Strong bones require a combination of nutrients, including calcium and magnesium. The best supplements have a good ratio of calcium to magnesium and contain the forms of each mineral that are easiest for your body to absorb and least likely to cause side effects.
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Magnesium and calcium both work together, but they can also compete for absorption, so it's important to consume them in the right proportions. Aim for a 2-1 ratio, getting twice as much calcium as magnesium each day, recommends Harvinder Sandhu, M.D., writing for SpineUniverse. Because you can only absorb about 500 milligrams of calcium at a time, your supplement shouldn't have more than 500 milligrams of calcium and 250 milligrams of magnesium per pill.
Best Form of Calcium
Of the two main types of calcium found in supplements, calcium citrate is more easily absorbed. Calcium carbonate can't be absorbed unless you've got stomach acid available, so if you choose a supplement containing this type of calcium, you'll need to take it with food to improve absorption. Calcium citrate also tends to cause fewer side effects than calcium carbonate, such as bloating, gas and constipation.
Best Form of Magnesium
Look for a calcium and magnesium supplement that contains magnesium citrate, magnesium chloride, magnesium aspartate or magnesium lactate, instead of magnesium sulfate or magnesium oxide. These forms of magnesium dissolve better in liquid, making it likely you'll absorb more of the magnesium contained in your supplement, according to the Office of Dietary Supplements.
Look for supplements with the U.S. Pharmacopeia symbol. This symbol shows that the supplement has the amounts of calcium and magnesium on the label, doesn't contain harmful levels of contaminants and will break down quickly during the digestive process so you can absorb as much as possible of the calcium and magnesium in your supplement.