Carbohydrate is the only macro nutrient that humans can live without eating but in order to keep the brain and other organs functioning normally, the carbohydrate depleted human body must begin producing a kind of brain-fuel called "ketone bodies". The production and circulation of ketone bodies is, by definition called "ketosis".
All low-carbohydrate diets like Atkins cause ketosis. Starvation causes ketosis. There are some reasons to believe that ketosis may have been the norm for our pre-agricultural ancestors and there is abundant evidence in the bio-medical literature that ketosis is not, generally, a dangerous or even harmful condition.
Ketone bodies are volatile chemicals not too different from acetone (nail polish remover). I don't however mean to imply that they are dangerous like acetone, just that they are volatile. And the volatility of these molecules causes them to "come-off" in people's breath creating a strange, vaguely fruity chemical odor. This is often called "ketosis breath".
Ketosis also causes duiresis (water loss) and this is why people starting the so-called "induction phase" of a low-carbohydrate diet lose dramatic amounts of weight; they lose dramatic amounts of water. Sometimes this water loss is accompanied by the loss of too much potassium from the body and this CAN be dangerous. Fortunately it is relatively rare.
Another important side effect of ketosis is appetite supression. People in long-term ketosis report feeling very little hunger, even when they are losing weight rather rapidly. This lack of hunger, above all, probably best explains the popularity of low carbohydrate diets.
People in ketosis report little hunger as long as they remain in ketosis, but if they consume enough carbohydrate to shut-down ketone body production, then the appetite suppressing effects of thos chemicals are withdraw and people tend to go a little crazy. So, unfortunately, ketosis is "all or nothing". Even small amounts of carbohydrate are enough to wreak havoc.
For whatever reason, few people seem able to live in a permanent state of ketosis. Again, it's not that they CAN'T but rather that they simply choose not to. America and most of the western world is a "high-glycemic" realm and it it's pretty easy for a low-carb dieter to "slip-up" and eat a ketosis-ending amount of carbs.
(after consulting with a doctor).
FAT SCIENCE > Nutrition and Weight Loss > Diet and Dieting > Macronutrient Restricted Diets > Low Carbohydrate Diets >