I don't understand why anyone in this day and age would recommend tums as a calcium supplement. It is for heartburn. It weakens stomach acid - you need stomach acid to digest and break food down so i can be absorbed. So why take as a calcium supp if you don't have heartburn. It also has aluminum salts. I copied this right from the tums website:

Antacids are the fastest-acting heartburn relief available because they start to weaken the acid in your stomach the second they reach it. The weaker the acid in your stomach, the less likely it is to give you heartburn. The longer an antacid stays in the stomach, the longer it works; and having some food in your stomach may actually prolong an antacid’s effect.1

Most antacids contain at least one of these key ingredients: calcium carbonate, magnesium hydroxide, aluminum hydroxide and/or sodium bicarbonate. While all these ingredients work to neutralize your stomach's acid, it's important to know more about them so you can choose the one that's right for you.