Our metabolism, like it or not, will always act in an extreme way to the stimulus we give it. What do I mean by this? When we are sleeping our body repairs and reconstructs our cells. A person who does not eat breakfast will put the body into a catabolic state (cell de contstruction) which will prevent our cells from getting the energy and fuel to regenerate thus using protein from our muscles as its source construction. This creates a circle of gaining and losing that is entirely preventable with good nutrition prior time to start our daily activities.
Conversely, the body understands that there is not enough food in the body, so here decreases the metabolic rate, which prevents the body metabolically operate optimally during future food intake. Ultimately we will have less ability to process macronutrients and we run the risk of generating fat retention in our body.
This same catabolic process is generated during and after our training. After a workout we deplete our glycogen stores and thus if not replenished damage our muscle tissue and affect muscle recovery for any future workouts, this similar process happens when we leave too much space between meals, your metabolism slows to counteract the lack of food, which makes food digestion also slow down which prevents us from absorbing all macronutrients efficiently.
Another negative to this is that a slow metabolism does not consume as many calories in state of rest. Many more examples still do conclude that small imbalances in our diet can lead to loss of muscle mass, fat gain etc. ultimately the opposite of what we want when we go to a gym.
Why NO cheat food? What happens when we restrict certain foods in our diet? Usually the food we restrict are foods high in glycemic index, by restricting these types of food we keep our insulin levels low (insulin regulates blood sugar), this helps keep the body from storing excess fat.
Otherwise by consuming high glycemic index foods, which generates a spike in blood sugar which leads to accumulation of fat tissue and increased weight gain after been kept low for some time. Ultimately all the effort we make to diet we then throw away with that cheat food.