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Metabolic changes after 72 hours of fasting

I had 2 RMR tests done to see how my metabolism changed. Figured I’d post here! 26M/5’11”/SW250/CW205/GW170

The tests were taken around the same time of the day, in the morning. Test 1 was done on a Monday and I had eaten poorly over the weekend. I was well hydrated both days.

Test 1, \~8 hours fasted, 209LBS: RMR - 2002kcal/day, Energy from fat - 44%, Energy from carbs - 56%

Test 2, \~72 hours fasted, 202LBS: RMR - 1910kcal/day, Energy from fat - 72%, Energy from carbs - 28%

Energy production from fat increased 63% by my second test, while my RMR actually decreased by about 5%.

The increase in fat oxidation didn’t surprise me, however I was expecting to see an increase in my RMR over that time, not a decrease. Perhaps the 5% change is negligible and my metabolism neither sped up or slowed down in those 72 hours. I felt fine at the end, just hungry. I chose 72 hours as it seems to be the most sustainable for me.

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Answer

Metabolism does not increase when fasting. It’s a calorie deficit, an extreme one technically. Metabolism down-regulates when the body is in a net energy deficit. The amount over time is relative to the body composition changes that have occurred, among other factors.

This is why maintenance eating periods and reverse dieting are important tools for the longevity of fat loss.

Answer

RMR doesn’t change much under normal conditions, fasting included. You might only see a change in RMR of 150-200 calories (around 10%) per day between a fit triathlete and that same triathlete who is completely overweight and out of shape. This is the case for all mammals, e.g., a wild, free-range lion and that same lion living in a zoo. Or a hunter-gatherer with no domesticated crops or livestock, no modern tools or electricity and that same person who has adopted a western lifestyle living in a mcmansion.

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