Every time you lick a stamp, how many calories are you consuming?

It depends on the class of postage. In the UK as an inticement to use the more expensive first class postage, the Post Office used low fat glue.Second class was full fat and had more taste.

I was trying to find a serious answer to this and came across the following:-
'And the glue on Israeli postage stamps is certified kosher'
and mdw you are officially labelled as extreme as I also foundthis:-
'They tend to eat low-calorie food in very specific quantities, e.g., 10 green beans or one tablespoon of ketchup. Some even go to the extreme of counting the calories in the glue on postage stamps as part of their daily food intake.'

At last found some:-
a)Open the countless invitations to galas, fashion shows and luncheon that arrive in the mail each day. The tearing process will work your wrists and strengthen your hand muscles. Just be careful to use self-adhesive stamps and a letter-sealing machine when sending your reply card: Glue on stamps and envelopes may have as much as two to five calories each.

b)I just found out that every time you like a stamp, it's 1/10 of a calorie!!
c)Yes, I have heard that. I also have heard that licking an envelope is 3-5 calories! There were times back in the old days when I would lick hundreds of envelopes in a day at work. I don't do it any more! Yuck. https://www.livestrong.com/article/330369-how-many-calories-do-you-consume-while-licking-a-stamp/

d)Q. It has to be sticky enough to hold yet maneuverable briefly once set down, drying then into firmness, tasting not too bad, offending no one's religion, not too high in calories, and cheap. Whose job is it to meet these specs?
A. Postage stamp glue makers worldwide, says David Bodanis in The Secret House. The solution is a mixture of Elmer's or other glue, plus powdered baby food starches (from U.S. corn or European potatoes) to lend last-secslideability.
Usually, the little that is swallowed is either digested in the case of the starch or dissolved in the case of the tiny glue balls leftover in the stomach or esophagus. It's reported that even the most thorough licking of a typical U.S. stamp will add only 6 calories, though very large stamps may deliver a gut-busting 15.
https://en.wikipedia.org/wiki/Postage_stamp_gum#Types_and_application