Ok I'll expand on my previous post a little bit...
Firstly, we need to understand that there is NO solid scientific evidence examining muscle mass preservation between low vs high protein, calorically restricted, ketogenic diets in active, healthy, individuals.
When I say "solid scientific evidence" I'm referring to an RCT with ALL nutrition being provided by the lab itself, body comp measurements done accurately (NOT skin calipers, BIA, etc) and a suitable duration of time to allow adaptation to the ketogenic diet to occur (as opposed to looking at the period of adaptation itself).
When I say "active, healthy individuals" I mean people involved in a controlled resistance training programme (not endurance sports, aerobics, etc) that are NOT newbies.
Therefore, due this lac of solid scientific evidence, we need to examine exactly what happens within the body during caloric restriction, caloric restriction with a ketogenic diet and caloric restriction with a ketogenic diet and suboptimal protein intake.
Caloric restriction (CR) results in a decrease in practically all anabolic signalling and this includes but isn't limited to testosterone, insulin, igf-1, mTOR, satellite cell proliferation, etc. This is accompanied by an increase in potentially anti-anabolic signals like AMPK, cortisol, SHBG, etc.
A ketogenic CR further inhibits insulin and increases AMPK through LKB1. AMPK directly inhibits mTOR, which is bad for protein synthesis. Of course this is also accompanied by an increase in FFA oxidation, limited glycolysis and so on.
Now by adding protein restriction to the ketogenic CR we are further inhibiting IGF-1 and mTOR through a new pathway that ends up further inhibiting IGF1-R>PI3K>Akt>mTOR.
That adds up to a lot of inhibition.
Now to see what all this means in reality, let's take 3 case reports focused on natural, competitive, bodybuilders dieting down for a show:
Natural bodybuilding competition preparation and recovery: a 12-month case study. - PubMed - NCBI
Case study: Natural bodybuilding contest preparation. - PubMed - NCBI
A nutrition and conditioning intervention for natural bodybuilding contest preparation: case study
These cases show pretty dramatic results when it comes to muscle preservation despite the fact that they all met the 2.3-3.1g/kg protein recommendation set out by Helms & co in this excellent review paper:
Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. - PubMed - NCBI
Specifically, the Robinson case report showed a total of 25.7lbs lost, 11lbs of which came from LBM - that is a fucking awful result. One of the key influencing factors here was the lack of carb intake.
The Rossow & Kistler case reports maintained fat intake at around 15-30% while Robinson had it in at 37%, which meant that carb intake was limited to 100g/day on average with no refeeds (another big difference between Rossow & Kistler vs Robinson). As far as I'm concerned, that is all the evidence you should need to show that carbs are more protein sparing than fats.
I will concede that none of these case reports involved a direct ketogenic diet (because its a fucking stupid idea and no sensible bodybuilder does it during prep) but my speculation is that the loss of LBM would only be further exacerbated in that condition.
So yes, keto CR with suboptimal protein intake sucks for muscle preservation and body recomp in general
