Does gut flora composition influence food cravings and dietary habits?

It’s well established in the scientific literature that diet affects gut flora compostion, but my hypothesis is that it also happens the other way around; that gut flora influences food cravings and dietary habits.
It’s often suggested that humans are designed to desire sweet, fatty and salty foods. Naturally, these cravings come and go throughout the day, but there also seems to be an overall degree of individual variability. While some people claim to have an intense sweet tooth, others rarely feel the need to consume fruits, cakes, sodas etc.
It also seems that someone can “permanently” alter their food preferences and food cravings by changing their diet or supplements. While some people attribute these changes to hormonal factors, blood sugar etc., gut flora also seems to be a vital part of this whole puzzle.
 
Content:

• The second brain
• Gut microorganisms aren’t working for us
• Anecdotal reports and studies supporting the hypothesis
  • Individuals with chocolate cravings have different gut flora composition
  • Dysfunctional gut flora and gut dysbiosis (candida overgrowth?) are linked to “intense” cravings
  • Obese and lean individuals have different gut flora composition
  • Low-carbohydrate diets linked to initial carbohydrate cravings, but no cravings following an “adaptation phase”
• Conclusions

The second brain

Studies show that there is a strong link between the gut and the brain, and the gut is often considered the second brain. Gut microbiota influences mood and behaviour (Clark et al., 2012; Collins, Surette, & Bercik, 2012; Diaz Heijtz et al., 2011).

Gut microorganisms aren’t working for us

Contrary to popular belief, the microorganisms hosted by a human body aren’t working for us. Rather, they are interested in their own survival and reproduction. By design, this is a mutual beneficial symbiotic relationship with the human host. E.g., certain microorganisms are able to ferment specific polysaccharides that in turn become energy that we can use.

Different microbial species are also able to suppress the growth of other microorganisms (i.e., by altering the gut pH, pH in fermented foods) in order to optimalize their own living conditions. E.g., it has been shown in several studies that the growth of bifidobacteria in the intestines is linked to suppressed growth and/or death of other microorgansims.
While more bifidobacteria is usually considered beneficial, other cases of this “microbial war” are deterimental to human health. This is seen when specific members of the opportunistic flora get a chance to overgrow (e.g., antibiotics, poor diet) and are able to suppress beneficial microbes.

Is it possible that gut microorganisms aren’t just able to alter their immediate environment, but also influence the behaviour of the host to benefit themselves? This could potentially happen by some type of biochemical/metabolic response, and Petro Dobromylskyj believes that it happens when microbes alter peptide neurotransmitters in our brain.

Several gut microbes seem to thrive on glucose (e.g., certain LPS-containing bacteria, Candida albicans), and the “overgrowth” of these microbes could potentially result in cravings for carbohdyrate-rich foods such as bread, fruit, cupcakes etc. These cravings might happen between meals, but can also be also be “intesified” during or right after meals.
Someone who consumes carbohydrate-rich foods often feel the need to consume more of the specific foods. Several explanations have been proposed for this continued “glucose” craving, but gut flora’s role has been mostly unrecognized. Is it possible that the growth of certain intestinal microorganisms (upper GI tract?) following the consumption of carbohdyrate-rich foods leads to a desire to eat more “sugar”?

Fat cravings (?) probably result from the same mechanims, but involve other microbial species.

Someone who’s already got a dysfunctional gut flora or gut dysbiosis is probably more susceptible to these food cravings both between and during meals.

Anecdotal reports and studies supporting the hypothesis

Individuals with chocolate cravings have different gut flora composition

A study done by Rezzi et al. (2007) showed that individuals who have an appearant desire for chocolate have different colonies of bacteria in their intestines compared to people who are “indifferent” towards chocolate.
Blood and urine of 11 men who ate chocolate on a daily basis were compared against 11 men who never ate chocolate. When the researchers looked at the byproducts of their metabolism they found several differences between the two groups. The specific substances that differed are linked to different types of microorganisms. Although this isn’t a very good study, it gives us some hints about the relationship between food, bacteria and cravings. Regular chocolate consumption results in an altered gut flora composition, but this study suggests that certain bacteria also influence the desire for chocolate.

I suggest that “chocolate lovers” don’t actually have an altered gut microbiota prior to introducing chocolate into their diet, but rather that chocolate consumption leads to the growth of specific microorganisms, which in turn promote regular “cravings” for chocolate.

This explains why so many people have a hard time quittting chocolate all together and why removing chocolate from the diet completely is often necessary to get rid of the cravings. Both sugar and cacao (found in chocolate) have been shown to be highly addictive, and this is probably influenced by gut bacteria. Certain compounds found in sugar and cacao (e.g., alkaloids in cacao) probably also contribute to the regular cravings for foods that contain these products.

Dysfunctional gut flora and gut dysbiosis (candida overgrowth?) are linked to “intense” cravings

A dysfunctional gut flora and Gut dysbiosis are often accompanied by an increased desire or craving for specific foods. (e.g., foods high in sugar) A typical western diet will promote the growth of certain bacterial species in the intestines. Building on the information in the previous paragraph, it seems possible that the subsequent growth of certain microbial species following antibiotics, poor diet etc. results in an increased desire for foods that contain substances that feed these microbes.

