Diets high in animal protein result in an acid overload that could lead to a higher risk of kidney stones and osteoporosis. Results of the only study to examine kidney function were striking. When subjects switched to Atkins from their regular diet, they excreted over 60 percent more calcium and their urine was markedly more acidic, placing them at higher risk of kidney stones. Shalini Reddy, MD, at University of Chicago, launched the study because, she says, "we had patients on the Atkins diet saying that they were getting kidney stones." However, during the eight-week study, Reddy and her colleagues could measure only risk factors, not actual kidney stones, which can take quite a while to form. As for osteoporosis, there are no long-term studies associating it with a low-carb diet. The NIH project will begin to address this concern.