It may seem surprising that there is such a strong connection, but when you learn about the physiology of the stress response, it makes much sense. When you are stressed, your body activates its physiological response "fight or flight". Part of this answer means that your body solves blood sugar in the bloodstream so you can use it immediately in an emergency. For example, if you run away from a dangerous situation, you will need the fast energy provided by glucose in your blood. However, a problem arises when you are always stressed. When this happens, you get a constant blood sugar release, causing more insulin to be released.
This high insulin state, called hyperinsulinemia, basically causes your body to try to force the glucose back into cells. Insulin is also one of the hormones that signals to your body to store fat, which explains why people often gain weight during a stressful time in their lives ̵
If you were to look inside the body while all this was happening, you would see the brain perceive stress or anxiety and release cortisol from the adrenal glands as a result. Then you will observe a message from the brain's journey to the body to release blood sugar and increase liver gluconeogenesis – a process that regenerates glucose. When the stressful event is over, the signal stops.
That's fine if it rarely happens, but for most of us this happens sometimes a week, daily or even hourly! It can leave your body really confused and with very unnecessary glucose floating around the bloodstream, as the muscles and body do not actually need.