These cravings depend on the severity of the condition, and individuals with a total gut flora disruption (severe gut dysbiosis) will usually have the most appearant food preferences and cravings. As mentioned in the previous paragraph, these cravings might be intensified during or directly following meals.

Two groups with moderate-severe gut dysbiosis who experience an appearant desire for certain foods:

- Candida albicans seems to thrive on glucose, and Candida overgrowth is “always” linked to an increased craving for “sweet foods”.

- Mental disorders such as autism and ADHD are linked to gut issues, and Dr. Natasha Campbell-McBride has worked with hundreds of children with a wide range of mental problems. These children suffer from gut dysbiosis, and Campbell-McBride says in her book (Gut and Psychology Syndrome) that all of the children also seem to have very specific food preferences, food cravings etc.

Have you ever meat a child with autism or ADHD that doesn’t have very “unusual” dietary habits?

Obese and lean individuals have different gut flora composition

It’s well established in the scientific literature that obese and lean individuals have different gut flora composition. Some studies show reduced numbers of Bacteroidetes in obese subjects, while others point to lower levels of beneficial bacteria like Lactobacillus.

Ian Spreadbury proposed in his excellent paper (Spreadbury, 2012) that a typical western diet, high in refined, acellular carbohydrates, promote the growth of specific intestinal microorganisms such as LPS-containing bacteria. He further hypothesise that this altered microbiota is a crucial part of inflammation and obesity.
So, it’s well established that a poor diet leads to altered gut flora and that it’s possible to establish if a person is lean or obese by looking at the gut microbiota. Is it also possible that the specific gut flora composition in the overweight and obese is part of the reason why they are often drawn towards refined, sweet and/or fatty foods? Will an established inflammatory microbiota further increase the satiety for “unhealthy” foods?

Low-carbohydrate diets linked to initial carbohydrate cravings, but no cravings following an “adaptation phase”

Anecdotal reports suggest that most individuals who switch from a high-carbohydrate diet to a low-carbohydrate diet have to go through an initial adaption period where they experience dizziness, fatigue, carbohydrate cravings etc. These symptoms seem to correlate with the degree of cabohydrate reduction, and someone who switches from a typical western diet (50-60% energy from carbohydrates) to a ketogenic diet (<20-40 grams of carbohydrates each day) usually experience some initial “sugar” cravings. One of the explanations for these carbohydrate cravings is that the body is switching from glucose to fat as the major energy source, and that the initial drop in blood glucose somehow results in an increased “glucose craving”; which is the body’s attempt to continue using it’s regular energy source.

However, someone who is properly “fat-adapted” often experience no or only sporadical carbohydrate cravings. Stable blood glucose and insulin levels are often considered the primary reason why low carb is so effective at silensing those cravings for fruits, cupcakes, bread etc.

While hormones, reduced glucose intake etc. probably influence carbohydrate cravings on a low-carbohydrate diet it seems possible that there is also a “microbial component” involved. The adoption of a low-carbohdyrate diet will shut down the “energy supply” for certain microorganisms involved in carbohydrate digestion.
Microorganisms living in the anaerobic conditions in the colon are able to extract energy from carbohydrates (e.g., fiber), but oxygen is necessary to extract energy from fat (especially saturated fat). So, transitioning to a low-carbohydrate diet is essentially starving some of the bacteria in the colon. These microorganisms who are used to a generous supply of energy get hungry, and they get host metabolism switched to fat burning. So when these “starving” microbes are able to control our weight to their advantage, it also seems quite possible that they can influence our food cravings/desires/preferences.

Individuals eating a low-carbohydrate diet often report that they seldom experience any cravings for carbohydrate-rich foods. It suggests that the fat-adaption phase is over and that the number of microorganisms thriving on a high-carbohydrate diet is significantly reduced; thereby not influencing food preferences to any extent.

Conclusions

It has already been shown that gut flora impacts mood and behaviour, and they are also able to influence host physiology (e.g. fat storage) to their own benefit. Although no well-controlled studies (to my knowledge) have been done on the connection between gut flora and food cravings/desires/preferences, there seems to be a lot of anecdotal evidence suggesting a relationship. Whether these dietary cravings result from the actual microorganisms involved in a “dysbiosis” or if the “dysbiosis” leads to some type of physiological change (e.g., hormonal), which in turn promotes an increased desire for specific foods, remains to be seen.

It seems to be a viscious cycle where a poor diet promotes a dysfunctional and inflammatory microbiota, which in turn increase the satiety for “unhealthy” foods. While gut flora adjusts according to diet, it also seems that microorganisms influence you in a way so you desire the foods you eat a lot of (e.g., chocolate study). A prolonged consumption of refined, sweet foods promotes the growth of certain microbial species, which in turn increases the desire for these exact foods.

A “drastic” dietary change shuts down or reduces the energy source for certain microbes. This “starvation” probably influences the cravings some people experience when they change their diet.

Humans are more like living ecosystems, and only time will show to which extent the microorganisms in and on us influence our desires and preferences.

References