The milligrams you take are not necessarily the milligrams your body absorbs.
“What’s a good calcium?” is actually somewhat complicated. But a simple place to start is by recognizing that all calcium types are either organic or inorganic salts.
The inorganic forms:
- Calcium sulfate – 5-10% absorbed
- Calcium phosphate – 5-10% absorbed
- Calcium carbonate – 5-10% absorbed
The organic forms:
- Calcium gluconate
- Calcium lactate
- Calcium citrate – 30 to 35% absorbed
- Calcium amino acid chelates – 65-80% absorbed
- Calcium orotate – 90 to 95% absorbed
- Calcium aspartate – 85% absorbed
- Calcium ascorbate
Each of the above is obviously not entirely calcium there’s a percentage of ‘the other stuff’ attached to the molecule. So, the percentage of the compound that’s ‘elemental calcium’ is an issue.
The most common form of supplement, by far (of all types), is calcium carbonate. It’s also the cheapest. What’s more, it also has the most elemental calcium (40% of the total molecule). Seems like that might pretty much settle the selection issue, right? Unfortunately, there are two problems with the carbonate form:
1) Like the other inorganic forms, it’s the most poorly absorbed (only 5-10%)
2) Unlike the other inorganic forms, calcium carbonate requires (and binds) the most acid.
The latter problem above is appealing if you’re trying to sell an antacid product (like TUMS) but it’s very much a double-edged sword. More acid is now required for the digestion of proteins, or else malabsorption (and indigestion!) can occur. Since you take the antacid for indigestion, you can see where this is headed.
Calcium ascorbate, which gets you the benefit of vitamin C as the other part of the molecule, along with the fact that it’s no longer an acidic form of vitamin C a neat solution to several problems.
Calcium citrate – the citric acid reduces the amount of stomach acids required for absorption.
You can’t generally take calcium alone without making biochemical trouble for the body. Calcium is not found in nature (in edible form) without magnesium, and they therefore should be given together. Studies show that calcium alone may even be preferentially laid down in arterial walls rather than in bones. Plus, phosphorous is also needed with calcium. The problem here is that phosphorous is one of the few minerals that’s over-supplied in the modern (crappy) diet. Excesses of phosphorous in the absence of the other minerals can create a problem with balance and possible leaching of other minerals.
What’s not mentioned in those cute major-media ads for calcium and antacids is that for bones, the calcium must also have not only magnesium (okay, and phosphorous), but also manganese, silica, boron, strontium, and vitamin D (and that last one in high doses), vitamin C, vitamin B-12, at the very least.