Extensively naturally occurring as chalk, limestone, marble, fieldspar, dolomite, eggshells (consist of 94% calcium carbonate), pearls, coral, stalactites, stalagmites, and the shells of many marine animals.
Can be found in biscuits, bread, cakes, ice cream, sweets, vitamin and other tablets and to firm canned fruit and vegetables, it is sometimes used for to deacidify wine. Also used in toothpastes, white paint and cleaning powders.
Should not be a problem at food additive levels but at higher levels may cause flatulence, constipation, haemorrhoids and bleeding anal fissures. Because of its' solubility, prolonged high levels may result in high quantities in the blood producing confused behaviour, abdominal pain, weak muscles and kidney stones.