Always Hungry? Your Gut Bacteria Are Probably to Blame

eating-ice-creamConventional health & fitness wisdom says that the three main ingredients that are needed to produce weight loss are self-discipline (“Resist your cravings!”), willpower (“Get off your butt and start moving!”), and calorie counting (“Just eat less!”). The fat loss advice that has grown out of this belief system is “eat less, move more”, which is typically what we tell people who want to lose weight that they should do. Whether or not the complex machinery that is the human body is on board with this course of action is rarely considered; what’s emphasized is that the “driver” of the vehicle – generally thought to be the conscious human mind – has made a choice to stay the course.

Fitness magazines, television shows such as the Biggest Loser, and online weight programs have all contributed to spreading these beliefs out to the general public – installing them in the minds of millions of people. Moreover, they’ve made us believe that fat loss is a battle. If you’ve ever watched the Biggest Loser, you know what I’m talking about. The people on that show are miserable: they run many miles every day, take in very little calories, and are pushed to the brink of complete exhaustion by their instructors. Basically, they are going through a living hell.

These types of TV shows have made some people believe that weight management involves a ruthless fight between mind and body, in the sense that you constantly have to override the signals your body is sending you in order to lose fat. You shouldn’t eat more than a specific amount of calories a day, regardless of how hungry you are; you should exercise, even if your body screams that it needs rest; and you should count every calorie you take in.

Furthermore, these types of shows have probably contributed to spreading the popular belief that the key to fat loss and successful dieting is to be mentally strong, and that those who are fat and unable to lose weight are simply weak-minded and lazy. If they just pulled themselves together and got off their butts, they would lose weight in no time.

There are a lot of inherent problems with this belief system. There’s no doubt that willpower, self-discipline, and calorie restriction are all important parts of the weight loss puzzle. However, they are not the only parts – far from it. In order to understand why so many people find it hard to lose weight and what causes us to store excess fat, I would argue that we have to turn our attention away from simplistic concepts such as “calorie counting” and instead examine what controls our appetite and eating behavior and what causes us to make the choices we make.

It’s time we started paying attention to the part of the iceberg that is covered in water

Many years ago, before I started digging into the science on health and nutrition and began actively thinking about the mechanisms of body fat regulation, I was very much on board with many of the conventional ideas outlined above. I thought weight loss was almost exclusively a matter of self-control and mental stamina.

I know now better…

Imagine that all of the different hormones, behaviors, nutrients, and lifestyle factors that play a role in body fat regulation and obesity make up a large iceberg. Like most other icebergs that are floating around in the sea, this iceberg is largely covered in water. Only a small part of it is over the surface.

The things I mentioned in the beginning of the article are important components of this visible part of the iceberg. Unfortunately, this is the only part of the iceberg that a lot of people pay attention to. I would argue that it’s time we also started paying attention to what’s beneath the surface.

The human body, including its associated microorganisms, is not a machine that is under complete control by an operator – often perceived to be the human mind. Rather, it’s a complex biological system that has evolved over eons of time. This system has mechanisms that regulate appetite, body fat levels, and energy expenditure integrated into it. These mechanisms “don’t care” about the wishes of the person they are a part of – whether he’d like to lose weight or gain it, or whether he’s happy with his current taste preferences or not. They simply react to the types of stimuli and environmental signals they are subjected to. Their responses to different cues are largely determined by their evolutionary programming.

Beyond that, it’s important to remember that the human body doesn’t operate in isolation; it is attached to trillions of other life forms. All of these life forms have their own agenda; an agenda that sometimes conflict with the one of the human host.

The microbes that colonize the human body are obviously not acting for the benefit of the human host; they are acting in their own self-interest. They need energy to survive and reproduce and will do everything in their power to obtain that. Whether or not their interests are aligned with that of the other organisms of the ecosystem they are a part of is largely irrelevant.

Over the past decade, it has become increasingly clear to me that the human gut microbiota is a key regulator of host appetite. A lot of people seem to think that the brain, by itself, is in charge of overseeing and controlling energy expenditure, appetite, and eating behavior. I think this notion is too simplistic.

The brain obviously plays an important role in regulating how much we eat and the amount of calories that we burn off; however, it’s not the only operator controlling these things – far from it. Many other parts of the ecosystem that is the human body are involved as well. Particularly the gut microbiota seems to play a critical role in all of this. In some respects, it’s even more important than the brain.

The bacteria in your gut exert control over your brain and eating behaviour

The brain doesn’t operate in isolation. No bodily organ does. Rather, it sends and receives signals to and from the rest of the body. Many of the signals it receives are of microbial origin. They were produced by microbes that live deep in the gut, in the mouth, or in other bodily compartments.

The microbes that live in the human gut are capable of producing an impressive range of cytokines, neurotransmitters, nutrients, and other substances. Many of these substances can pass through the gut wall and enter into the systemic circulation of the host, where they act upon various organs. Some attach to receptors involved in regulating immune functions, others are used as energy, and yet others travel all the way up to the brain, affecting appetite regulation and eating behaviour (1, 2, 3)

It has been shown that some of the compounds that are produced by microbes in the gut are identical or similar to appetite hormones that are produced by the human body (1). Moreover, we know that gut bacteria can affect a variety of different bodily systems and organs that are directly or indirectly involved in appetite regulation. For example, they can stimulate the vagus nerve; they impact insulin signaling; and they seem to play a role in regulating leptin production and sensitivity (1, 4, 5).

My belief is that the microbiota makes up a large part of the aforementioned iceberg. The “problem” is that this part is found under the sea; it’s not easily visible. Hence, most people don’t notice it.

