Since its fitness related, i’m sure they focus heavily on the Muscle Mass reduction part. But even that is sketchy:
We generally see greater lean body mass (LBM) loss in ketogenic diet groups. Note that lean body mass contains water, glycogen, and muscle protein, by definition. It is hard to say with certainty that LBM loss implies greater “dry” muscle protein loss. “Wet” LBM can come and go quickly because it consists of water and glycogen.
This statement makes sense, which would drive people to immediately say, “It causes muscle loss!”. But, as its noted, its difficult to find out if you’re actually losing muscle. Two members on here @richard and @Brenda have done DEXA scans and have not loss muscle mass, even though they fast.
I think keto as a whole is still evolving. There are still no concrete guidelines to what a keto diet is when studies are performed. Some people just say high-fat diet and it ends up being high-fat and high-carb which is horrible. Others say 100g of carbs or less is keto. Also, they called out a Dr. Phinney reference from 1983? really? 35 years ago?:
The graph below shows what happens before and after 5 people were put on a ketogenic diet. Their endurance, as measured by time to exhaustion, varied from person to person (Phinney et al., 1983).