When you fast for a long time, your body becomes super sensitive to a stress hormone called epinephrine that tells fat cells to break down—but this doesn’t happen because there’s more of the hormone in your blood; it’s because your fat cells just get better at responding to the same amount.

When you fast for 3.5 days instead of just a day and a half, your body becomes more sensitive to the 'fat-burning' signal from adrenaline—even if the adrenaline level in your blood is exactly the same. It's like your fat cells get better at listening to the fat-burning command.

When healthy adults don’t eat for 3.5 days, their bodies start breaking down fat for energy—even though the usual fat-burning hormones and blood sugar levels stay pretty much the same.

When you don't eat for about 3.5 days, your body switches to burning fat for energy—so it stops listening to the hormone that tells fat cells to stop breaking down, and starts listening more closely to the hormone that tells them to break down faster.