Yah, I didn't know either until I had to take calcium daily. The
cheapest and most dispensed variety, Calcium carbonate is the kind
that makes you fart. So I had to ditch that and go with the next
variety, Calcium Citrate. The only disadvantage is that you have to
ingest a bigger pill, which I call a horsepill because it's so
huge. When I first started on this variety, the only way I could
swallow it was to halve it. After a couple of months, I got so sick
and tired of swallowing twice, I started swallowing it whole. So
proud of myself. 
