People can live on extremely low calorie diets, and, in fact, there is evidence which supports the idea that people who do so, generally live longer. However, even extremely low calorie diets consist of the minimum number of calories it takes to sustain bodily functions, and a modicum of activity. If enough calories aren't consumed to meet your body's basal metabolic rate, you begin to starve, and your body starts consuming it's own muscle and fat to keep your organs functioning. If this condition persists, eventually, your body starts to consume its own organs, which will lead to death.