Magnesium is Vital for Good Health useful article from Natural news
Glad to see the comments on calcium, but he should have made the point that calcium uptake is also dependent on Vitamin d status. You've got to have a 25(OH)D above 32ng 80nmol/l to absorb the maximum amount of calcium from your diet/supplements. If you actually look at the plots they use to come up with that recommendation you will see there are lots of "outliers" that require much higher D3 status, perhaps around 110nmol/l 42ng/ml before they reach their particular maximum for calcium absorption &/or BMD. The trouble with minimum threshold levels is that people tend to interpret them as targets to aim for or optimum levels rather than the lowest acceptable level.

We wouldn't say the same if the threshold we were talking about was income.

Certainly sufficient to cover the daily bills is a reasonable minimum threshold, but common sense tells us that more than the bare minimum affords greater safety and better quality of life.