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As i have said several times without your seeming to notice, i have long had a diet of outstandingly good calories - but ate too many of them. Restricting my calorie intake on 2 days a week with no other changes of significance has take off 2 stones with no additional exercise.
Wrt why not a 1500 deficit? You seem to have missed the idea that long term gross deficits change metabolic rate AKA switching the body to starvation mode. It is you who has a hang up about a linear relationship, no one here is arguing there is. Another straw man of your own invention.
You rant against calorie counting yet advocate carb counting, protein counting and veg counting viz do you not recognise that counting food components is a proxy for counting calories?
My approach to the 5:2 diet involves no ongoing calorie counting - I now know what a roughly 500 or 600 calorie meal looks like, and it does not matter if i am out by 100/200 calories sometimes as there is still a big deficit on that day and one day at a time does not kick the body into starvation mode.