True or False about calories and gainage

I am trying to remember a comment that was made to me when I was beginning my journey. 

That one day of eating a lot and/or some bad calories will not equal a 2-3 pound jump in a day. 

That you have to eat that way consistantly to gain that type of poundage. 

Which most likely the 2-3 pound increase is more water weight and not so much true weight. 

I am asking as I told this to my sister who is 6 weeks out from RNY WLS and I started second guessing myself! 

I think you're right.  The "bad" foods generally cause me to retain fluid - so I will see a couple pounds gain after indulging. 

The problem is making sure that the good days far outnumber the bad.... and occasional treats can be a slipery slope. 

1 pound of fat = 3500 non-burned calories.  So if you've eaten 3500 calories over and above what your body uses for fuel, you'll gain a pound.  To gain three pounds, you would need to have consumed 10,500 extra calories.

If she didn't eat that much, the it is probably water retention from too much sodium or just from normal cyclical fluctuations.