Starting in the 1970s, a hypothesis called the "acid-ash" or "calcium-leaching" theory proposed that high protein intake produces metabolic acid, which the body neutralizes by pulling calcium out of bone — implying high protein diets should weaken bones over time. This idea circulated widely for decades. The more recent, larger, and longer-duration research has not supported it.
What the calcium studies actually found
Early short-term studies did find that high protein intake increases urinary calcium excretion, which seemed to support the leaching theory. But later research clarified the mechanism: this is mostly explained by improved intestinal calcium absorption on a high-protein diet, not by bone breaking down and losing calcium into urine. More calcium is absorbed from food, and correspondingly more is excreted — the bone itself isn't the source.
What longer-term bone density research shows
Multiple systematic reviews and meta-analyses — including research specifically looking at bone mineral density over years, not just urinary calcium over days — have found higher protein intake is associated with equal or better bone density outcomes, not worse. A well-cited 2017 systematic review in the American Journal of Clinical Nutrition found no evidence that higher protein diets harm bone health, and some evidence of benefit, particularly in older adults.
Why higher protein may actually help bone health
Bone isn't pure mineral — it has a significant protein matrix (mostly collagen) that protein intake helps maintain. Higher protein intake is also linked to greater muscle mass and strength, and stronger muscles place more mechanical loading stress on bone, which is itself a known stimulus for bone density maintenance — this is likely a meaningful part of why the older adults in the sarcopenia research (see our older adults guide) who eat more protein and stay resistance-trained tend to have better bone outcomes, not worse.
The one real caveat
This research generally assumes adequate calcium and vitamin D intake alongside protein — a high-protein diet that's also very low in calcium-containing foods (dairy, leafy greens, fortified foods) isn't automatically protected just because protein itself isn't the villain the old theory suggested.