Acetyl groups are made of carbonyl and methyl groups. To improve bioavailability of a compound, attempting to add on either a methyl or acetyl group may help.
Depending on the compound, skipping this step may cause the compound to be relatively inert. This is why, for example, calcium carbonate is a poor source of calcium as a nutritional supplement, but calcium citrate is readily absorbed- the citrate itself is an organic compound, so the body more readily takes it up out of the digestive system and the calcium can be used.
Maybe a dumb question, but from this should I conclude that calcium citrate does not fully dissociate in solution the way e.g. NaCl does? Because otherwise how would it matter what the counter-ion was for the calcium? Then again I've always been hazy on why ions seem to behave differently depending on what they originally dissolve from, so if there's something weird going on there I'd love to know about it.
Calcium carbonate is entirely insoluble. Calcium citrate is only slightly soluble in water.
Calcium carbonate will react with stomach acid to form calcium chloride (along with CO2 and water). When given intravenously, calcium chloride is perfectly bioavailable. However, within the small intestines, it has very poor uptake, both through the intestines' active and passive mechanisms. Any remaining calcium carbonate has no uptake at all.
May I ask why?