Having now read through a few articles with links to studies involving genuinely low fat diets (~10%) and genuinely low carb diets (~4%) I think I'm convinced enough to give ketosis and fasting a go.
In many ways I'm liking the ND, really it just takes 2-3 days of adjustment and then all is fine. I'm only 4 days in and already I've done a couple of walks, don't have sugar cravings in spite of the sugary shakes, and am feeling pretty mentally alert. What I don't like is the regularly feeling that my teeth are rotting and needing to brush my teeth, and in particular, the ND is an approach where exercise is not recommended (particularly in the weight loss phase). While I find I can do an hours walking no problem on it, there's no way I'll be going into the mountains on 800 calories a day.
It felt very strange today to be looking out at a winter wonderland, and desperately wanting to be out on a big walk, but not feeling it was safe to do so. I don't think the ND plays well with my strengths at the moment, I'd much rather have a bit more energy and be getting out and about more. It feels like whatever help it may be doing in one way, it's taking away from something else I do which is healthy.
I do have reason to suspect I have an issue with genuinely very low carbing. When I went below 20g a day a while ago just because I was eating to my meter, I ended up in hospital with stabbing chest pains. And again just recently, when consciously making sure I added fat to compensate for the missing carbs, I was needing about 4,000 calories a day because of the walking I was doing, which translated to a lot of fat, and I didn't like the mild chest pains I was getting, or much else about how I was feeling. So I've had minimal carbs in two contexts: calorie restriction, and weight maintenance. It didn't go well.
So it's something I need to approach with great caution, but I think I'll ease myself into it much more carefully this time.
There's also one marker that I haven't been able to change in 4 months of weight loss. Yes my HbA1c has dropped dramatically, I've lost a ton of weight, and more satisfyingly my fasting bg when maintaining weight has come down, but when I do a regular modest carb tolerance test, I can either get a good result by making the most of the "last meal effect", or a bad result by not. That bad result has stayed the same in 4 months: I get a 2 hours postprandial rise of almost +3 mmol/l.
I'd have hoped to see that drop a little as the fat came off. But it hasn't. And the one thing I haven't tried is squarely combating insulin resistance. I am curious to know if that will have an effect on this number which I can't seem to change!
Fasting may also work well for me at the moment, if I can time the non-fasting days with getting out into the hills at the weekends.
Anyway, more reading to do to see how to go about it sensibly.