Food that is processed with additives and chemical alteration or packaged in plastic containers, prepared with tools made of known endocrine disruptors may present a far greater challenge to any weight loss goal than any diet. In other words, it's not simply how much we eat, but what kind of food we eat and how we prepare it.
For example, it is not safe to reheat food in Styrofoam or wrapped in plastic wrap. It is not safe to cook on petrol-coated nonstick surfaces, and it is not healthy to eat food that was created in lab instead of a field or barn. It is not safe to eat food that was nurtured with chemically altered food or dosed with massive amounts of chemical fertilizers. This sort of food manipulation may disrupt the endocrine system of the body which can affect metabolism as well as fat absorption.
All these issues factor into the body's ability maintain a healthy weight. If you've found yourself or your child gaining weight while following a healthy diet, consider how you prepare your food and where your food comes from. It may well be another cause.