Diet which is high in protein generates a large amount of acid in body fluids [2]. The kidneys respond to this dietary acid challenge with net acid excretion, and, concurrently, the skeleton supplies buffer by active resorption of bone resulting in excessive calcium loss [2]. Moreover, acid loading directly inhibits renal calcium reabsorption leading to hypercalciuria in combination with the exorbitant bone loss [3, 4]. In a metabolic study an increase in protein intake from about 47 to 112 g caused an increase in urinary calcium and a decrease in calcium retention. The data indicated that protein-induced hypercalciuria was due to an elevation in glomerular filtration rate and a lower fractional renal tubular reabsorption of calcium, the latter of which caused by the increased acid load on the renal tubular cells [8]. Another study on subjects consuming diets containing 48 g protein daily to 142 g showed that urinary calcium doubled, while the calcium balance became negative [9]. In addition, the effect of dietary protein on markers of bone turnover has been evaluated [10]. In this study the subjects were on a well-balanced diet for 2 weeks which was followed by 4 days of an experimental diet containing one of three levels of protein (low, medium, or high). Urinary calcium excretion was significantly higher, and urinary N-telopeptide excretion (indicator of bone resorption) was significantly greater during the high protein than during the low protein intake. Data suggested that, at high levels of dietary protein, at least a portion of the increase in urinary calcium reflected increased bone resorption [10]. Additionally, subjects on a low-carbohydrate high-protein (LCHP) diet for 6 weeks had increased urinary calcium levels, decreased calcium balance, and decreased serum osteocalcin concentrations [11]. In a prospective study, protein was associated with an increased risk of forearm fracture for women who consumed more than 95 g per day compared with those who consumed less than 68 g per day. Women who consumed five or more servings of red meat per week also had a significantly increased risk of forearm fracture compared with women who ate red meat less than once per week [12]. Furthermore, the effect of high-protein diets on the excretion of calcium in urine was evaluated in normal persons and patients with nephrolithiasis. All subjects were given diets containing 0.5 g protein/kg/day, while, during the experimental phase, each person received an additional 1.5 g protein/kg/day. There was a consistent increase in urinary calcium with the high-protein diet averaging 88% above control in the normals and 82% in the patients [13]. Moreover, in a study where protein intake was varied from 47 g/day (low protein diet) to 95 g/day (medium protein diet) and to 142 g/day (high protein diet) the urinary calcium increased significantly with each increase in protein (168, 240, and 301 mg, resp.) [14]. In addition, it has been shown that increasing the protein intake from 48 to 141 g daily caused a highly significant elevation in urinary calcium, the mean daily values being 175 and 338 mg, respectively [15]. In another study the relationship of animal protein rich diet to calcium metabolism was investigated during a 12-day dietary period. An increase in urinary calcium excretion was found indicating that the animal protein-induced calciuric response could be a risk factor for the development of osteoporosis [16]. Notably, it has been shown that the consumption of high calcium diets is unlikely to prevent the negative calcium balance and probable bone loss induced by the consumption of high protein diets (protein-induced hypercalciuria) [17]. In this experiment (a 95-day metabolic study) subjects received formula diets supplying 12 g nitrogen or 36 g nitrogen, and approximately 1400 mg calcium per day. Overall calcium balance was −37 mg/day on the 12 g nitrogen diet, and significantly lower at −137 mg/day in subjects consuming the high protein diet [17]. Additionally, dietary excess (2 g/kg/day) in animal protein for 1 week led to significant changes in urinary calcium excretion rates [18]. Furthermore, in an interesting study the effects on urinary calcium levels of increasing dietary protein from 50 to 150 g protein were compared with those of increasing the sulfur amino acids to simulate the amounts present in the 150 g protein diet. The increase in protein intake caused urinary calcium to double, while sulfur amino acids added to the low protein diet also caused urinary calcium to increase [19]. Moreover, a prospective cohort study showed that a high ratio of dietary animal to vegetable protein increases the rate of bone loss and the risk of fracture in postmenopausal women. Animal foods provide predominantly acid precursors, whereas protein in vegetable foods is accompanied by base precursors not found in animal foods. Imbalance between dietary acid and base precursors leads to a chronic net dietary acid load that may have adverse consequences on bone. An increase in vegetable protein intake and a decrease in animal protein intake may decrease bone loss and the risk of hip fracture [20].