ and ‘fix your microbiome’, today I’ve decided to talk about how your diet can influence the composition of your gut bacteria, and whether it is even worth trying to change it.
Research has shown that diet is one of the quickest and most reproducible ways to alter the human microbiome. This is great news- but doesn’t necessarily mean it’s an easy or quick fix. We know a few ways different foods can alter your gut environment, however we don’t really understand how that affects your overall health, or what the best way to alter the microbiome is. In this article I’ve chosen a few examples of ways the food you eat alters your microbiome: fibre, probiotics and prebiotics and finally overall diet composition.
Short-chain fatty acids: your gut bacteria helps you digest food!
It is relatively common knowledge that fibre is good for your gastrointestinal health and also for keeping you regular. But why is that? There is actually three types of fibre, and one type, insoluble fibre, absorbs water, helping to soften bowel contents and keep bowels moving. Another type, resistant starch, moves to the large intestine, where it is fermented by gut bacteria into products called short-chain fatty acids (SCFAs). SCFAs include acetate, propionate and butyrate and have lots of beneficial effects. For example, butyrate is the major energy source for cells in the tissue of our large intestine and can alter gastrointestinal pH, which affects nutrient uptake. They’ve also been shown to have a positive effect in treatment of gut disorders like Crohn’s disease and some types of diarrhoea.
There is some research at the moment showing that nutrients in a Western diet is mainly absorbed in the small intestine, leaving nothing for the large intestinal bacteria. This means changes to our SCFA and gut bacterial population. On the other hand, in rural areas, SCFAs and presence of SCFA-producing bacteria are significantly elevated. This suggests that a good intake of fibre leads to a balanced microbial profile and healthy level of SCFAs.
If you’d like to know more about SCFAs and their wide-ranging effects on our health, this review is a good starting point.
So why is kimchi healthy?
You may have heard that fermented foods like kimchi, kombucha and yoghurt have health benefits- but you may not know why. Fermented foods are made by controlled microbial growth and using enzymes to convert parts of the food. Studies have shown good correlations between consumption of fermented foods and things like weight maintenance, reduced risk of type 2 diabetes and improved mood. Fermented foods will sometimes still contain bacteria as you eat them, and also contain compounds that help keep your existing gut bacteria healthy. They therefore may be considered a form of probiotics and prebiotics. Probiotics are microorganisms that have health benefits when taken. Probiotics can be found in foods such as yoghurt, supplements in a tablet or capsule, and other forms like Yakult.
Prebiotics have lots of different definitions, but are generally agreed to be indigestible food substances that act as ‘food’ for gut bacteria. There are lots of examples of prebiotics- some are compounds called lactulose, beta-glucan and inulin, which come from foods like oats, bananas and asparagus. We are still working out exactly how pre and probiotics work, and which exact strains of bacteria are the most beneficial. Probiotics are sometimes prescribed after an infection or gastrointestinal complaint. We still don’t fully understand which types of probiotic or prebiotic works best in certain situations, and whether they are useful to consume when you are healthy, or if they are better suited to improve a bacterial imbalance.
This video looks at using oats as a prebiotic.
Does your microbiome love fast food as much as you do?
There are multiple pieces of evidence to show that a Western-style diet with lots of fast-food leads to a dramatically different microbiome compared to a fresh-food based diet, or even an average Western diet with less fast-food. One article (not peer-reviewed, but interesting nonetheless) showed that a male who ate only McDonald’s for a week lost 40% of his microbial diversity. Microbial diversity is the total amount of species of bacteria you have, and has been shown to be an important indicator of health, and ability to recover from disease.
Another study compared the microbiome of children living in urban Italy to children in a rural village or a city in Burkina Faso. The rural children had some unique types of bacteria that are known to ferment fibre and vegetable matter. Another study looking at a rural diet compared to a Western diet showed increased diversity and new bacterial types in the rural diet. These changes may be due to newer ways of food processing and some food additives.
It seems clear that our Western diet plays a role in changing our microbiome. Maybe eating unprocessed foods is a good way to counteract this? Still, more research is needed to understand if these changes to our gut microbiome are problematic and leading to disease, or if other factors such as genetics are more important.
If you’d like to know more about the Western diet and its changes to the gut microbiome, these very quick audio clips from NPR are great: https://www.npr.org/sections/goatsandsoda/2017/08/24/545631521/is-the-secret-to-a-healthier-microbiome-hidden-in-the-hadza-diet, https://www.npr.org/sections/goatsandsoda/2015/04/21/400393756/how-modern-life-depletes-our-gut-microbes.
Is it worth buying diet books to ‘supercharge’ and ‘heal’ your gut?
Overall, on the whole - no. There is as yet no reproducible, concrete evidence of a particular ideal bacterial profile. Just eating a food that can increase one type of bacteria will probably not be able to increase your overall health. Additionally, we know that your gut microbiome is personalised to just you - so why would there be one way to improve everyone’s gut microbiome? Future research may be better able to show how overall microbial profiles link to health or disease, but we are not quite there yet. The boring answer is that it seems that a varied diet full of fresh fruits and vegetables and resistant starches, low in highly-processed foods and maybe with a few fermented foods thrown in is the best way to keep your gut bacteria happy!