Nearly 10 years ago, I wrote Everyone is Different. Back then, I warned about excessively-rapid weight loss, as catabolising muscle results in 5.8 times faster weight loss than losing bodyfat alone.
Recently, I was directed towards the following two studies with very similar titles:-
Change in Body Composition during a Weight Loss Trial in Obese Adolescents
Changes in Body Composition During Weight Reduction in Obesity: Balance Studies Comparing Effects of Fasting and a Ketogenic Diet
In the first study, the following string of text stood out:- "Although absolute LM increased in boys and decreased in girls.." Boys gained absolute lean mass and girls lost absolute lean mass. Teenage boys have much higher testosterone levels than teenage girls. Ditto for men & women.
In the second study, the full version was behind a pay-wall. There are ways to bypass pay-walls, but they're illegal, so I'm not going to include a link to the full version. I took a couple of screen-shots instead.
Here's Table 2.
Note: The Patient was a 104kg (228.8lb) man.
1. 1000kcal/day mixed diet (60%C, 20%P, 20%F) resulted in a mean daily weight loss of 0.38kg/day (0.836lb/day). Mean potassium balance was positive, so there was some net gain of muscle, even though there was weight loss. Muscle, also glycogen+water gain probably explains the low average weight loss
2. 1000kcal/day ketogenic diet (4%C, 14%P, 82%F ≡ "Nutritional Ketosis") resulted in a mean daily weight loss of 0.82kg/day (1.804lb/day), of which 6.4kg was bodyfat and 0.2kg loss was LBM. Mean potassium balance was negative, so there was some net loss of muscle.
3. 0kcal/day total fast resulted in a mean daily weight loss of 1.28kg/day (2.816lb/day), of which only 3.4kg was bodyfat and 6.2kg was LBM. Mean potassium balance was very negative, so there was large net loss of muscle.
Here's Figure 1.
Can you spot what's going on with the Nutritional Ketosis diet? The 14%P made me suspicious, as it seemed unnecessarily precise. There's a reason for that!
At the end of day 4, weight loss due to glycogen+water depletion is starting to slow down as glycogen becomes exhausted, then it speeds up again because falling blood glucose level to ~3.3mmol/L stimulates the pituitary gland to secrete ACTH, which stimulates the adrenal cortex to secrete cortisol, which catabolises muscle into amino acids for the liver & kidneys to use for gluconeogenesis. This speeds up the rate of weight loss for 3 days.
The Energy Balance Equation explains why losing LBM results in 5.8 times faster weight loss.
From Circulating Nutrients in Starvation, here's the Figure.
This is why protein % was set to precisely 14%. If it had been lower, weight loss would have been greater, but muscle loss would have become significant, which would have made Nutritional Ketosis look bad. If it had been higher, weight loss would have been less, which would have made Nutritional Ketosis look bad.
In conclusion, don't obsess over scale weight. It's just a rough guide. The irony is that the very group who can't regain lost muscle is the same group that often tries every fad extreme weight loss diet going - females.
In females, muscle mass is like a bargain at Tesco - When it's gone, it's gone. Don't do excessively-rapid weight loss diets &/or not eat for more than 24 hours.
There's only one safe rapid fat loss diet and it's The Protein-Sparing Modified Fast (PSMF)