In a previous discussion, soaking beans in salted water prior to cooking was a suggestion to soften skins. Does this result in less calcium and magnesium content of the cooked beans?
See Common Bean Myths from Captious for a survey of several bean related issues. They report that Cook's Illustrated has indicated (emphasis added):
[...] during soaking many nutrients leach out of the beans, more so with a quick soak (presumably due to the brief heat) than with a long cold soak. Apparently heat breaks down cell membranes within the beans, and increases the solubility of water-soluble nutrients, such as calcium, magnesium, thiamine, riboflavin, and niacin. As a result, quick soaking tends to leach somewhat more of the nutrients out of the beans than do slow soaking methods.