Macro partitioning
During total starvation, fat loss is significantly reduced (averaging 75 g/day) and lean tissue loss is significantly increased (averaging 576 g/day) compared to caloric restriction (fat loss 553 g/day, lean loss 224 g/day).
If your goal is to lose fat while keeping muscle, do not fast completely. This 1967 study found that a moderate 800-calorie diet burned fat 7 times faster than total starvation, while total starvation caused 2.5 times more muscle loss. Rapid weight loss on a fast is mostly water and tissue breakdown, not fat.
In contrast, the average rate of fat loss was uniform and greater during caloric restriction—553 g/day—than noted during total starvation, wherein fat loss was variable and averaged only 75 g/day. Loss of lean tissue averaged 576 g/day during starvation as compared with variable loss, averaging 224 g/day, during the initial period of caloric restriction.
Why this rating
Small sample size (n=9 total across two studies), short duration (16 days), and lack of long-term follow-up, though methods are rigorous for the era.
Source
Comparative Effects of Caloric Restriction and Total Starvation on Body Composition in Obesity
Michael F. Ball et al. · Annals of Internal Medicine · 1967
DOI 10.7326/0003-4819-67-1-60
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