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Are Bacteria Making You Fat ?


Nurturing the microbes that make us (P1)


The miracle of life becomes most apparent when you look at its smallest details. Entwined to our very existence are trillions of microorganisms that have integrated themselves into our bodies biological functions. This cellular world includes bacteria, fungi, and a plethora of other uncountable communities of microscopic organisms that, in its collectivity, constitute what scientists now call the “microbiome”.

There are many different types of microbial organisms that constitute the microbiome, but bacteria are the most studied among them. (1)Most of these microbes populate the insides of the intestine. The large intestine contains a “pocket” called the cecum in which most of these microbes can be found.

Microbial organisms

There are more microbial organisms in your body than human cells! (P2)

The Gut Microbiome

The average human being contains around 30 trillion human cells and 39 trillion microbial organisms (2) (Abbott, 2016) While this number can vary significantly between different individuals, the implications and impacts of having such a significant amount of microorganisms in our bodies is just being understood. Owing to the symbiotic relationship these microorganisms have with our body, such as; performing important physiological functions such as harvesting nutrients and developing our immune system, scientists have put forward the proposition that in collection they could be considered as a “microbial organ”. (3)

These microbes can be highly diverse, and different types of these organisms compete with one another for survival inside us. A healthy microbiome is one where the diversity of the microbes within it are high, as more of their energy is spent in competition amongst each other. (4) Characteristics of a less diverse microbiome include; dominant species with a high amount of resources available to them which it uses to further its growth independent of the benefit to its host, ie us.(5)

Different diets

Different microbes thrive on different diets (P3)

Specialist Microbes

Different microbes have different “preferences” of nutrient intake. Some are seen growing the most on a diet of carbohydrates, whereas others have shown to thrive on fats or fibers. (6) People who identify themselves as “chocolate desiring” have been found to have different microbial metabolites in their urine than those who identify as “chocolate indifferent” despite having identical diets.

Another study discovered specialist microbes that digest seaweed in Japanese people, demonstrating how distinct traits can be evolved among microbes. (7) Even microbes that have a generalist survival strategy tend to do perform better with certain nutrient combinations than others. (8)

In their competition for survival, research is showing that these microbes actually influence our diet through our bodies various systems.


Microbes influence our habits through stimulating pain and pleasure centers in our brain (P4)

The Microbial carrot and stick

These microbes have shown that they can manipulate pain in us through the production of virulence toxins that are triggered by a low concentration of growth-limiting nutrients. Some microbes directly manipulate the intestinal epithelium, tissue lining the surface of the gut, when certain nutrients are absent. Through the production of virulence toxins and the direct manipulation of our gut tissue, microbes have been shown to activate pain receptors inside our body. (9) These are made possible by a complex connection between our brain and the gut known as the gut-brain axis.

In addition to stimulating pain receptors the microbiome can also be observed controlling the body through its reward mechanisms. More than 50% of the dopamine and serotonin in the body can be traced back to an intestinal source. Dopamine is the neurotransmitter that controls the brain’s reward and pleasure centers and serotonin is a neurotransmitter most well-known as a regulator of mood, appetite, and sleep. (10) This implies that the microbiome has a greater influence on people’s actions and behaviors than previously thought. 

Microbes and Obesity

Research is indicating that some microbes even influence obesity (P5)

Microbial Obesity

In addition to having an impact on a person’s diet, certain types of microbiota have also been shown to influence obesity. The gut microbiome and its evolutionary conflict with the host may be an important factor in generating obesity. This was highlighted in a study in which mice who were genetically predisposed to obesity were observed to remain lean when they were raised without the suspected microbiota. The germ-free mice became obese mice when fed a fecal pellet from an ordinary obese mouse. (3) Similar results were observed when microbiota from obese humans were introduced to germ-free mice. (11) 


Studies suggest that these microbes are contagious! (P6)

Microbial contagion

Another intriguing implication of some of these studies is that microbial induced cravings and preferences for certain foods can be contagious. Fecal and oral microbiota of cohabiting family members were more similar than those who were not living together. (12Food preferences can sometimes be determined by a single member of the household. A specialist gut microbe adapted to such an environment could spread through all members of a household. More alarming is the high probability of obesity inducing microbes being transmitted from person to person.

Probiotics in yoghurt

Probiotics, beneficial microbes are found in yogurt (P7)

The Probiotic counter

Microbial activities can be managed with the administering of probiotics. Probiotics are microbes that are beneficial for the body and helps decrease food intake. Probiotics demonstrate that greater diversity in the gut microbiome can limit overeating. Ingesting probiotics such as lactobacillus acidophilus, a bacteria found in yogurt, has shown to reduce fat mass, increases sensitivity to insulin and improve tolerance against glucose; but such effects are not universally reported for all lactobacillus species.

Probiotics have also shown to have a role in weight loss in several trials. In one study, a probiotic yogurt was shown to affect weight loss independent of the changes in energy intake and exercise. (13) Yogurt was also shown to be the food associated with weight loss in another study that monitored the diet and health of 120,000 nurses for over 12-20 years. (14) 

Living organisms in the body

These studies show rich world inside us (P8)


Scientists have put forward a number of hypotheses for pertaining problems such as obesity, unhealthy food cravings, addiction, environmental mismatch, and nutrient deprivation. What such studies have shown is that a microbial cause is not mutually exclusive from such conditions; that is to say that microbial activity can be linked, but is not the reason for the aforementioned conditions. Therefore rather than providing something new to blame, in this case your gut, for the habits you’ve developed, these studies help paint painting a richer picture of what it means to human.




1. Robertson, R., 2017. [Online]

2. Abbott, A., 2016. [Online]

3. Backhed, F. et al., 2004. The gut microbiota as an environmental factor that regulates fat storage. PNAS, 101(44), pp. 15718-15723.

4. Alcock, J., Maley, C. C. & Aktipis, A. C., 2014. Is eating behavior manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms. Bioessays, 36(10), pp. 940-949.

5. Gonzalez-Rodriguez, I. et al., 2013. Factors involved in the colonization and survival of bifidobacteria in the gastrointestinal tract. Microbiology Letters, 340(1), pp. 1-10.

6. Wu, G. D. et al., 2011. Linking Long-Term Dietary Patterns with Gut Microbial Enterotypes. Science, 334(6052), pp. 105-108.

7. Hehemann, al., 2010. Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota. Nature, Volume 464, pp. 908-912.

8. Mcnulty, N. P. et al., 2013. Microbiota Containing Bacteroides cellulosilyticus WH2, a Symbiont with an Extensive Glycobiome. PLoS Biol, 11(8).

9. Casadevall, A. & Pirofski, L.-a., 2000. Host-Pathogen Interactions: Basic Concepts of Microbial Commensalism, Colonization, Infection, and Disease. Infection and Immunity, 68(12), pp. 6511-6518.

10. Deans, E., 2011. [Online]

11. Ridaura, V. et al., 2013. Gut Microbiota from Twins Discordant for Obesity Modulate Metabolism in Mice. Science, 341(6150).

12. Song, S. J. et al., 2013. [Online]

13. Kadooka, Y. et al., 2010. Regulation of abdominal adiposity by probiotics (Lactobacillus gasseri SBT2055) in adults with obese tendencies in a randomized controlled trial. European Journal of Clinical Nutrition, Volume 64, pp. 636-643.

14. Mozaffarian, D. et al., 2011. Changes in Diet and Lifestyle and Long-Term Weight Gain in Women and Men. New England Journal of Medicine, 364(25), pp. 2392-2404.



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