I'll post this excerpt from Lyle McDonald's "The Ultimate Diet 2.0":
One problem may be that lean individuals can't make enough ketones to exert a protein sparing effect; this is a consequence of the difficulties in mobilizing fatty acids in the first place. Even during total starvation, when you'd expect ketosis to have the greatest impact, ketones aren't protein sparing in lean individuals (<15% bodyfat or so). Perhaps this is the shining moment for MCTs (Medium-Chain Triglycerides), by producing ketones in larger amounts, we can exert a protein sparing effect beyond simply providing quick fat energy. Assuming protein intake is sufficient in the first place, I still tend to doubt ketosis has any huge advantages in this regards. If it does, it simply hasn't shown up in real world experience.
When you have less fat to begin with, it becomes more difficult to mobilize the body fat because the body needs it as a survival mechanism. In turn, the body resorts to using protein in order to produce the glucose needed. Ketosis is more of a "side-effect" of fat loss for lean individuals due to the high rate of glycosis due to low insulin levels on a low-carb diet.
MCTs are mentioned because they break down faster than Long-Chain Triglycerides (commonly found in meat), and can be mobilized into energy much faster. The former can be found commonly as Coconut/Palm Kernel oil, and butter has a small amount of it as well.
A small amount of glucose provides a small sparring effect (as well as limiting ketosis), as well as increasing protein intake. The former is more efficient. That's why "no carb" is relatively worse than "low carb".