Hi Emanuel and welcome:

You first have to recognize that a day is not 24 hours, it is a Monday or Tuesday or whatever that goes from midnight to midnight. There are only seven of them in a week. And 5:2 requires that you eat only 5/600 cal. during two 36 hour periods each week – not 24 hour periods. See the FAQ, above on the page, for an example.

If you choose to do the 24 hour variation, what you have is a reduced calorie diet. It is not 5:2. Using 2 p.m. Tuesday to 2 p.m. Wednesday as an example, that means that you have to make sure you eat less than your TDEE before 2 p.m. Tuesday and less than your TDEE after 2 p.m. Wednesday to have any hope of losing weight. If you eat your TDEE before 2 p.m. Tuesday, you will not have cut any calories from your diet on Tuesday, and if you eat your TDEE after 2 p.m. Wednesday, you will not have cut any calories from your diet on Wednesday so you will not lose any weight unless you eat less than your TDEE on the other days of the week. And if you actually eat calories during the 24 hour ‘fasting’ period, you will have over eaten and will gain weight over time. To determine how you are doing, given your example, you would add all the calories eaten on Tuesday, including the 400 for dinner, and all of the calories eaten on Wednesday, including the 200 for breakfast, to see how that matches up with your TDEE for those two days.

I have no idea why the book offered the ’24 hour option’, but it is not fasting and it does not automatically lead to weight loss. So if you choose to do it, just understand it is merely a 24 hour reduced calorie diet and how much you lose will depend on how little you eat before, during and after the 24 hour period – and the other five days of the week.

Here are some 5:2 tips: https://thefastdiet.co.uk/forums/topic/the-basics-for-newbies-your-questions-answered/

Good Luck!