In a post down below I shouted at someone to not get MDF wet and that started a discussion.
I took a scrap of MDF that was cut out of a shelf that came from HD a couple of days ago. The scrap was about 1.8" wide and .75" thick and 8" long. Last knight I stuck it in a cup of water and soak the end about 2 1/2" up overnight, about 24 hours. I occasionally drizzled a little water on the top end.
The submerged end swelled .3" in thickness but almost nothing widthwise. The top end swelled about .05" in thickness.
I thought it was interesting that it swelled in thickness dramatically but not at all in width. It started swelling within an hour.