More protein in the diet has been linked to more calcium in the urine. Two reasons have been suggested to explain this phenomenon:
- Your body draws from its calcium stores (in bones) to buffer the acid load caused by dietary protein. This has led researchers to suggest that higher protein intake could increase bone loss.[1]
- Most studies that looked at protein intake and calcium excretion list dairy products as a protein source,[2] so higher urinary calcium could simply be the result of higher calcium intake (i.e., more calcium in, more calcium out).
Therefore, looking only at calcium excretion wasn’t enough. Subsequent studies showed that dietary protein promotes dietary-calcium absorption[3] and that high protein intake “promotes bone growth and retards bone loss whereas low-protein diet is associated with higher risk of hip fractures.”[4] High-protein diets have also been shown to modestly suppress the decrease in bone mineral density caused by weight loss.[5]
What happens is that when you ingest more protein, you absorb more of the calcium in your food, so less calcium ends up in your feces. Later, your body gets rid of the calcium it doesn’t need, so more calcium ends up in your urine, but not as much as would have otherwise ended in your feces.[6] Therefore, an increase in protein intake leads to an overall decrease in calcium excretion, which points to an increase in calcium retention. High-protein diets also raise your insulin-like growth factor-1 (IGF-1),[7] which notably promotes bone growth.[8]
All in all, current evidence suggests that protein’s effect on bones is either neutral or beneficial.[6][9]