Although the evidence is accumulating, the scientists conducting the studies continue to be "surprised" by the result that more protein in the diet at the very least causes no harm, and in most studies improves bone density rather than causes bone loss. This makes some intuitive sense, since bones are one of the most protein-dense tissues in the body. Several of the studies suggest that increased protein intake improves calcium absorption from food.
Whether adding protein improves bones may be partly a function of how much protein the person was eating to begin with. But it is interesting that in several studies comparing the bones of people eating the standard “recommended daily requirement” of protein (.8 grams of protein per kg of body weight) with those eating more protein found that those who ate more protein than the standard recommendation had less bone loss.
A 2006 study focused on low-carb diets, rather than protein intake per se. Subjects were limited to 20 grams of daily carbohydrate for one month, and to 40 grams of carb for an additional two months. There was no problem with increased “bone turnover” (a short-term indication of potential bone loss) during this period of time. The lead scientist on the study, John L. Carter, has been quoted as saying that he was “shocked” at the results.
Of course, eating enough protein is not the only way to protect our bones. Sufficient intakes of calcium and Vitamin D, weight bearing and strengthening exercise, and avoiding smoking and excess alcohol can all help keep our bones strong throughout our lives.
Information about Osteoporosis and Its Prevention from About.com’s Guide to Arthritis
Dietary protein, calcium metabolism, and skeletal homeostasis revisited. , Am J Clin Nutr. 2003 Sep;78(3 Suppl):584S-592S.
Effect of dietary protein supplements on calcium excretion in healthy older men and women. , J Clin Endocrinol Metab. 2004 Mar;89(3):1169-73.
Low protein intake: the impact on calcium and bone homeostasis in humans. J Nutr. 2003 Mar;133(3):855S-861S. This chart is of special interest.
Protein consumption is an important predictor of lower limb bone mass in elderly women. Am J Clin Nutr. 2005 Jun;81(6):1423-8.
The impact of dietary protein on calcium absorption and kinetic measures of bone turnover in women., J Clin Endocrinol Metab. 2005 Jan;90(1):26-31
The effect of a low-carbohydrate diet on bone turnover. Osteoporos Int. 2006 May 23.