This answer about calcium supplements and strength-building mentions calcium helping the body to absorb protein, a claim I have also heard several times before.
Can anyone offer a deeper explanation for how/why this works, or any reliable sources?
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When you eat excess protein, your body leaches calcium from other sources in order to process the unnecessary protein. This means it will take calcium from your bones, if needed. The calcium loss can result in many health problems, including osteoporosis. According to this article from Michigan State University, the average Western diet includes double the protein needed for the body. The article also states,
"Several studies show that extra calcium beyond the minimum requirement of 400 mg does little or nothing to stave off osteoporosis.", claiming the repeated studies have shown that bone loss is not related to calcium intake, but calcium losses due to above-RDA protein intakes. Studies have shown that doubling your protein intake causes a 50% higher rate of calcium loss.
If you want to try digging more deeply into the real science behind all of the interactions between protein and calcium, this article from The American Society for Nutritional Sciences has some less cryptic information about the calcium-protein interactions in the body, and this page has lots of diagrams of various chemical reactions in the body. The last mentioned page ends with the sentence,
"In actual fact, signaling pathways in the cell are much more complex.", and the following image: