Cheat Day Science!
There is a lot of differing opinions regarding both the actual concept of the 'Cheat Day,' and exactly how effective/useful it is in the context of achieving fitness results through dieting and training. However, irrespective of whether or not it will help you achieve your results faster or indeed if at all - let us first discuss what the 'Cheat Day' and/or 'Cheat Meal' is in the eyes of those who use it:
THE CHEAT DAY
Typical Use: All macro-nutrient ratios on a diet plan are ignored for the entire day, and the trainee eats what they want to eat at any time they want, without recording any nutrients or calorie amounts. The Frequency of the Cheat Day is usually at most one day per week.
THE CHEAT MEAL
Typical Use: All macro-nutrient ratios on a diet plan are ignored for one meal, and the trainee eats what they want to eat, without recording any nutrients or calorie amounts. The frequency of the Cheat Meal is usually one to two times per week (instead of one big Cheat Day).
NOTE: The 'Cheat' aspect of the Cheat Meal or Day is what is important to note. When you are 'Cheating' you are not following the rules, and in this case it means you are not following the rules of what is most likely a very strict diet of a particular macro-nutrient ratios from clean sources. So this means that a 'Cheat Meal/Day' approach is not the same as a 'Flexible Diet' or 'IIFYM' approach, where your macro-nutrients ratios are the only rules to follow, and the sources from which you eat them are less strict.
So Why Does The Cheat Day Exist?
A few reasons; First and foremost it is for alleviating cravings for 'dirty' foods or, at the very least, foods that are not on your diet plan. The idea is that if you have a short binge of eating junk food once per week, you can then go another 6-7 days without those cravings coming back. However, from a scientific point of view the Cheat Day/Meal can in theory actually help us achieve 'better' results than if we were to stay clean the entire time.
So What Is The Science?
When on a energy restrictive clean diet the body's hormones involved with feeling hunger (Ghrelin) and satiation/ fullness (Leptin) start to invert. Ghrelin levels rise, causing hunger pangs and cravings for more food. Leptin levels subsequently fall as less energy is consumed in the diet. Leptin levels correlate with your metabolism speed - how much energy you consume at rest - so as these levels fall, so does your potential fat burning. As a result, less body fat is used as energy. This is a simplified explanation as to why your energy levels will fall while on a restrictive diet and your cravings increase.
However, it only takes a short period for Leptin levels to rise back up and start suppressing Ghrelin again - allowing stored energy (body fat) to be burned again. It is done with having a meal high in energy (usually high in Carbohydrate). As the body registers a sharp increase in energy consumption Leptin levels rises and Ghrelin levels drop allowing you to stave off cravings and keep metabolic speed high until the next Cheat Meal.
The hope is that the metabolic increase will last long enough to not only cancel out the energy consumed in the cheat meal, but also increase the overall amount of body fat burned as energy compared to keeping metabolism slightly lower on a strict diet.
Again, the jury is out on the exact relevance of incorporating a strategic Cheat Meal / Day. What we see is that the trainee is better off staying clean on the diet for as long as possible before any cheat food is introduced. This is because it initially takes a long time for the metabolism to be suppressed.
Once we know that the effectiveness of a diet is intrinsically linked to the diet immediately preceding it, then we can start to understand if a Cheat on the diet is useful. Either way, a cheat meal will most likely put the body into a fat storing mode for several hours.
Is The Cheat Meal/Day For You?
Generally speaking, someone with a fast metabolism can better tolerate a Cheat Meal but at the same time does not need one to boost metabolism. This trainee will get lean quickly with little trouble while on an energy restrictive diet, and the odd cheat meal will not make a significant impact. Conversely, someone with a slower metabolism might be less tolerant to high energy meals, but at the same time might benefit long term from a diet high in energy in order to repair a particularly slow metabolism. These individuals may be better suited to a carbohydrate cycling diet in order to stimulate Leptin more. Then once metabolic speed has recovered they can switch to a lower carbohydrate diet.