4 key points

As discussed in my interview with Joe Alcock, a pioneering researcher in the microbiome field, there’s still a way to go before we have a good understanding of the role gut microbes play in shaping the eating behavior of their host. With that said, we do know quite a bit. Moreover, we can make some educated guesses based on our knowledge about evolution, human behavior, and biology. The following points summarize my current view of the microbiome-host appetite connection. The statements I make are based on everything I’ve read about this phenomenon, my knowledge about evolution/biology, my own experience, and the observations I’ve made over the years.

1. The interests of the gut microbes don’t necessarily align with the interests of the human host

Just like other organisms, microbes act in their own self-interest. They are concerned with their own survival and reproduction. The interests of a microbe may conflict with that of its host. For example, some microbes, such as Streptococcus mutans, a bacterium known for causing oral cavities, proliferate when the human host eats sugary, processed food. From the perspective of these microbes, it’s good to get an influx of sugar; however, from the perspective of the human host, it’s probably not. A high intake of sugar could make the host fat and sick and  impair his libido and reproductive capabilities.

2. There’s a positive feedback loop
– Your microbiota is influenced by what you eat
– Your microbiota influences what you crave/eat

The microbes that occupy the long tube that runs through your body depend on you – the human host – to provide them with energy. If you don’t eat anything, some microbes may start digesting you – beginning with the lining of your intestine. As the nutrient supply declines, they will gradually wither and die. This is not a happy scenario, so let’s assume that are you eating something. If this is the case, the microbes in your gut will break down and utilize some of the nutrients that you take in. Whether or not a specific gut microbe is able to survive and reproduce in your gut is largely determined by what types of foods you’re eating. Some gut microbes do well on a meat-heavy, fat-rich diet, others thrive if you eat a lot of refined sugar, and yet others do their best work if you take in a lot of fiber.

Your dietary choices produce a positive feedback loop. For example, if you eat a lot of sugary, processed foods, “sugar-loving” microbes will proliferate in your mouth and intestine, whereas microbes that don’t do so well in the sugar-rich environment wither or die off. Due to their expanding numbers, the “sugar-loving” microbes will attain greater control over your appetite and eating behavior and will cause you to crave more sugary, processed food, because that’s the type of food that’s optimal in terms of their survival and reproduction.

3. Dysbiosis sets the stage for abnormal eating behavior and undesirable food cravings

One of the main reasons I started to suspect that there is a link between the microbiota and host appetite is that I had observed that people who harbor a severely dysbiotic microbiota exhibit abnormal, unhealthy eating behavior. For example, people with mental health disorders such as autism and ADHD, conditions that tend to go hand in hand with microbial imbalances in the gut, often experience regular cravings for processed, unhealthy food.

The exact types of foods that people with dysbiosis will be drawn towards will vary depending on inter-individual differences in microbiota composition. For example, someone with a lot of Candida albicans in their mouth and/or gut may crave and seek out a lot of sugary foods; whereas someone who has been eating a very fatty, fiber-depleted diet for a long time may have a preference for bacon, butter, potato chips, and other foods with a very high fat density.

The problems that a person with dysbiosis experiences will be particularly severe if his microbiota is rid of diversity, as this may allow certain types of hostile microbes to take over the system and profoundly impact the eating behavior of the person in question.

4. The keys to getting rid of undesirable food cravings, attaining better appetite control, and losing weight are likely to make lasting diet changes and manipulate the microbiota (The icing on the cake: Stress management, better sleep, and exercise)

A lot of people regularly experience cravings for unhealthy foods and find it difficult to take control over their appetite, eat healthier, and lose weight. For the sub-group of these people who harbor a diverse microbiota that has not become severely infested with weeds, diet changes (imprudent diet–>fiber-rich, species-appropriate diet) may be sufficient to remedy the problem, as long as they are lasting.

Simply changing your diet for one or a couple of days may not be sufficient to reconfigure your microbiota. Just a single meal can have significant impact on the composition of bacteria that live in your gut, but it’s obviously the totality of your diet that matters the most. If you for example eat an imprudent diet for many days, weeks, or months, your microbiota will settle into an unhealthy state and it may take quite a bit of willpower and time to turn things around.

Not everyone will find success with this type of plan though. People who harbor a microbiota that is severely degraded and disturbed may have to take additional steps beyond diet changes to bring about a significant improvement in the state of their microbiota. Fermented foods may prove very useful in some of these cases.

Besides diet and microbiota composition, stress levels, sleep quality, and physical activity levels are important determinants of eating behavior.

Okay, that was everything I had to say for today. Let me know in the comment section if you have any thoughts on the article.

Picture: Creative commons picture by Photo and Share CC. Some rights reserved.


  1. I got hungry early and often when I was eating a lot of grains and sweets. The first thing I did after getting out of bed was make a couple of slices of buttered toast. I was never heavily overweight, but after years of this type of eating, I was noticing my weight creeping gradually upward, which I’d never had a problem with previously.

    After floundering around with various diets that didn’t really work for me, I happened onto the Paleo lifestyle, which my body loved immediately. Lo and behold, I lost the cravings for sweets within a week. Grains were replaced by an increase in protein, fruit, and veggies. That was several years ago. I haven’t eliminated sweets and grains from my life totally, but my consumption of those foods is very infrequent. I no longer get hungry early in the day and routinely fast following an early dinner until close to noon the following day.

    This might not work for everybody, but it seems to come naturally for me. Meanwhile, I feel good and my weight has stabilized at a normal level. I have no idea if all this is due to better gut microbiota, but it’s as good a theory as any.

  2. So true! keeping hormones in check is huge.

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