Fasting for 12 hours doesn't change how fast your body burns energy, but fasting for 24-36 hours makes it burn energy faster, and fasting longer than that doesn't make it burn even faster.

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 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.

Prolonged or frequent fasting can cause a decrease in metabolic rate after an initial increase.

When people don't eat for 3 days, their noradrenaline levels go up, but not until after the second day of fasting.

When people don't eat for 3 days, their adrenaline levels go up, but not until after the second day of fasting.