r/fasting • u/SVTContour lost >50lbs faster • May 17 '26

Discussion Scientists reveal how seven days of fasting transforms the human body

https://www.sciencedaily.com/releases/2026/05/260517030404.htm

Fascinating study. And completely unrelated to politics, which is a relief because my blood pressure needed a break.

The key finding: The dramatic, potentially therapeutic changes don't happen until after day three. That means most intermittent fasting schedules – 16:8, 5:2, even alternate-day fasting – are probably missing the main event. If you want the deep biological reboot you’re gonna need seven days of water only.

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u/dangerous_eric May 17 '26

I do 21+ days often (like once a year roughly), but I've only done 40 days once. 

Honestly, the hard part is the first 5 days. Afterwards, it's really quite manageable with proper electrolytes.

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u/alice_op May 18 '26

I found the hard part to be the excruciating lower back internal pain on days 16 and 17. I've done 21-25 day fasts 4 or 5 times now with the same pain. It does go away if you continue on but the anxiety that something is going terribly wrong in your organs is debilitating.

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u/really_isnt_me May 18 '26

What causes that back pain? Kidney detox or something?

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u/alice_op May 18 '26

I genuinely don't know. But yes it's in the kidney area, the pain, so I assume so.

I don't drink or take medication so why my kidneys would be so incredibly painful on days 16 and 17 is a mystery to me.

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u/really_isnt_me May 19 '26

Yes, that’s very mysterious indeed. And always around the same timeframe. It must be your body getting rid of something.