Two processes are involved in the intestinal absorption of calcium. Vitamin D . Moreover, calcitriol-mediated adaptation in calcium absorption appears to decline with age.
Food is another source of vitamin D in addition to exposure to sunlight.
Dietary protein increases glomerular filtration rate and decreases renal calcium reabsorption, both of which increase urine calcium. An effect of protein on bone, if it exists, is likely to be small. The calciuretic effect of dietary protein is no reason to reduce intake of this nutrient. As mentioned above, dietary phosphorus reduces urinary calcium excretion and counterbalances, in part, the calciuric effect of dietary protein. Protein intake increases urinary calcium excretion.