I think this explanation of the effects of “excess protein” is a common overstatement made by proponents of veganism and raw foodists. I used to believe and make this same overstatement, myself, 24 years ago.
My current understanding is that protein metabolism may use minerals as catalysts, but no catalyst is used up in the process. (That’s kinda the definition of a catalyst.)
It’s true that if the body is dealing with a true excess of protein, a lot of urea is produced, and in flushing out the urea, the kidneys inadvertently may flush out other minerals in the blood, including calcium. The body uses calcium from bone to keep the blood in balance, and this is the basis for the theory that excess protein leads to calcium loss from the bones.
But calcium loss to the urine also leads the gut to absorb more calcium! So when you’re excreting more calcium, you’re also absorbing more calcium. It’s a system with many things balancing.
The whole “bone equals calcium” equation also conveniently forgets that healthy bone is about 50% protein, by weight (think marrow, collagen…) Without the protein, bone cannot deposit the calcium.
Studies have shown that when the diet has plenty of calcium, extra protein does not, in fact, lead to bone loss, it leads to bone gain - especially in the elderly, if I recall properly. But only if there’s plenty of calcium in the diet as well as protein, and the person is able to absorb it (that is, if they’re not deficient in Vitamin D, for example, without which they can’t absorb calcium well.)
There’s a good article about this whole balancing process here; a quote:
As urinary calcium rises, a potential hypocalcemic stress is created, to which the parathyroid glands respond with elevated parathyroid hormone (PTH) secretion. PTH in turn acts on 3 end organs: bone, gut, and kidney. The gut effect is mediated by increased renal synthesis of 1,25-dihydroxyvitamin D, which in turn leads to improved calcium absorption. At the same time, PTH enhances bone resorption, so that some of the calciuric loss is offset from the bony reserves, rather than exclusively from the diet. It is in this way that high protein intakes, leading to incompletely offset urinary calcium loss, might produce bone loss.
In practice, my understanding is that within the context of an appropriate calorie diet, with plenty of vitamins and minerals, the body is very flexible about how it deals with different protein amounts. That is, if 2000 Kcal is correct for you, the body can deal quite well with getting 200 Kcal from protein, or 700 Kcal from protein. That’s 10% to 35% - a very wide range.
But if 2000 Kcal is correct for you, and you’re consuming 2600 Kcal, then it’s a different story. If you’re overeating, you will tend to suffer different long-term problems, based on what you’re overconsuming most. So, just speculating wildly now, if you’re overeating by 600 Kcal a day, and you’re eating a lot of excess protein, you might start having problems associated with excess protein, like mineral loss or kidney problems. But if you’re overeating by consuming a lot of refined carbs, you might start showing signs of metabolic syndrome or head towards diabetes. And if you’re overconsuming fat, you might see problems with fat in the liver or with fatty plaques in your arteries. And in all three cases, you’ll probably be gaining weight and getting fat.
Interestingly, if you need 2000 Kcal and you only consume, say, 1800 Kcal a day, then you aren’t likely to show any of these problem, even if you intake is rather wildly tilted towards being very high protein, or very high fat, or very high carb… because your body pretty much has to use up all of it to meet your needs.