By Dr. Robert Heaney

Protein, particularly of animal origin, is caught up in political correctness. Some argue, in fact, that we consume too much protein and that, for example, we would not have such a high calcium requirement if only we would eat less meat.

It has been known for nearly 80 years that increasing protein intake increases urinary calcium loss.

Because urinary calcium loss is the most important physiologic determinant of calcium retention, excess protein intake should be bad for bone, other things being equal. Some studies have indeed found this, but others found exactly the opposite.

Interestingly studies show that with increased protein intakes, rather than being harmful to bone, those with osteoporosis substantially improve recovery after hip fracture and reduce age-related bone loss in the other hip.

A positive effect of protein on bone is not surprising, as bone tissue is nearly 50% protein. However, a large fraction of the amino acids in bone collagen cannot be reused in new protein building. New bone turnover thus requires continuous intake of new protein.

If we don't eat enough protein, bone rebuilding is low on the body's priority list. But the other 50% of bone is mineral, and here calcium plays the crucial role. Without a diet containing sufficient calcium and protein, new bone formation will be limited.

There appears to be a homeostatic normalization that occurs which explains the paradox of the increased calcium lost in the urine with extra protein resulting in increased bone mass.

The homeostatic feedback system controlling the calcium balance does not regulate calcium intake and has no mechanism for assessing it. The system simply uses the combined calcium input from the kidney, gut and bone to prevent or offset a decrease in serum ionized calcium resulting from an increased calcium leak from the body.

In this way, increased protein intake which causes increased calcium loss in the urine, would be predicted to affect bone negatively, mainly at low calcium intakes.

Thus, it appears that protein and calcium act synergistically on bone if both are present in sufficient quantities in the diet. However, protein may seem to be detrimental to bone density because of its ability to cause one to excrete calcium in the urine when one's calcium intake is low.

American Journal Clinical Nutrition April 2002;75(4):609-10

Dr. Mercola's Comments:

One of the arguments vegetarians are found of stating is that increased protein intake, especially animal protein, results in loss of calcium from the bone. As Dr. Heaney explains this is not entirely true. If one has inadequate protein intake it is clearly quite detrimental to bone density. There are many studies that clearly demonstrate this. However, if one has excess protein intake and does not adequately have sufficient calcium in the diet, the protein will cause loss of calcium from the bones into the urine to buffer the system. Having large amounts of raw vegetables which are high in calcium and other acidic buffering agents will clearly compensate for this. Additionally, sufficient quantities of vitamin D in the summer from sun and in the winter from cod liver oil will also maximize calcium absorption to more than compensate for the loss of calcium from protein intake in most people. So if you have osteoporosis, or osteopenia you will clearly want to have sufficient.