600 calories per day is absurd unless you are the size of a toddler and very, very sedentary.
Your body needs food to function, the exact amount of which depends upon your size and lifestyle, but should never dip below 1,000. And 1,000 is even too low.
Remember such an extreme weight-loss regiment can result in hypoglycemia, hair loss, amenorrhea, constipation, low blood pressure, poor cognitive functioning, weakness, and chronic fatigue.
Just in case you are rationalizing it will all be worth it so long as you lose weight, I will add that restricting caloric intake to such a degree would result in a slowed metabolism which would make weight-loss impossible to maintain.
If you are indeed desperate to lose weight, please consult your doctor and get his/her advice on a proper diet and exercise plan.