During starvation, and high fat or high protein diets, where not enough carbohydrates are being eaten, the body's carbohydrate reserves are quickly used up. As a result, there is an accelerated use of fat for fuel. When we burn fat as our primary energy source, it is not completely metabolized, and chemical compounds called ketones are produced. Ketosis is the accumulation of these ketones in the body.
Ketones have calories, like the fat from which they are derived.
Ketones are needed by the brain which cannot use fat and normally relies on carbohydrate for fuel. Also, ketones are used to a minor extent by the heart, skeletal muscles, intestines, and kidneys for energy. The reason staying in ketosis is required on low carbohydrate diets is that those monitoring the diet can determine whether you are in fact burning mostly fat by measuring the ketones in your urine. It is a very effective way to tell if you are sticking to their required diet regime. The calories lost by ketone excretion would at most amount to a weight loss of one pound a month.