Cortisol and Muscle Loss
The main culprit for muscle loss while dieting is chronically elevated cortisol levels. Cortisol has many functions, but two important ones are: mobilizing stored energy to provide fuel, and increasing blood glucose levels.
Your body wants to maintain a stable blood glucose levels. Around 4.0 - 5.0mmol/L and up to 7.0mmol/L after a meal. If it dips down below that (hypoglycemia) you normally get cravings, which compel you to eat and elevate blood sugar, or you'll release stored glucose into the bloodstream. The hormones that increase blood glucose levels are cortisol, glucagon, and (to some extent) growth hormone. All three of these will tend to be more elevated during periods of caloric restriction.
Low-Carb Makes It Worse
Things can get even more problematic when you're on a low-carb diet because you could increase protein breakdown (breakdown of muscle tissue) into amino acids to produce glucose (gluconeogenesis) from these aminos to maintain stable blood sugar levels.
This means that the more restrictive your diet, and the longer the dieting period is, the more likely you are to have chronically elevated cortisol levels because you constantly need to mobilize more fuel and elevate blood glucose levels. This is especially true when carbs are very low and fats aren't high enough to compensate.
And since chronically elevated cortisol levels can be destructive for natural individuals, it makes it almost impossible to build more muscle, and even harder to prevent muscle loss.
Steroid Users, Cortisol, and Muscle
When you're using steroids you're inhibiting the negative action that cortisol can have on muscle – both by reducing its action at the cellular level and by increasing the protein synthesis rate to compensate for the increased rate of protein breakdown.
If someone is using growth hormone, the need to produce cortisol might be reduced because GH also increases blood glucose levels and mobilizes stored energy. If growth hormone is taking care of that, there's less need to release cortisol.
Because of that, enhanced athletes have much less risk of losing muscle while dieting and can use a much stricter diet. They can even build muscle while losing fat.
How Natural Lifters Can Control Cortisol
Natural lifters need to minimize excess cortisol if they want to grow. Ingesting some fast absorbed carbs like highly-branched cyclic dextrins before and during workouts will go a long way in preventing excessive cortisol output.
If you have carbs readily available for fuel while you train there's less need to mobilize your reserves. If you don't need to mobilize as much fuel, you don't release as much cortisol. Plazma™ is the best product for that purpose.
You can also use other strategies like using vitamin C pre or post workout (around 2000mg) as well as glycine post workout. Something like Z-12™ which will increase GABA levels (a neurotransmitter that helps fight stress and anxiety) and magnesium in the evening can keep your cortisol levels low at night, which will help you sleep better, recover, and grow.
Rhodiola is a great adaptogen that improves your overall capacity to handle stress and might reduce your daily average cortisol output, which will improve your immune system and muscle growth.