I think an interesting question is: if the Biggest Loser contestants in the well known study did everything they did except instead were on keto diets, would that be enough to make their outcomes so different in terms of RMR and weight regain? In a nutshell, they lost lean mass while taking part in the show, but then regained a fair bit but that didn't help raise their RMRs. Would keto have made all the difference?
I think there could be a lot more to it than purely muscle mass.
I'm sure there is, and every little helps. I'm also not sure that a low RMR is necessarily a bad thing as it can be a measure of how efficient the body is. So if you take a group of people and give them 24hrs on a Kreb's cycle, the ones that can do the most work for the fewest calories would be the most efficient and arguably fittest. But if they overconsumed calories, would store those as fat because that's just what the body does.
I think you can see some of these effects if you look at different athletes. So compare say a long distance runner with a 100m sprinter. The sprinter would have more muscle mass because they need it for that explosive power. But they'd be at a disadvantage against a marathon runner because they're carrying a lot more weight and need to fuel it.
They're extremes though, and so are 'bread and circuses' stunts like the Biggest Loser. Contestants are incentivised to lose the most weight, not end up the healthiest and especially because that has to fit with the production schedule. For everyone else, the goal should be to find a sustainable and enjoyable way to eat and live.
For me, I'd struggle with the ND simply because I really don't like shakes, and especially artificially flavored shakes. If Optifast came in sausage and bacon flavors, I'd probably enjoy them more than strawberry or chocolate. I think there's an irony in diet shakes being dessert flavored, when for dieting, desserts should be reduced or avoided. ND appears to work though, for it's intended purpose, ie a supervised crash diet.
I think that's also one of the challenges, ie an NHS approved process vs something we can choose to do and experiment on our own bodies. If it's official, it needs to be safe given the 'first, do no harm' rules. If it's unofficial, it just needs to avoid falling foul of health or medical marketing and labelling restrictions. I think this is where LCHF and keto fall at the moment, especially as they're contrary to the official 'Eatwell' guidance. But there have been more studies and trials showing benefits, and often no clinically significant harm, so there's growing pressure for official acceptance.
Part of that would be defining terminology, ie recommended carb levels for LC or keto, and most importantly how and when to advise people to follow those diets, ie a keto or high protein diet probably wouldn't be advisable for someone with renal problems. Part of that should be preserving or increasing muscle, simply because muscle helps burn calories.