Magnesium oxide absorbs a well as any other form of magnesium. And better than most.

Just as calcium carbonate absorbs as well as any other form of calcium.

The problem with those two compounds is..they are cheap!! So it is assumed that they are poorly absorbed which is completely false.

I used to make vitamins and raw material companies call and tell me about their new fangled compound( which are very expensive) I ask for studies and they are usually pending or done by their own study group. Not a third party researcher.

Mag citrate is the most know magnesium for constipation. It draws water to the bowel thus helping with constipation.

I have seen bottles of liquid mag citrate at those dollar stores and in pharmacies. You can chug one of those and you are good to go.

But, mag sulfate(epsom salts) is even better. It is the least absorbable magnesium..it will not be used by the body for much except it's laxative properties, so you get a concentrated dose for laxative purposes only.

and though I hate to provide links..here is all you need to know in regards to magnesium. Scroll down and you will see absorption rates for the different magnesiums.

http://ods.od.nih.gov/factsheets/magnesium.asp