Fasting for different lengths of time affects how fast your body burns energy, and fasting too often can slow it down instead of speeding it up.

Frequent or prolonged fasting triggers adaptive thermogenesis, causing the body to reduce metabolic rate to conserve energy.

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.

Metabolic rate increases during 24-36 hour fasts due to norepinephrine and adrenaline release, but does not continue to increase beyond 36 hours of fasting.

Intermittent fasting without daily fasting maintains higher metabolic rate while still providing metabolic benefits.

Prolonged fasting (72 hours) impairs glucose tolerance, resulting in higher blood glucose and insulin responses to carbohydrate intake.