The above figure is an estimate of what is referred to as your calorie ‘ maintenance’. In order to gain weight, one must increase intake to overcome this maintenance and eat in a calorie surplus (typically just referred to as a surplus) and must eat below this maintenance in order to create a calorie deficit (or just deficit) in order to lose weight.
Simple, right?
Yes, but also no.
The human body is a dynamic being, and it has a number of fail safes and mechanisms in place to allow it to maintain a state of equilibrium known as homeostasis. As you increase or decrease your energy intake your body receives a number of signals that something is going on, with leptin being the main one we will focus on here. Leptin is a hormone with a multitude of functions, but in this context, it is released from fat tissue to ‘let your body know’ whether or not you have eaten enough. Eat a lot, leptin secretion increases, with the opposite being true if you under eat. Leptin is so impactful, that a disregulation in either its secretion or in the sensitivity of receptor sites that allow cells to register its presence, is one of the key factors in obesity. Simply, if your leptin secretion or sensitivity is lower than it should be, your body doesn’t get the message that you have eaten enough, and your desire to eat more will be increased.
While the calories in vs. calories out theory of weight management is undeniable, it’s not as simple in practice as this statement would have you believe. Each time you eat away from this maintenance level, your body will combat it somewhat in the following ways.