Anything below ~1000 calories for the average person, and your body will not be capable of sustaining itself. Ultimately you will shift your metabolism toward the breakdown of fat, and peripheral protein, i.e your muscles to ensure your brain receives enough glucose as do your erythrocytes. Yes you'll lose weight this way but you're also much more likely to die than if you were a bit overweight. A good diet has barely anything to do with calories, it's more about obtaining the correct nutrition. You could eat 1000 calories of a butter a day - and die 10 years later, or if you could eat 1000 calories of nuts, legumes, wholegrains, vegetables, fish and fruits and become much healthier.