I got this from a university webpage...
. Calcium carbonate (TUMS) vs. calcium citrate (Citracal)
Either form of calcium supplement is appropriate after gastric bypass surgery.
No calcium pill is pure calcium. The calcium needs to be combined with something else to make a pill: citrate, gluconate, carbonate…What is important is the amount of “elemental” calcium that you are getting in each pill. Calcium carbonate contains about 40% elemental calcium. This means that a TUMS tablet containing 500 mg of calcium carbonate actually contains 200 mg of elemental calcium. You should not take more than 500 mg of elemental calcium at a time. Your body will absorb large amounts less efficiently. Larger doses of calcium should be divided throughout the day. The recommended dose of calcium carbonate after gastric bypass surgery is 3 TUMS or one Caltrate (600 mg elemental calcium in each) per day.
Calcium citrate provides 21% elemental calcium. This means that you need to take more of it to get the same dose of elemental calcium. Studies have shown that it is a more easily absorbed form of calcium (more bioavailable). A 950 mg tablet of Citracal will provide 200 mg elemental calcium. You should take 1-2 tablets twice a day if you choose this form of calcium supplementation (generally more expensive).
Viactiv is another form of calcium supplementation. This is actually a soft calcium chew that comes in several flavors (milk chocolate, mochaccino, caramel, orange cream). Each Viactiv chew contains 500 mg of elemental calcium as well as 25% of the RDA (recommended daily allowance) of Vitamin D and 50% of the RDA of Vitamin K. Each Viactiv chew contains about 20 calories and 0.5 grams of fat. Taking two Viactiv chews a day is also an acceptable method of calcium supplementation.
Another article went on to say, is maybe an easier way to understand that citrate is acid based carbonate is alkaline based. calcium is best absorbed in an acidic environment. So due to the decrease of acid in our new tummies we would be better off taking the citrate. It says that the carbonate is best absorbed when taken with food or some acidic juice like OJ. So i am just going to keep taking both..