Eating Less=No Constipation?

So, a few years ago I did something really stupid and went a diet where I ate only like 900 or 1000 calories. The food I was consuming was all healthy, but I didn't realize this was dangerous at first.  I was able to stop doing this a few weeks later after research  indicated this could cause anorexia.

Still, one thing I realized while going on this diet was that constipation wasn't an issue any more.  But when I stopped eating 900 calories and went to 2000 calories after realizing what I was doing was dangerous(still on the diet)I had problems with constipation.  The food was mostly healthy; I'm sure some of you realize that reaching 2000 calories with oatmeal, apple, salad,  rice, etc. is impossible, so it wasn't as healthy.  Then again, it wasn't like I was going to McDonald's or drinking soda.

I'm still having problems with this now.  Any ideas on why this would happen?

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