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Gut bacteria: effects on weight & what happens when you go on a diet

When you think of what determines our weight, the first two things that likely come to mind are calorie intake and calorie expenditure (diet and exercise). However, there is so much more to it than just these two factors! Though you may shower daily and consider yourself a clean person, you are colonized with little colonies of bacteria, all over the surface and inner cavities of your body. Despite how the word “bacteria” may have a negative connotation, they are crucial to our survival and have many important functions. A growing stream of research investigates how bacteria in our digestive system, also called the gut microbiome, plays a role in weight, health and disease states, and overall well-being.

Weight & gut bacteria

The bacterial makeup of your intestines has an impact on your weight. One of the main reasons behind this is that the gut microbiota (the bacteria in your intestines) plays a big role in energy regulation. Some bacteria are better at wringing calories out of your meals (and thus, you won’t absorb all of them), while others leave all the calories for you to absorb them. For example, people at higher weights tend to have a significantly greater proportion of one of the two main types of bacteria found in the gut, known as Firmicutes, than the other, known as Bacteroidetes. Detailed molecular analyses show that Firmicutes are much better at helping you absorb calories from food (1).

This is one of the reasons why “calories in, calories out” is inaccurate. Depending on the composition of your gut bacteria, for the same meal, a different amount of energy is absorbed between individuals. For example, if 3 friends ate a banana, one may absorb 75 calories, one friend may absorb 80, and one may absorb 90. Sure, it may sound like small differences, but it adds up throughout days, weeks and years.

A note on mice studies

There have been numerous studies on mice to demonstrate that gut bacteria composition affects weight. But first, we have to understand how mice studieswork. When researchers are trying to examine the role of the gut microbiome on health in mammalian physiology, they use what is known as Germ Free (GF) mice. The name itself implies its function; microbiologists are able to rid the mouse from any organisms in its body, whether that be from the skin, the digestive system or the lungs. Using this method, researchers can observe how a GF mouse may react differently to certain stimuli compared to a control (normal) mouse.

One study on mice showed that the microbiota of a higher weight individual allows them to have an increased capacity to harvest energy from the diet. They took the gut bacteria from the higher weight mouse, as well as the gut bacteria of the lean mouse, and injected them (separately) into two recipient GF mice (also called colonization). The mouse that was colonized with the bacteria from the higher weight mouse then gained body fat – a significantly greater percentage compared to the mouse that was colonized with the lean mouse’s bacteria. They relate this phenomenon to a greater abundance of Firmicutes than Bacteroidetes in the higher weight mice (2).

In another study, GF mice were colonized with bacteria from a conventional control mouse and saw an increase of 60% body fat in just a two-week period although their overall food consumption was reduced (3). From an outside perspective, it doesn’t seem rational. One animal eats less but puts on more weight? This contradicts the traditional "calories in, calories out” mantra.

So, can we just remove the bacteria from the gut microbiome that’s making individuals gain weight? Given our current understanding of the gut microbiome, physicians are unable to target specific species and remove them from an organism. Although antibiotics (bacterial killers) do exist, they are often classified as ‘broad spectrum’ killers which comes with the risk of eliminating Lactobacillus, for example (a type of anti-inflammatory bacteria), just as much as the Firmicutes bacteria.

Dieting & gut bacteria

Interestingly, restrictive dieting shifts the balance of bacteria so you’re more likely to absorb more calories from your food, playing a role in the common post-diet weight regain. Why? Because the body think’s it’s starving, so it needs to try to get as many calories as it can. To demonstrate this phenomenon, researchers studied heavier weight individuals as they tried to lose weight with low-calorie diets. They discovered that the proportion of Firmicutes (bacteria that make you absorb more calories) in their digestive tracts rose and the proportion of Bacteroidetes (bacteria that make you absorb less calories) fell (1). In general, diets can also reduce microbiota richness when they eliminate one or more dietary macronutrients such as carbohydrates (think about the keto diet, everyone!) (4). The bacterium in your gut is going to be dominated by organisms that can get the most out of the foods you eat – so if you’re eating a limited variety of foods, you’ll have a limited variety of bacterium.


In summary, calories in do not equal calories out. Your gut microbiota plays a big role in energy regulation and weight, and so far, we have no way of controlling the ratio of Firmicutes to Bacteroidetes. Of course, gut microbiota is only one part of the picture, and weight does not determine health. It’s much better to focus on lifestyle behaviour changes to better our health rather than worrying about the ratio of bacteria in our intestines.

There are, however, some proven ways we can adapt our diets to benefit the gut microbiome and subsequently, our health. For example, one study in 2012 found that the consumption of polyunsaturated fatty acids, which are derived from fatty fish, oils, nuts and seeds, favors the growth of Lactobacillus, a bacterium working against inflammation (5). Other ways you can change your diet to favor gut bacteria can be found in my (Olivia’s) post on “gut bacteria, gut health and how to improve it” or Spencer (my co-author)'s post on Microbiota Accessible Carbohydrates!

A special thank you to Spencer Abbott, a microbiology intern at PnuVax Incorporated, for helping me write this post! Check out his other blog posts on his website,!

Works cited

  1. Ley, R. et al. Human gut microbes associated with obesity. Nature 444, 1022–1023 (2006).

  2. Turnbaugh, P. et al. An obesity-associated gut microbiome with increased capacity for energy harvest. Nature 444, 1027–1031 (2006).

  3. Bäckhed, F. et al. The gut microbiota as an environmental factor that regulates fat storage. Proceedings of the national academy of sciences, 101(44), 15718-15723 (2004).

  4. Heiman, ML. & Greenway, F. A healthy gastrointestinal microbiome is dependent on dietary diversity. Molecular metabolism vol. 5,5 317-320 (2016).

  5. Santacruz, A et al. Interplay between weight loss and gut microbiota composition in overweight adolescents. Obesity, 17(10), 1906-1915 (2009).


3 комментария

14 февр. 2022 г.

the only nutritionist I trust


14 февр. 2022 г.

Excellent article


14 февр. 2022 г.

Olivia you are so smart

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