Gut bacteria make a huge difference and they adapt to your diet. There are companies working on transplants, so far it's pretty much consuming excrement... It's the only treatment for some diseases, notably some antibiotic resistant ones. Gut bacteria will change what you consume depending on what they consume and they can change how full you feel based on what they give off (gases...).
Antibiotics kill a lot of gut bacteria so they probably have an impact on weight gain and weight loss. They almost certainly reduce the variety of intestinal flora and that is probably a bad thing.
Recent studies show eating fat does not make you fat. The fat gets broken down and used. Eating sugar makes you fat, it gets stored quickly. Drinking soft drinks (even zero cal zero sugar) makes you fat, don't know why, studies are consistent.
Fruits, vegetables, and exercise are all good.
Walking briskly is good exercise. Walking slowly isn't.
If you want to lose weight: weigh yourself often, cut sugar, exercise. Not necessarily fun, but not bad if you can cook and find sports you enjoy, and/or learn to enjoy walking fairly fast and far.