Do you experience food cravings, often for foods that are less than healthy? If so, you’re not alone. Research has shown that 68% of men and an incredible 97% of women experience food cravings. So what causes food cravings? Well, it’s a complex mixture of cultural, physical and psychological factors that historically have been very hard to pick apart. This is a problem. Food cravings are a huge health concern because of how big a barrier they are to weight loss and the management of diseases where your diet plays a big role, such as diabetes and heart disease. However, in recent years a growing body of research has started to lay bare the relationship between our gut microflora, diet and overall health. (Gut microflora is the collective name for all of the species of bacteria that call our gut home. Our guts each contain trillions of bacteria of countless different species. In fact, believe it or not, but the human body actually contains more bacteria than human cells!) Read on to find out how the gut communicates with the brain, and how gut bacteria hijack the process.
The Enteric Nervous System and the ‘Gut-Brain Axis’
The human body is controlled by a vast network of nerve fibres known as the nervous system. You can think of the nervous system as a computer, which is a more accurate description than you might think! The central nervous system is comprised of the brain and the spinal cord, which is where all of the information processing takes place. Think of the central nervous system as your computer’s processor, which receives input from all of the other computer chips, crunches the numbers and then sends commands back out to the rest of the computer, such as telling the screen what to display and the speakers what sounds to make. The central nervous system is similar, receiving information in the form of nerve impulses, hormones and neurotransmitters from the rest of the body, processes the information and then sends commands back to the rest of the body, such as telling your diaphragm to contract and relax, so that you can breathe in and out, or coordinating the different muscles in your arms and legs, so that you can walk. The rest of the nervous system outside of the ‘core’ central nervous system is known as the ‘peripheral nervous system’. This is the collective name for all of the other bits of the nervous system that send information to the central nervous system.
One large part of the peripheral nervous system is known as the ‘enteric nervous system’. This is a huge mesh-like structure of nerve fibres that surrounds the gut and controls its function. You can think of it as the gut’s own computer chip. And it is huge. It contains 500 million nerve fibres (also known as neurons), which is five times as many nerve fibres as the whole spinal cord. Have you ever been caught short, suddenly needing to use the bathroom when you’re out and about, with no idea where the nearest bathroom is? Well you can blame the enteric nervous system for that, because it’s the enteric nervous system that tells your large intestine to contract, signalling that it’s time to use the bathroom! In fact, the enteric nervous system is so extensive that it is often referred to as the ‘second brain’. And this description may be more apt than you might think, due to a growing body of research showing how the enteric nervous system and the brain communicate with one another. This is known as the ‘gut-brain axis’.
There are two main ways in which the gut and the brain communicate with one another. The first is via the nervous system. The enteric nervous system is connected to the central nervous system by the vagus nerve, which is one of the longest nerve fibres in the body. (Research has shown that blocking the flow of information up and down the vagus nerve leads to significant weight loss, while stimulating the vagus nerve leads to an increased appetite.) The second way in which the gut and brain communicate is through the circulatory system, in other words via the bloodstream. Research has discovered countless hormones and other signalling molecules that travel via the bloodstream between the gut and the brain to relay information. (One of these molecules is known as ghrelin. Ghrelin is produced when the stomach is empty, and travels to the brain via the bloodstream. Once in the brain, it connects to a receptor that causes feelings of hunger when activated. That empty stomach feeling that you sometimes get, or that feeling that your stomach is eating itself? The feeling isn’t coming from the stomach at all – it’s ghrelin in the brain tricking you into feeling that your stomach is eating itself, so that you go and have something to eat.)
Gut Bacteria Hijacking the ‘Gut-Brain Axis’
So we now understand how the gut communicates with the brain, but what do gut bacteria have to do with this? Up until recently, even scientists didn’t think that gut bacteria had much of an effect on the body. People thought that gut bacteria just sat there, and gut microflora was only of interest when somebody got food poisoning from food contaminated with a ‘bad’ species of bacteria, such as Salmonella. However, research over the past decades has shown that gut bacteria actually heavily influence communication between the gut and the brain, in ways that influence our mood, food cravings and overall health.
When gut bacteria digest food, they produce lots of different molecules as by-products. Some of these by-products are able to mimic the hormones, neurotransmitters and other signalling molecules that the human body uses to communicate between the gut and the brain. For example, certain bacterial by-products are able to mimic the hormones that the body uses to control feelings of hunger. One of these is ghrelin, a hormone that I already mentioned that makes you feel hungry. The opposite of ghrelin is known as ‘peptide YY’ and makes you feel full. In another example, two studies on chocolate cravings showed that people who crave chocolate have different bacterial by-products in their urine than people who don’t crave chocolate. (However, these studies may not be trustworthy, because I have never heard of somebody that doesn’t crave chocolate!)
A group of bacterial by-products caused by the fermentation of fibre in the gut are SCFA (short chain fatty acids). Some of these SCFAs have been shown to affect the way in which your body’s cells use certain genes. Even though our genetic code is fixed, all of our genes are not in use at the same time. At any one point in time, some genes are active, while others are inactive. Genes are like little instruction booklets that tell our cells how to make cellular components like proteins and enzymes. Many genes contain the instructions for making proteins and enzymes that have an effect on our weight, such as proteins and enzymes that affect our metabolism, food cravings, the storage of fat etc.
There is much interest in how these particular genes are activated and inactivated, because if we better understand the factors that affect them, we may be able to come up with better preventions or treatments for diseases related to our diets. Therefore, the fact that bacterial by-products can have an effect on the way that the body’s cells use genes is an important discovery. In one study, a particular SCFA produced by gut bacteria was shown to influence the use of genes by certain cells of the immune system in a way that had an effect on mood and behaviour. It is well-known that your mood is a huge factor affecting food cravings, with anxiety and depression strongly linked with cravings for unhealthy foods.
