If you eat atleast 500 Calories in a Day, Your Basic Functions Will Continue (Although Not very Healthy).
If You're Going To Each One Meal Only, Make It in the middle Of the day, and Watch Your Calories. However, As Always It's Recommended That you eat 1,500 Calories + And Work Out To The Point That You Loose Almost As Much As You Receive.
Eating 500-800 Calories a day, Or Burning Off Enough To Sustain A 500-800 Calorie Intake, Will Loose You About 5 pounds A Week, You Make Your Choice.
As For the "Starvation Mode" Theory, It Isn't Really True In Practice. You Will Eventually Reach A Plataea, And Will Have To Each More, Then Quickly Switch To less, But it In No Way Can Change The Way Your Genes Perform.