Each and everyone of us has a rich culture of different bacteria in our gut. Their numbers will be different in different people, as will the type of bacteria. Both these aspects are affected by the type of food we eat. Certain food will prevent certain bacteria from growing while other food will hep it prosper. Don't worry though, these bacteria are pretty vital to health and fitness. They help with digestion and help get rid of waste products too. If you look at ruminants (cows etc) their whole digestion relies on gut bacteria to break down their food for them - a symbiotic relationship.
So now we know different people have different levels of gut bacteria we can investigate how much of an impact this has on our wieght. Scientists took the bacteria from either obese or slim people and introduced them to mice that had grown up in a sterile environment (and as such had no bacteria in their gut to begin with). They then measured the change in wieght with the mice. To keep things fair the mice were fed the same amount of the same food. The results are interesting!
The results show that the mice that were given the bacteria from the obese people put on more weight than the mice that were given the bacteria form slim people. With these results you can say that the bacteria are Implicated in weight, but the difficulty is telling which one caused the other. Did the bacteria cause the people to become obese, or did the diet of the people mean that only those bacteria could survive? This a more difficult question to answer, and we don't know the answer yet.
However, if we were to use our knowledge to suggest the most likely scenario we'd say that the diet of the obese people had an impact on their gut bacteria, leading to an imbalance of 'healthy' vs 'unhealthy' bacteria which subsequently lead to contributing to weight gain. It's true that certain types of food (mainly processed, high sugar food that contain anti-nutrients such as gluten and gliadin) will harm bacteria in your gut. We're suggesting that this may be the cause of the imbalance that caused the mice to gain weight.
Regardless of the scientific evidence to back up this theory there is already evidence to show that keeping a healthy level of the right type of bacteria in your gut is vital for health and fitness. The best way to do this is to eat unprocessed, natural food that is prepared properly at home and not in factories. Eating certain types of raw or fermented food will also help. Avoid the processed food with additives too and you'll be onto a winner. Lack of a healthy gut bacteria profile is implicated in many health issues, so it's important to keep your little helpers healthy!