Ketosis is actually a pretty rad thing.  It's thought to be an evolved mechanism that allowed humankind to stay (relatively) healthy and functioning in times of famine or during long northern winters that were inhospitable to growing carbs (ie: fruits, vegetables, and grains).  When the body is starved of carbohydrates, blood glucose -our body's primary energy source- dries up (figuratively).  When glucose is severely limited over an extended period of time, the body responds by concocting little balls of energy called ketones, manufactured from fatty acids.  These fatty acids come both from the diet and from the body's stored fat.  What's unusual here isn't that the body is burning some fat, but that this particular form of fat is able to cross the blood-brain barrier and provide energy to the brain.