Does running cause weight gain?

A few months ago (in February, actually) I stepped on the scales to see that I had suddenly gained 5 pounds since my previous weigh-in.  This came just a few days after the longest run of my life (13.1 miles, which I ran on a whim during my Saturday morning run).  I immediately suspected that body was probably hoarding energy after the shock of the long run.  But I decided to look into this weight gain a little further to see if I could learn why I had randomly put on weight.

First, I made no changes to my diet (I didn't eat more than usual); additionally, aside from the long run, there were no other changes to my lifestyle that might prompt a weight gain.

There are, essentially two schools of thought: 1) you gain weight because you overeat in response to the additional hunger from running, and 2) the second school of thought is that your body gains weight because it's storing energy and/or water (and here we have a few divergent theories).

Anyone who tracks what they eat and acts responsibly, isn't going to overeat--nor is this review meant to address such issues, at any rate.   So let's talk about what's actually happening to a healthy person who goes for a long run and gains weight without actually stuffing their face.

Coach Jeff at Runners Connect has an interesting article about running and weight gain. "Drink a gallon of water and you’ve instantly gained 8.3 pounds."  His key points are that you will:

• Your body will store extra water to "repair damaged muscle fibers and to deliver glycogen to the working muscles" (short term).
• Your body will build muscle [by running] (long term).
• If you eat too much...

As an aside: I've noticed that despite the addition of about 6lbs this year, my waist has only minutely expanded.  In other words, my size 32 pants continue to fit.  I believe the cause of my own weight gain has been in doing less walking and cross-training this year as compared to last year.