One study found that 0.85 mg of calcium was lost for each gram (1 g) of protein in the diet. A meta-analysis of 16 studies in 154 adult humans on protein intakes up to 200 g found that 1.2 mg of calcium was lost in the urine for every 1g rise in dietary protein. A small but more focussed study showed a rise of 40 mg in urinary calcium when dietary animal protein was raised from 40 to 80 g. Urinary calcium to dietary protein ratio is 1 mg to 1g. The empirical observation that each 1 g of protein results in 1 mg of calcium in the urine agrees very well with the phosphorus content of animal protein (about 1 percent by weight).
This means that a 40 g reduction in animal protein intake from 60 to 20 g would reduce calcium requirement by the same amount as a 2.3 g reduction in dietary sodium, i.e. from 840 to 600 mg.
How animal protein exerts its effect on calcium excretion is not fully understood.