PJ, there wasn't much detail on the study. I get very angry when my sister who is extremely overweight and recently diagnosed with diabetes, tell me how dangerous Atkins is. Meanwhile, she eats PopTarts!

Anyways, here is the article:

"Ill Effects of High-protein Diets: More Than Just Theory"

Developing kidney stones or osteoporosis as the result of a low-carbohydrate, high-protein diet might sound like a remote possibility put forth by those who "believe in" a higher-carbohydrate regimen. But a meticulously controlled new study suggests that the possibility is more real than many have supposed. The diet triggers biochemical changes that set the body up for kidney stones, and bone loss--and the process starts after just 2 weeks.
Researchers recruited 10 people who wanted to lose weight, asking them to follow the popular high-protein Atkins diet. For 2 weeks, the volunteers followed what Dr. Atkins calls his "Induction" plan, eating an average of 19 grams of carbohydrate a day (about half of a 2 1/2 ounce bagel) and as much protein (and fat) as they wanted. They then switched to his more liberal, but still low-carbohydrate "maintenance" plan for 4 weeks. The volunteers were monitered in a high-tech metabolic ward, where the scientists carefully tracked their food intake and measured different substances in their urine and blood.
What was found was that their urine had become markedly more acidic, and there was a "striking increase" in the amount of acid their kidneys had to handle. The more acidic the urine (a result of both the high animal protein and very low carbohydrate content of the diets), the more likely kidney stones are to form.
An outflow of excess acid also promotes excretion of calcium, throwing the body into a defecit that could lead to bone loss. Indeed, the investigators found that not only did calcium levels in the volunteers' urine rise dramatically, but also that the losses weren't offset by increased absorption of calcium from food.
The study was a short one, but "acid excess will be sustained as long as carbohydrate restriction and high-protein intake are maintained," the authors note. WE note that the Atkins maintenance diet isn't meant to be temporary--which means that there's lots of time for any extra acid to exert its negative effects.