@chrisjohnh thanks for bringing this study to our attention. As a scientist myself I agree with you that "the science" is important. As long as LCHF is considered anecdotal evidence with sample sizes of as small as one, it will never become NHS or government advice. We might not like it and can get angry, but that won't change it.
Regarding the DIRECT study, I had a look and agree with you that the information you want is not available. What did they eat in these two years? Did they continue to take medication?. What I also consider relevant is that you had to be very obese with BMI > 37 to be included in the trial, so the findings might not apply to T2s like you and me who lost 10 to 15 kg.
For participants in the DIRECT study, who maintained their weight, I would apply what I call the "There is no free lunch" theorem (pun intended). Any diet, not only the Newcastle diet is not sustainable in the long run. Basically if you go on a diet of just reducing calories, you lose weight but you also lower your metabolic base rate, so your body will require fewer calories. At this point will stop losing weight and feel hungry and miserable all the time. At some point you will give up and gain weight again. On the other hand fatty food is more satiating, so increasing the fraction of fat in your diet will allow you to not feel hungry and you'll be able to keep your weight. LCHF is based on this. Thus to make weight loss sustainable you need to change your diet. I claim that "There is no free lunch" also applies to the DIRECT participants.
My hypothesis is that the DIRECT participants who maintained their weight, had to significantly change their diet. As you know there is low hanging fruit, such as cutting out fizzy and sugary drinks. IMHO it is very likely, that they reduced the carb fraction in their diet by a significant fraction. Why would they do this? The study indicates that the participants increased their happiness considerably. So if you were at a BMI of 37 and lost around 20kg , possibly more, you will be able to do things, such as sport, play with children, ... again. This is a powerful motive for people to find a way to keep their weight. So it is possible that they managed to reduce carb intake whilst maintaining or possibly increasing fat in their diet without being told. This is only a hypothesis and we do not have data to corroborate or disprove it.
@chrisjohnh if you have connections to medical researchers, please suggest such a study.
My own story (again a sample size of one) is consistent with this. In 2019 I did Michael Mosley's 5+2 diet with 600 calories on fast days and lost 10 kg. This brought down my HbA1c to 42. It is now three years in which I managed to keep this weight. I don't count carbs on normal days, but have continued fasting for 2 days a week, as this works well for me. However I noticed that I've significantly reduced portion size of anything carby, e.g. potatoes in my diet. Pasta, which I love, is now a rare treat. I might eat a half slice or a slice of bread a couple times a week, but no more. Thus my carb intake could be around 100 to 150 h similar to
@chrisjohnh To compensate I've increased my fat intake, mainly cheese and oily fish, and eggs. Thus my diet has changed considerably from 4 years ago. Since end of 2019 my HbA1c has stayed in the pre-diabetic range, which is great, as I didn't take this for granted.