Whoo baby! There is alot to unpack here
The DATA and results of the meta-analysis itself: Keto and Omnivore in well adapted body builders showed no meaningful difference in fat free mass over studies of 8 weeks -> 12 weeks.
The author's OPINIONS and discussion... so much.... so much that needs to be addressed. Bias, bias everywhere.
The catchy line in the abstract
Ketogenic diets, taking into account the possible side effects, can be an alternative for increasing muscle mass as long as energy surplus is generated; however, their application for eight weeks or more without interruption does not seem to be the best option due to the satiety and lack of adherence generated.
are straight up opinion, and not based on the meta-analysis or results of any of the studies included in the meta-analysis... This is just the author getting on their soap box. But get this... because eating a ketogenic diet is so satisfying, people find it difficult to over-eat to hit a calorie requirement, and THEREFORE keto is not the best option because of "lack of adherence".... that is gold star mental gymnastic bullshit. None of the studies show that, his discussion doesn't reference anything demonstrating that.. this is 100% the dudes OPINION based on the faulty assumption that FFM is directly driven by excess calorie consumption (i.e. a totally different hormonal and dietary context then keto).
i.e. if you eat only sugar to hit your calorie target, how do you think its going to affect your FFM? What about protein powder plus sugar? What about protein and fat? Obviously (at least for the people reading the keto literature) we know that these are driven by hormones and substrates - not calories
In a high carb eating pattern, excess calories means lots of extra insulin which is famously a anabolic hormone... but in a ketogenic (or zero carb) metabolism eating extra fat isn't going to drive insulin... sooo.... this guy needs a new opinion on FFM mechanisms that takes into account hormones
One problem with these studies is they keep calories equal, where I think it would be more interesting to keep protein equal and let the rest of the food (fat) just be as desired.
The author warns of keto side effects (i.e. the adaption window), then admits they go away in a few days, but uses those side effects in the conclusion as a basis for cautioning the keto diet.
However, due to the satiating effect and the consequent lack of adherence that a KD generates, it does not seem to be an optimal nutritional strategy, especially if it is maintained continuously for eight weeks or more.
The very last sentence in the paper - Keto is too satisfying, lack of adherence (no references, so straight up personal opinion), not optimal? - based on what target - this paper demonstrates the same FFM in omni/keto... Especially if more then 8 weeks - why? they never specify what is magical about 8 weeks or what bad thing happens, the only complaint they have is people are very satisfied and full when eating keto.
This paper is a great demonstration of why you need to read the whole paper, not just the abstract and conclusions (which are opinions).. the data is very positive for keto, the opinions are very negative for keto. So you will see this paper referenced in many papers about keto supporting it, because the data is the key value in a paper not the opinion.