When most people think of low carb diets they automatically think Atkins. For obvious reasons. Dr. Robert Atkins formulated original low carb diet in 1972 and was met a with a hail of ridicule and negativity from medical community at large.The Atkins Diet, since renamed to Atkins Nutritional Approach, works on a very simple principle. Carbohydrates are what our body normally use to produce energy. If you restrict carbohydrate intake of a normal person their metabolism changes. Changes? Yes your body looks to next available energy source which is....fat. So now instead of burning carbohydrates for energy your body does same thing with your existing fat deposits.
In a low carbohydrate diet you actually increase your intake of fat and pure protein as opposed to reducing it. In theory more fat you eat more fat you burn because your body is now using fat for energy. It's common to see low carb dieters eating enormous steaks smothered in butter with fried eggs and mushrooms on side. A very high fat meal but also a perfect low carbohydrate meal.
The fat burning process is called lipolysis. As fat is being broken down for energy a secondary phase called ketosis occurs. Ketones are fragments of fat being broken down in process of lipolysis hence name ketosis.
Ketosis has a single nasty side effect -bad breath. For whatever biological reason conversion of fat to energy can lead to really, really bad breath. This is a common complaint amongst followers of Atkins and similar low carbohydrate diets.