A lot of people who pursue more extreme exercise programs also adhere to a low-carb, high-protein diet. Low-carb diets have a number of benefits, including rapid fat loss and a better cholesterol profile (generally speaking), but recent research has found evidence that there could be risks associated with this diet as well.
In the March 2010 issue of Muscular Development, their section entitled Fat Loss featured 3 short articles on low-carb diets, and two others that alluded to them.
The consensus among recent researchers seem to be that although low-carb diets have their benefits, they can also promote unsafe levels of arterial plaque. One study in particular pitted low-carb diets vs traditional low-fat diets, and found that although most of the aesthetic and general health benefits were the same between the two, there was a significant increase in arterial stiffness among those on the low-carb diet.
Another study also suggested that there is a significant increase in appetite after switching from a low-carb diet to a traditional one. Perhaps this is the reason that almost 90% of people who lose weight on low-carb diets end up gaining it back within 12 months.
This study was performed on rats, and not humans, so the results could be different in humans since the physiology is different. Nevertheless, it could shed some light on this ongoing debate.
Low-carb diets work and have been proven to work, but perhaps they are not for everyone. Each person reacts differently to a lack of carbs, so the ketogenic approach should be used with discretion.
It should also be noted that many, many professional athletes and bodybuilders have successfully eaten traditional diets and become extremely lean, so the evidence does not clearly point to one style or another. Always consult a trainer, nutritionist, or physician before making any significant changes to your diet.