Unless you weigh somewhere in the vicinity of 50lbs, then yes 600-800cals is way too little. If you were to consume this little amount of energy from food, yes, you'll lose weight quickly. Much of it will be a loss of muscle tissue in addition to some fat and water weight. But guess what happens very quickly thereafter...your metabolism slows (your body begins to burn less calories at rest) and your body begins to go into fat (energy) conservation mode. This is a biological consequence of evolution as the body seeks to conserve as much energy as possible. When you resume eating above 800cals/day guess what happens then? Your body begins to convert the surplus calories into bodyfat to protect itself from the next 'period of starvation'. Add the loss of muscle tissue now, you will be back to square one minus two paces. If you're going to reduce your caloric intake to lose weight slowly, then use an online BMR/RMR calculator to determine your daily base caloric needs (factor in an estimate for cals burned for daily activity level like your job and exercise) and then subtract about 300-400cals from that. It's te simplest rule of weight loss...consume less calories than your body require and you will drop the pounds.But you have to be a bit patient as well as you want to drop body fat and preserve the lean body mass (muscle that you have.