I did a 7 day fast, and I mostly ended up losing fat. While I did not do any scientific measurement, i was able to exercise the same post the 7 day fast and my belly fat came down big time.
Maybe body builders lose lean body mass when they fast, but it does not seem to hold true for the average human.
How would you know, without the "expensive" densitometry scan?
My anecdotal experience: I lost weight via alternate day fasting and did indeed shell out the 120 euro that two scans cost me. I lost mainly fat, my lean body mass after 6 months was down 4 pct, within error range I'd assume. I worked out normally through the entire period and even did things like a 200km bike trip during a 72hr fast. (I did a few of those to compensate for not fasting on holidays). Great experience, never felt better tbh.
One thing I think I learned was the exact threshold of power that allowed me to keep burning fat. Its hard to describe the feeling but after doing this for months (over a year by now) I think I know exactly when I can still push and continue on fat, and when I need to slow down to prevent lbm loss. Doing sprints, short intervals etc. would likely cause me to break proteins in the body and indeed be counterproductive - but here more research is probably needed, and individuals will likely have different thresholds.
If you can't get a DEXA scan there is an equation published in December of 2021 which approximates it well:
The new equation [%BFNew = 6.083 + (0.143 × SSnew) - (12.058 × sex) - (0.150 × age) - (0.233 × body mass index) + (0.256 × waist) + (0.162 × sex × age)] explained a significant proportion of variance in %BF5C (R2 = 0.775, SEE = 4.0%). Predictors included sum of skinfolds (SSnew, midaxillary, triceps, and thigh) and waist circumference. The new equation cross-validated well against %BF5C when compared with other existing equations, producing a large intraclass correlation coefficient (0.90), small mean bias and limits of agreement (0.4% ± 8.6%), and small measures of error (SEE = 2.5%).
Generalized Equations for Predicting Percent Body Fat from Anthropometric Measures Using a Criterion Five-Compartment Model
Zackary S Cicone, Brett S Nickerson 1, Youn-Jeng Choi 2, Clifton J Holmes 3, Bjoern Hornikel 4, Michael V Fedewa 4, Michael R Esco 4
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PMID: 34310492 PMCID: PMC8785250 (available on 2022-12-01) DOI: 10.1249/MSS.0000000000002754
>Maybe body builders lose lean body mass when they fast
Well, highly fit people certainly lose lean body mass when they fast, which you're probably not (even if you're fairly fit)
And bodybuilders DON'T lose fat when they fast because they take supraphysiological doses of hormones (steroids) that prevent the body losing muscle - and usually eat protein throughout their fast too. So not a good example.
Their whole argument was around body builders not losing fat when fasting, so if you're right, this is just irrelevant. No one expects people to lose fat when they're not caloric restricted.
Maybe body builders lose lean body mass when they fast, but it does not seem to hold true for the average human.