With 100 trillion microbes in your gut, there are about 10 times as many microbial cells in the body as there are human cells.
With an estimate of more than 10,000 different species of microbes, they provide a metabolic capacity which exceeds the liver by a factor of 100.
And while the human DNA is very similar between individuals, there are huge differences in the composition of the microbiome between different people.
This might explain while somebody becomes overweight or develops a disease but somebody with the same DNA stays perfectly healthy.
Now imagine the following scenario:
Two genetically identical people decide to lose weight by going on a diet. The only difference between the two twins is that one of them was treated for 10 days with an antibiotic due to an infection, which kills big parts of the microbiome.
Will the twins lose weight equally?
Based on the evidence found by various studies, absolutely not!
In fact, there is no shortage of studies, showing how antibiotic usage leads to abnormal weight gain [3, 4].
One reason antibiotics are used so much in livestock is that at some point farmers discovered that by using antibiotics the animals grew fatter .
Just using low concentrations of antibiotics is enough to alter the composition of gut bacteria and promote fat gain! 
The Obese Microbiome
But even if you’ve never taken antibiotics, your microbiome could still be promoting weight gain. Researchers found that the microbiome of overweight people is already very different compared to lean people, regardless if they take any medication.
Scientist even invented a special name for it: “The Obese Microbiome”
To exclude the factor of genes, researchers analyzed the microbiome of twins that were quite physically different from each other - one twin was lean while the other one was obese. After taking a look at each twin’s individual microbiome, they found that their microbiomes were just as different as their physical appearance was:
The obese individuals had a far less diverse microbiome and diversity of our microbiome is thought to be critical for our health and a less diverse microbiome has been associated with many diseases. 
Another group of researchers looked into the phylums of gut bacteria found in lean and obese twins. They found that the obese twins had 20% more of one group of bacteria called Firmicutes and nearly 90% less of another group called of Bacteroidetes. 
Researchers use the ratio of these two groups as a biomarker for obesity. The higher the ratio of Firmicutes to Bacteroidetes, the higher the risk of becoming overweight.
The study also questions whether the ratio of firmicutes to bacteroidetes was set in stone. The obese volunteers were set on a diet for one year and their percentage of bacteroidetes nicely increased while they lost weight. This shows that the microbiome diversity is apt to change with alterations in diet.
Transfer of "obese microbes"
The next study is a great example of how the human microbiome can have a huge influence your waistline!
Researchers sucked microbes from the guts of lean mice and obese ones and injected the microbes into the intestines of animals whose own microbes were deleted due to sterile caging. The mice injected with the 'obese' microbes gained roughly double the quantity of fat than those that received the 'lean' microbes !
Another paper published in the Journal Science went one step further with this research by extracting the microbiome from human twins, one obese twin, and one lean twin, and then transferred the microbes into different sterile mice.
I guess by now you won’t be surprised hear the results of the study showed the mice who received the obese microbiome gained more body mass, especially fat, than the mice with the lean microbiome.
Let me cite the chief editor of the renowned journal, Nature:
“Fat people harbor fat microbes.”
“The obese are often blamed for their own corpulence. But perhaps, just perhaps, some of the blame should be placed on another type of organism entirely: bacteria.”
Here is another example of how easy you can improve your odds:
In 2015 scientist gave a group of volunteers 21 grams of fiber extra per day or just a placebo. After just 21 days the abundance of bacteroidetes increase by 13% in the group that received the fiber.
With this knowledge, one might consider that the first step of a diet should be to reset our microbiome!
For the start, it would be the best to focus on less processed food as they contain naturally more fiber. Maybe even include some probiotic food like yogurts, kombucha, kefir, kimchi or sauerkraut - or even take some probiotic supplements to get a fast start.
For my last diet, it certainly helped me a lot to focus on a diet with high fiber intake and good fats - and I never lost fat so easily while staying mentally sharp.
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Kekwick and Pawan, Calorie intake in relation to body-weight changes in the obese, The Lancet, 1956
Bouchard et al., The response to long-term overfeeding in identical twins, The New England Journal of Medicine, 1990
Angelakis et al., Abnormal weight gain and gut microbiota modifications are side effects of long-term doxycycline and hydoxychlroquine treament, American Society for Microbiology, 2014
Scott et al., Administration of Antibiotics to Children Before Age 2 Years Increases Risk for Childhood Obesity, Gastroenterology. 2016
Cho et al., Antibiotics in early life alter the murine colonic microbiome and adiposity, Nature, 2012
Turnbaugh et al., A core gut microbiome in obese and lean twins, Nature, 2009
Ley et al., Human gut microbes associated with obesity, Nature, 2006
Turnbaugh et al., An obesity-associated gut microbiome with increased capacity for energy harvest, Nature, 2006
Ridaura et al., Gut Microbiota from Twins Discordant for Obesity Modulate Metabolism in Mice, Science, 2013