people have a tendency to either not adhere to the diet over time or not partake in the diet at all by listening to people say: all these other magical factors effect your weight, why bother? negative platitudes if you will. 'its not that 4000 calorie a day diet that is making you fat, its genetics!' etc etc
while its true gut bacteria makes a difference, it is also true and proven that gut bacteria adjusts to your food intake. it is more likely that a persons weight problems can be moderated by diet and exercise than anything else. diet is most easily moderated by good eating habits.