If you’d like to learn more about how diet and lifestyle affect the way in which your genes work, read our article ‘How Lifestyle Affects Your Genes’.
Some of the by-products produced by gut bacteria mimic human neurotransmitters, molecules that neurons use to communicate with one another. Other bacterial by-products mimic molecules that affect the way in which the nervous system is wired. It might sound odd, but the wiring of the nervous system isn’t actually fixed. It changes as we age in response to many factors, some of which are diet and lifestyle-related. Think about how you’ve changed over the last ten or twenty years. Think about how your personality has changed, how you have different likes and dislikes when it comes to music and movies and how your food preferences have changed. This is due to the wiring of your nervous system changing with time. This ability for the nervous system’s wiring to change with time is known as ‘neuroplasticity’ and research has shown that your gut microbiome can have an effect on it. A molecule known as BDNF strongly stimulates neuroplasticity and a study in mice showed that mice lacking a gut microbiome (i.e. mice with no bacteria in their gut) had much lower levels of BDNF than normal. Another study in mice lacking a gut microbiome also showed that they had much higher levels of anxiety than normal.
Some Gut Bacteria Are Picky Eaters – And How They Get What They Want
Our guts contain trillions of bacteria of countless different species. And just like humans, not all gut bacteria have the same food preferences. Research has split gut bacteria into two groups when it comes to food preference: the ‘generalists’ are able to survive on pretty much any diet, while the ‘specialists’ require a particular food source to survive. (Some gut bacteria even feed on us, with one species of bacteria found to survive solely on the mucus that the gut produces to lubricate itself!)
So some bacteria in your gut are picky eaters and are only able to survive on certain diets. Does that mean that their survival is at risk if you decide to change what you eat? Well, perhaps the choice of what you eat isn’t entirely up to you. We learnt earlier in this article that gut bacteria are able to hijack the communication between the gut and the brain through the various by-products that they produce. Some bacterial by-products affect how hungry you feel. Some by-products cause you to have cravings for certain foods. Some by-products affect your mood and behaviour, both in the short-term and, due to neuroplasticity, in the long-term. This can also affect your diet and food cravings, since the way you feel is one of the factors that effects your eating choices.
Therefore, in recent years researchers have started to propose that gut bacteria may be able to alter your food cravings in order to make you eat the foods that they need in order to survive. Even though the scientific community hasn’t discovered many direct examples of this happening, it is a sensible proposition. This is because it fits in perfectly with the principles of natural selection and evolution. We know that gut bacteria are able to have an effect on our cravings, due to the by-products they produce hijacking the communication between the gut and the brain. Therefore, bacteria that are able to make you crave the foods that they need to survive are going to have a much better chance of surviving than those bacteria that aren’t able to make you crave the foods that they need to survive.
Are We At The Mercy Of Our Gut Bacteria?
At this point, you may be feeling a little disheartened. We’ve not only learnt that our gut microbiome is able to affect our food cravings, but we’ve also learnt that the principles of natural selection and evolution make it fairly inevitable that bacteria are able to make you crave the foods that they need to survive. Since gut bacteria are also able to affect the way that your nervous system is wired via neuroplasticity, they may also be able to change your food preferences over the long-term, making bad habits a lot harder to break. All of this very much makes it seem like we are at the mercy of our gut bacteria. So is this true, or are there ways to break the cycle?
Well, research has shown that a varied gut microbiome tends to be associated with better overall health. In fact, a less varied gut microbiome tends to be associated with weight gain and obesity. It may be the case that as your gut microbiome becomes more varied, the different species of gut bacteria use more of their resources to compete against one another for space in the gut, with fewer resources being available to hijack the communication between the gut and the brain. Therefore, a good place to start may be changes to your diet to try and support a more varied gut microbiome. A varied diet rich in micronutrients will help to support many different species of gut bacteria. You can also try taking a supplement such as sense* for gut health, to help build up your body’s micronutrient stores. It may be challenging at first to make these changes to your diet, due to strong cravings for unhealthy foods caused by certain species of gut bacteria. However, it is possible to overcome this hurdle and break the cycle. People often find that cravings for unhealthy foods tend to subside after eating a more varied and balanced diet for a few months. This is likely due to changes in the gut microflora and subsequent changes to the wiring of the nervous system due to neuroplasticity.
Another way to help encourage a more varied gut microbiome is to incorporate more probiotics and prebiotics into your diet. The word ‘probiotics’ usually refers to foods or supplements that contain one or more species of ‘good’ bacteria. When you eat these foods, the ‘good’ bacteria can colonise your gut, thus helping to increase the variety of the gut microbiome. Earlier in this article we learnt that some bacteria are ‘selective’, meaning that they require certain foods and certain nutrients in order to thrive. Prebiotics are foods containing nutrients that help ‘good’ bacteria to thrive. Our sense* for gut health superfood supplement powder contains prebiotic nutrients such as inulin.
In this article, we learnt about the way in which the gut communicates with the central nervous system to influence food cravings, mood and overall health. We also learnt about how large a role the gut microbiome plays in this process and how certain species of bacteria can hijack your food cravings for their own benefit, leading to a vicious cycle that can be hard to break. However, we also learnt that it is possible to break the cycle. We learnt about the helpful role that probiotics and prebiotics can play. As the years pass and more research is done, we will certainly learn more about the factors that lead to food cravings and what steps we can take to minimise our cravings for unhealthy foods.