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.
Scientific Claim
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.
Original Statement
“After 12 hours, the metabolic rate barely changed. It was really just kind of neutral. I mean, there was more fat oxidation for sure, but metabolic rate wasn't changing. What they found is that after about like 24 to 36 hours, metabolic rate was increasing. Your body really starts ramping up norepinephrine. You get a major increase in circulating fatty acids in conjunction with the adrenaline, which just puts you into like fat burning mode. You really upshift into that fat burning gear. Then after about 72 hours, metabolic rate doesn't go any higher than the 36-hour mark.”
Context Details
Domain
nutrition
Population
human
Subject
fasting duration
Action
causes
Target
metabolic rate increase during 24-36 hours followed by plateau
Intervention Details
Evidence from Studies
Supporting Evidence (1)
This study measured metabolic rate changes during 12, 36, and 72 hours of starvation in 29 healthy subjects. It found that metabolic rate increased by approximately 6% at 36 hours of fasting but did not continue to increase at 72 hours, which directly supports the claim that metabolic rate increases during 24-36 hour fasts but plateaus after 36 hours. The study also documented increased catecholamine levels (norepinephrine and epinephrine) during fasting, which explains the mechanism behind the metabolic rate increase.