This book differs from the full 5/2 diet book and contradicts it. The original 5/2 diet book states you should eat 500-600 calories on your fast days and eat normally on the other days. This book says to lower your calories some percentage on all the days, and then also have the fast days. This is NOT what the original 5/2 recommends at all. I can see lots of ways this would defeat the purpose by making it really difficult. It would be like being on a regular diet (hard enough) and then having two days where you fast (harder) and then going back to (difficult) regular diet. I think you would probably slow your metabolism with the dieting all the time and start feeling really deprived. The whole point of the 5/2 is that you do not slow your metabolism because you are not dieting all the time and your metabolism doesn’t get used to less calories, and you get all the benefits of fasting because it is not hard to do it only two days a week knowing you will not be deprived tomorrow!