What happens when calories are too low?

1) Muscle mass is broken down for energy (catabolism).
2) Metabolic rate will begin to drop (typically) after 3 days of very low calories – this is related to, and compounded by the loss of muscle mass.
3) With very low calories you risk sluggishness, nutritional deficiencies, fatigue, and often irritability. You are completely set-up for a regain in fat if you suddenly return to your previous eating patterns.