Low GABA and low serotonin actually contributes to increasing cortisol levels.
Why? Because both of these neurotransmitters help you relax. Basically they reduce neural activity which relaxes you. If you increase neural activity (via adrenaline or dopamine for example) BUT cannot bring it back down (because you don’t have enough serotonin or GABA) you stay in “fight or flight” mode, even after the stress has passed. That keeps your body pumping cortisol.
Cortisol is a mobilization hormone. So technically it should make you leaner, not fatter… right?
Acute cortisol increase, yes. If it goes back down. But chronic cortisol elevation can make you fatter or make it harder to lose fat.
First because chronic cortisol elevation inhibits the conversion of T4 into T3. T4 is an inactive thyroid hormone while T3 is the one that has the greatest impact on metabolic rate. You have very little T3 and you produce what you need to maintain a normal metabolic rate by converting T4. BUT cortisol reduces that conversion. Meaning that chronic cortisol elevation can lower metabolic rate.
Furthermore chronic cortisol elevation can lead to insulin resistance. When you are insulin resistant it’s much harder to get leaner and build muscle. It’s also easier to gain fat.
Cortisol elevates blood sugar level. Insulin lowers it. Your body wants to stay in the 4.0 - 5.0mmol/dl. When cortisol is chronically elevated your blood sugar levels always goes up… if that happens when you are not physically active your body has to lower blood sugar levels by releasing insulin. So when cortisol is always high, you always have to compensate by releasing insulin to lower back down blood sugar levels. That can easily lead to insulin resistance.
So yeah, low serotonin/GABA and excess cortisol will make it much harder to get lean and build muscle