There are various reasons the stress response happens. Caffeine can lead to adrenaline spikes and so can unconsciously worrying about gaining approval from others. Generally, modern stress is triggered by the way we think, leading to chronic and constant stress which changes the chemistry of the body.

Initially, adrenaline increases. This alters blood pressure. It also deters your blood supply from your digestive organs to your arms and legs, which allows you to run away from a threat. This means that your digestion becomes compromised. Ever notice how when you’re stressed your stomach and digestive tract doesn’t handle food and elimination the same? This is why.

These constantly elevated levels of adrenaline also change the fuel that the body uses. Usually, the body uses a combination of glucose and fat for fuel. Yet, with elevated adrenaline, the body turns to burning primarily glucose, losing its ability to burn fat. 

High levels of adrenaline further leads to the creation of inflammatory compounds. And this leads to aging and degeneration from the inside-out. As such, the body begins to produce anti-inflammatory substances, including cortisol.

Cortisol is an indicator of long-term stress. It is a very catabolic hormone, which means it breaks down muscle to produce energy and this also slows down the metabolism via a reduction in muscle mass.

This is why long-term stress frequently leads to weight gain, no matter how much you eat or workout. In fact, excessive exercise can even add to this stress pile, leading to your body to put on even more weight. This can be very counterproductive when what you really need to focus on is de-stressing and finding ways to relax.