We know that our gut microbes are heavily involved in the activities of our nervous system, our hormonal system and our immune system. They are operating via many different pathways on numerous levels. Scientists are now questioning whether these microscopic power houses may be influencing our desires and cravings for different foods. We know they can influence our taste receptors, our appetite hormones and when our gut microbiome lacks diversity, it appears this may allow some species too much power and influence over their host (that’s you and me). Read on for more detail.
Scientists are only just starting to scratch the surface in terms of the enormous influence our gut microbes have over us. We know that our microbes influence our health and well-being via the by-products they produce (some nourishing, some not), via their interaction with our hormonal system (they are involved in the manufacture of hormones) via their interaction with our nervous system (they produce neurotransmitters and brain chemicals and communicate with our brain via the vagus nerve) and via our immune system (which they help programme, develop and modulate).
Evidence suggests that the vagus nerve is the communication route by which eating behaviour and body weight is regulated. And we know that most of the neuronal activity in the vagus nerve is from information being sent to the brain from the gut rather than in the other direction. Given that we know our microbes communicate with and influence us and other microbes in a variety of ways, it seems likely then, that as part of their own survival mechanism, they may well influence our desire for certain foods.
Depending on our diet, different enzyme-producing microbes thrive and different by-products are produced. Some of them nourishing, others damaging. What we eat influences which microbes flourish and the diversity of our microbial community (1 and 2). Whilst many microbes can manage on a wide range of nutrients, some thrive on specific ones. For example, we know that bifidobacterium and lactobacillus species thrive on prebiotic food such as onions, garlic, Jerusalem artichokes. Other bacteria thrive in a low carbohydrate environment and feed on the mucus lining of the gut. Microbes that digest seaweed have been isolated in Japanese people and African children raised on sorghum have been shown to have unique microbes that digest the cellulose it contains.
We know that our microbes influence gene expression in terms of how we metabolise food and whether we are obese or not (3 and 4). We also know that hormonal cascades govern our hunger levels and feelings of fullness and our microbes are intricately involved in this delicate dance. It is not a giant leap then, given their intricate involvement in all of these processes, to see how our microbes could be influencing our desires or cravings for certain foods in order to ensure they are supplied with the nutrients they need.
how are they pulling our strings?
Research in this area is very young but here are some of the means by which our microbes may be influencing our eating habits:
Research suggests that our microbes might affect our taste receptors both on our tongue and in our gut (yes, we have taste receptors in our gastrointestinal tract). This has been shown in various experiments on mice and it has also been observed in humans after gastric bypass surgery, a procedure that also changes gut microbes and alters food preferences and feelings of hunger and fullness.
We know that our microbes are involved in the activity of our appetite hormones such as leptin (fullness) and ghrelin (hunger). Many bacteria even make substances that mimic these hormones. To avoid the microbes running the show, our bodies can produce antibodies directed against these microbial substances. It is our body’s way of keeping hold of the appetite reins. However, these antibodies can also end up acting against our own hunger and fullness hormones rather than solely against the microbial ones. Effectively, when this happens, we have created an autoimmune response. This autoimmune response can result in our microbes basically having the capacity to manipulate our eating behaviour, not only by producing substances that mimic our hunger-regulating hormones, but also by indirectly stimulating an autoimmune response that interferes with our own appetite regulation hormones.
Our gut microbial community is a competitive place to be. There is constant competition over habitat and available nutrients. This is one of the reasons that a highly diverse microbiome is a healthy one, because it ensures there is no room for pathogenic bacteria to strong arm their way in. In addition, high diversity also means that our microbes are more focussed on competing for energy and resources compared to a less diverse microbial population. Any individual species will tend to occur at lower abundance when there is high diversity. A less diverse microbial tribe increases the chance that a particular species dominates and, therefore, has more resources available for manipulating us, the host, through higher levels of coordinated behaviour. I don’t have room to describe it here, but if you really want to mess with your head, google quorum sensing in bacteria to read about how, when bacterial communities reach certain sizes/density of population, they coordinate changes in their behaviour from colonisation to, say, invasion of tissue. Quorum sensing may be one way in which microbes coordinate their behaviour to manipulate our eating habits to enhance their own nutritional supply.
So, it is highly plausible that, via various mechanisms, our microbes could be influencing our desire for certain foods. Is this part of the reason why there tends to be a lag between cutting out or cutting down on sugar, for example, and our cravings ceasing? Perhaps this is due to the time it takes for the sugar-mad microbes to die back into better balance with others, thereby relinquishing their power over our mind?
Interestingly, some activities, such as exercise, meditation and yoga are thought to strengthen will-power and improve alignment of food consumption with energy expenditure. These behaviours also work via the vagus nerve.
I guess the question is, who do we want running the show? I am not sure it is as simple as us or them, but we certainly have some influence over whether it is a good range of diverse microbes in the hot seat in one of our most important control rooms or a poor range that lack diversity, thereby allowing some dominant, possibly troublesome voices to take hold of some of the controls.
the usual suspects
Not surprisingly, it is the same foods that reduce our microbial diversity and increase gut microbial dysbiosis that also snare us in the pleasure trap. Read my post on The Pleasure Trap. Limiting those highly processed foods that have little nutritional value will benefit us in both departments! BUT, it is not just a matter of cutting out the nutrient black hole foods, we also need to increase our intake of a diverse range of nutrient dense foods – vegetables in all shapes and colours, beans, lentils, peas, as good quality meat as you can afford (but not too much of it), the same goes for cheeses – unpasteurised where possible. Why? Because these foods are packed with the nutrients and fibre our body and therefore our microbes need to function healthily. Eating a diverse array of whole foods helps to ensure that we have a diverse gut microbiome and we know this to be important for avoiding puppet-like behaviours!
Shop to maximise the amount of natural whole foods in your house.
Cook to maximise the amount of natural whole foods in your colon.
- Diet rapidly and reproducibly alters the human gut microbiome: Lawrence A. David, et al Nature January 2014
- Prior Dietary Practices and Connections to a Human Gut Microbial Metacommunity Alter Responses to Diet Interventions Nicholas W. Griffin, Jeffrey I. Gordon et al Cell Host and Microbe January 2017
- Bacteria from lean cage-mates help mice stay slim: Jeffery Gordon et al Nature September 2013
- The impact of a consortium of fermented milk strains on the gut microbiome of gnotobiotic mice and monozygotic twins: McNulty NP, Gordon JI et al. October 2011
- Is eating behaviour manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms: J. Alcock et al NCBI (August 2014)
- Do Gut Microbes Control Your Food Cravings? Chris Kresser July 2016