There are several aspects at play here, which complicate things.
1. The First Law of Thermodynamics. There is no way around this. If you consume more Calories than you combust, you will gain weight. If you combust more than you consume, you will lose weight. The average person daily combusts approximately 2000 Calories ( = 2000 kcal = 2000000 cal) just to maintain basic existence. So if you consume more than that, and don't burn it all off with additional activity, you will gain weight.
2. Metabolic biochemical feedback. Briefly, if you lose weight by drastically cutting your caloric intake, your body will go into starvation mode in order to lower the basic maintenance threshold below the average 2000 Calories threshold. This can make it harder to lose more weight, and to keep what you have lost off. So the most effective way to prevent weight gain is to prevent going into starvation mode. There is a lot of controversy around this. Some weight-loss diets deliberately try to put you into starvation mode (ketosis).
3. Simple sugars (especially high fructose corn syrup) appear to trigger large amounts of weight gain, and accumulation of omentum fat, which is very hard to get rid of, and is regarded as the most dangerous type of fat. Stress also contributes to its accumulation. Once this causes diabetes, it is very hard, if not impossible, to reset the switch.
So, if you lose weight slowly and naturally (increase exercise, eating right), regulate your caloric intake, and your sugar intake, and you aren't diabetic, you should be able to reset the switch, but it isn't easy.