Not a cdc but the reason why most consultants won't give bars or water flavourings in first 2 weeks is they are slightly higher in carbs and they like to make sure the person is firmly in ketosis before introducing bars incase the person is carb sensitive unlikey to but may knock you out of ketosis or take longer to get in to.
2nd reason is complete break from food like products as sometimes chewing on a bar may make someone have food thoughts.
But saying that some consultants say you can have them from day 1 so just depends on consultant and you.
thanks I wasnt sure of the science of it. It doesnt worry me too much if it takes a bit longer to get into K as i'll get there eventually! Its what I thought tho - not really a BIG deal but it helps if you dont