Professor Taylor, who developed the Newcastle Diet (ND) had never suggested his diet is a long term solution, and agrees it is important to transition to a longer term eating plan in a considered way.
@Janine , Professor Taylor recommends having your Doc's support when following this approach, so that any other relevant medical conditions or indicators can be taken into account.
I didn't follow the ND myself, and achieved similar results to the aim of the ND by eating "normal food", with reduced carbohydrates. I wasn't aiming to lose weight. I was aiming to reduce my blood sugar numbers. My trimming up just sort of happened.
Prof Taylor suggests that reduction of the visceral fat (VF) is the important thing, but how much or little weight required to do that is very individual. He also states the means of VF reduction is unimportant.
A good number of members on another forum I participate in have followed the ND; some for shorter/longer periods than others. Some have had excellent results, and some found it just too gruelling or incompatible with other aspects of their lives, like family or work circumstances.
If you feel the ND, over the short term, is right for you, then do try to talk to your Doc; even if it's a telephone consultation, then you'll have covered all bases.
Good luck with it all