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.

“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.”