No one is overweight/obese because of gut bacteria. There may be marginal effects, but it's a bit like optimizing the tires on a soccer mom's mini-van.
So while it is fundamentally true that a calorie deficit will lead to weight loss, it's not trivial to create a consistent deficit while maintaining good nutrition.
http://www.iflscience.com/health-and-medicine/woman-becomes-...
Take someone 100 pounds overweight. That is 350000 Kcal more consumption in their lifetime than they should've had. Let's spread that over 10 years, or 35000 Kcal a year. That works out to about a 100 Kcal difference per day. Over 20 years, it would be about 50 Kcal.
Now, they could've made different choices, and there are some people who stay a healthy weight by making different choices. But I would not dismiss small changes as being irrelevant because given the time scale, those changes do add up.
50 kcal extra a day means steady state weight increases by about 3 pounds or 1.5 kg.
35 pounds gained from changing her stomach bacteria.
That is an excellent baseless assertion you have there!
Care to back that up with facts?
Or should we just take your word for it?