Hi Squidge,
The key is insulin levels. The “healthy” diet the program follows includes carbs and does not ensure that you are in ketosis.
If you are in ketosis then your body can access your fat stores, if your insulin levels are high then your body can not. – After a period of time, then your body says ok, food coming in has been restricted therefore to survive I need to get more efficient and metabolic rates drop. Thats why if you are following the program and recipes in the book which do not specify a level of carbs, then the guidance is to follow this way of eating and the 800 calorie intake for a maximum of 8 weeks. or sometimes 12 weeks.
Those of us who have a lot to loose have therefore done research and realise that its keeping the carbs down that protects your metabolic rate. In fact research shows that metabolic rates increase during fasting.
I know there are others on the forum who say if you are loosing weight at 50g of carbs then thats ok, but I don’t agree. – I know that I am sufficiently Insulin Resistant as a result of decades of a high carb diet that more than 25g of carbs a day puts me out of ketosis. So my advice is to stick to the 20g of carbs as a limit.