People are confused by cortisol and its role when it comes to leanness. On one hand, it's a hormone that should increase fat loss. It actually plays a role in the breakdown of stored energy (glycogen, fat, protein) for fuel.
As the stress hormone it gets your body ready to deal with a stressful situation like running away from a tiger. Energy mobilization is one of the most important elements of dealing with stress.
Furthermore, cortisol increases the body's release of adrenaline by helping with the conversion of noradrenaline into adrenaline. Adrenaline increases energy mobilization too. It also increases energy use.
Charles Poliquin often claimed that cortisol makes you fatter. He specifically said that elevated cortisol makes you store more fat on your belly and lower back. Sadly, this idea of spot-storing body fat discredited him in the eyes of some coaches. And the evidence-based crowd dismissed the impact of cortisol on fat loss/fat gain.
Cortisol is a mobilization hormone. And when it's released acutely and not chronically it does help with fat loss.
However, if it stays elevated chronically it can hurt your fat loss efforts. How? Mostly by reducing the conversion of the T4 thyroid hormone (mostly inactive in regard to metabolic rate) to the T3 thyroid hormone (which has a big role in setting metabolic rate). The more T3 you have, the higher your metabolic rate will be and the easier it'll be to lose fat.
Chronic cortisol elevation decreases the conversion of T4 into T3. And that's how it can decrease metabolic rate over time.
That's important for natural lifters because if you use a form of dieting (and training) that leads to excessively high levels of cortisol, you run the risk of slowing down your fat loss efforts in the long run.
Excessive caloric deficits can lead to chronic cortisol elevation, and so does complete deprivation of carbs.
Think about it. Cortisol's first function is maintaining a stable blood sugar level. So when blood sugar drops 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.
So the greater the caloric deficit, and the lower the carbs, the more you risk increasing cortisol.
For a steroid user, this isn't a huge problem because the anabolics can compensate for the increase in catabolism from the cortisol by the increase in anabolism from the steroids. And if they take fat-loss drugs, the impact of cortisol on metabolic rate also doesn't matter that much, especially if they take synthetic T3 like Cytomel.
But for a natural lifter, chronic cortisol elevation can not only slow down fat loss in the long run, but also make it harder to gain muscle or even maintain it while dieting down. (Hit the first related link below for more info.)