You're thinking of ketones, which are what trigger your body to burn fat for energy. (Another perk of ketosis is being able to go long stretches between meals w

You're thinking of ketones, which are what trigger your body to burn fat for energy. (Another perk of ketosis is being able to go long stretches between meals without getting hungry.) It should be noted that the first time moving to a low-carb, high fat diet, it's mildly traumatic and unpleasant. It takes 1-2 weeks of low energy and constant hunger before the body switches to ketosis mode. (This is sometimes called the "carb flu".)