Some studies say one thing, some another. The number of RMR test on humans is tiny and doesn’t include any large cohort of women. Studies on fasting were mostly were on mice, worms, and non-human critters. But if you go into the hypocaloric diet studies on humans and RMR, which are plentiful and go back 100 years, the effect starts pretty quickly, 36 hours. And women seem to suffer from the effect more than men. Some studies only look at 14 hour fasts, which is a typical night for a lot of people. A few look at 30-day fasts. So there’s a dearth of 18 and 36 hour human fasting studies.
It’s everyone else’s business what they do, but I already have a lowered RMR from years upon years of low-calorie dieting. I’m not risking losing another single point. Eating 1450 calories a day on whole foods, exercising daily, and still ending up over 200 pounds isn’t much fun. Everyone yells at you for “eating like a pig,” including doctors, who are shockingly ignorant about the RMR effect of the stupid diets they promote, but you’re actually hungry a lot of the day if you try eating like that for years. It’s sustainable (or is for damned stubborn people), but it’s not fun to always be hungry. And it can lead to refeeding (short term “binges” where your desperate body tries to get some calories in there to preserve little things like, oh, heart mass.)
And no, moving really heavy weights for years didn’t shift it for me. I had 142 pounds of lean body mass, tested the same day by water displacement that I had RMR tested, and it wasn’t helpful in the least. If the “muscle mass adds BMR” is actually a thing, it’s a tiny thing for women, or at least this woman, and I have a difficult time believing I’m all that weird in physiology.
Once RMR is gone, it’s gone. Wave bye-bye to it forever. It’s not like weight. You can gain 50 pounds and take it off. You lose 500 calories of RMR, and it’s just gone.
Eating lots of meat, keeping the protein over 100 (and better 120 g) seems to help raise the amount of food I can eat, as does exercise. But then exercise makes you hungrier, so that’s kind of a wash.
I grasp that people are very fond of fasting because it helps them lose weight faster. So does 500 calories per day. (and it’s ketogenic, 500 calories, so no, ketogenic diets don’t protect against the lowering-RMR effect at all.) But “fast” isn’t necessarily “good” or healthy or sustainable. If I could go back and talk to 20 year old me, I’d beg her to not starve. She probably wouldn’t listen because back then, being barely overweight, she was desperate for social approval, poor thing, but I’d give it a go.
So I shout into the wind on this one from time to time. I suspect in 10 years, the fad will have passed and we’ll grasp what all dangers there were with it, too late, beyond the ones we already know of (it triggers EDs in some; ED counselors would argue it is one, per se.)
One of my food mantras is: eat when you’re hungry. Nature gives us hunger for a reason. If we’re on sugar and grains, it’s hard to understand what hunger is, but once off them, it gets clearer. I listen to my body now, though I surely did not when I was young, and stupid from desperation for social approval. Thankfully, I’m grown up now, and don’t give a rat’s ass what anyone but my half-dozen dearest friends thinks of me.
The fasting talk is why I haven’t really engaged much in the low-carb world for the past several years. How I’d love to find a place where such talk was forbidden! Amy Berger, Keto without the Crazy, is one of the few people I can bear to listen to for long. She’s utterly sensible.