"Ketosis is a state in metabolism occurring when the liver excessively converts fat into fatty acids and ketone bodies which can be used by the body for energy."
"Most medical resources regard ketosis as a physiological state associated with chronic starvation. Glucose is regarded as the preferred energy source for all cells in the body with ketosis being regarded as a crisis reaction of the body to a lack of carbohydrates in the diet."
"Ketone bodies, from the breakdown of fatty acids to acetyl groups, are also produced during this state, and are burned throughout the body. Excess ketone bodies will slowly decarboxylate into acetone. That molecule is excreted in the breath and urine."
So basically, a person on Atkins is in a state of low-grade starvation, and is literally breathing the remnants of their fat cells into the air.
Following the above page to the one on low-carb diets, one learns that there is currently a debate over whether this state of starvation is normal for the human body. The argument goes like this: Before agriculture made carbohydrates abundant, people spent a lot more of their time in ketosis, burning fat and protein for fuel, instead of burning glucose derived from starch in their last meal. Basically, starvation was a way of life. As far as proof of the "benefits" or harmlessness of ketosis goes, this is a very poor argument, since by the same tack one could also consider it "normal" for inland peoples to develop goitre and for sailors to get scurvy.
The two long term studies cited by the article were not convincing either:
Their data shows that, beyond one year, weight loss is not significantly different between a low-carb diet or a "traditional" low-fat one. Based on this, I find myself agreeing with the Drs Arne Astrup, Thomas Meinert Larsen, and Angela Harper (RVA University, Copenhagen, Denmark), when they say:
"Weight loss on the low-carbohydrate diet is probably caused by a combination of restriction of food choices and the enhanced satiety produced by the high-protein content."
In other words, it works because, duh, people on it eat less than they usually do.
Seems the unifying thread here is the urge to eat. Whether physiological, psychological, or social.