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I'm not sure exactly either and don't find any immediate solid answers on a quick search. One loop hole that may exist is that 'no added sugar' means that it doesn't have any extra sugar added - but that doesn't discount the possibility that the high fructose concentrate is not reconstituted to exact proportions of its original dilution - which means high sweetness is gained by default, simply by adding less water - which doesn't then increase its nutritional value as well. Humans just shouldn't eat/drink things that sweet. Honey may be one exception but of course, in moderation.

Edit: Here's something from Canadian standards - which doesn't explicitly state how much water is or is not added to the concentrate in order to make it 'juice' again - https://cooking.stackexchange.com/questions/108885/can-juice-from-concentrate-be-diluted-100-juice

Anyways, sugar in foods and what is allowed under standards without labelling is a subject worthy of a book. lol

It seems there is a difference between fruit juice and juice drinks - I knew this but just to add to this discussion for someone else that might come along with interest - here was a paper I found on it for UK standards (I am in Australia by the way) I know the standards are different between countries but similar smoke and mirrors have been used - https://bmjopen.bmj.com/content/6/3/e010330