Caryn, good instincts on your guess that magnesium is important for calcium absorption! When I was researching magnesium a while back that's exactly what I was reading. Unfortunately, it's hard to find completely objective research online about vitamin supplements because so many "complemetary medicine" folks write a lot online, and I don't have the background to know for sure what is accurate and reputable. I did find a Harvard website
" You might decide that you donÃ¢â‚¬â„¢t, or canÃ¢â‚¬â„¢t, get enough calcium from food sources. (Hint: most children, adolescents women, and many men, consume less than their RDA.) Which calcium supplement you choose depends on other factors in your diet. Optimal calcium metabolism requires adequate amounts of vitamin D and magnesium as well. If your diet has little of these nutrients, you might want to include them in your supplement."
Most vitamin sites seemed to recommend a 2:1 ratio for calcium:magnesium supplementation, but again, I didn't find objective studies on that.
http://www.mgwater.com/calmagab.shtml is the "Magnesium Website's" explanation of how magnesium helps calcium absorption. It's a really interesting read, but I have no way of knowing if it's accurate [;)].
And thanks, Caryn, for the breastfeeding study! I knew mag sulfate delayed milk production / engorgement, but I was shocked to see regular magnesium supplements having an effect on breastfeeding!