People are confused by cortisol and its role in leanness. On the one hand, it is a hormone that should increase fat loss. It actually plays a role in the breakdown of stored energy (glycogen, fat, protein) as fuel.
As a stress hormone, it prepares your body to cope with a stressful situation like running away from a tiger. Energy mobilization is one of the most important elements in dealing with stress.
Cortisol also increases the body's own release of epinephrine by helping to convert norepinephrine to epinephrine. Adrenaline also increases energy mobilization. It also increases energy consumption.
Charles Poliquin often claimed that cortisol makes you fatter. He explicitly said that with increased cortisol more fat is stored in the abdomen and lower back. Unfortunately, this idea of storing body fat has been discredited in the eyes of some coaches. And the evidence-based mass rejected the influence of cortisol on fat loss / fat gain.
Here's the thing …
Cortisol is a mobilizing hormone. And if it is released acutely and not chronically, it helps with fat loss.
However, if it stays chronically elevated, it can harm your fat loss efforts. As? Mainly by reducing the conversion of T4 thyroid hormone (mostly inactive with respect to metabolic rate) into T3 thyroid hormone (which plays a major role in determining metabolic rate). The more T3 you have, the higher your metabolic rate and the easier it is to lose fat.
Increasing chronic cortisol reduces the conversion of T4 into T3. And so it can reduce the metabolic rate over time.
This is important for natural lifters, because if you use a diet (and workout) that leads to an excessively high cortisol level, there is a risk that you will slow down your fat loss efforts in the long run.
Too few calories and carbohydrates
Excessive calorie deficiencies can lead to a chronic increase in cortisol, as does the complete withdrawal of carbohydrates.
Think about it. The first function of cortisol is to maintain a stable blood sugar level. So when blood sugar goes down (when carbs or calories are too low), cortisol and glucagon are released to bring it back up. Cortisol is also released to mobilize other fuel sources.
The higher the calorie deficit and the lower the carbs, the more you risk increasing cortisol.
This is not a big problem for steroid users Anabolic steroids can compensate for the increase in cortisol catabolism due to steroids increasing their anabolism. And when they take fat loss drugs, the effect of cortisol on metabolic rate also does not matter, especially if they are taking synthetic T3 like Cytomel.
But for a natural lifter, chronic cortisol enhancement can not only slow down fat loss in the long run, but makes it harder to build or even maintain muscle while dieting. (Click on the first related link for more information.)
The best damn diet for natural lifters
The two faces of cortisol