In the immediate interim (perhaps 1-2 weeks) the severe deficit you’ve introduced will work, as your body will be able utilise the calorific value of your stored fat to fuel your endeavour and maintain your existence.
However, such a severe deficit will soon place your body into starvation mode and it’ll begin to hold on to the fat that you possess, as it seeks to maintain your existence, caring little for your desire to lose weight.
Furthermore, a sensible daily calorific intake allows your metabolism to continually fire. However, it can only fire if it has energy. Consider what happens when you remove oxygen from a burning fire – the flame is extinguished, as its energy source has been removed.
The same applies to your body if you restrict calories so severely, since calories are the body's fuel.
By continuing upon the course on which you’ve embarked, as already stated by Ishafer, it won’t be long before you crash.
In order to ensure that your metabolism continues to fire, I’d also recommend that you research BMR calculators, since they’re largely accurate in determining how many daily calories you need, based upon age, weight, height and level of activity.
Once you’ve attained your active BMR, you can begin to introduce a daily deficit of approximately 500 calories, which, combined with calories burned through regular, intensive exercise, should allow you to begin losing between 1-3lbs per week. Intensive exercise will also help to change your body composition, as it’ll improve the shape and definition of your muscles.
As such, if your fitness level allows, introduce HIIT into your exercise schedule 2-3times a week, in order to improve your level of fitness and maximise your fat loss; don’t expect to be able to execute HIIT on 800 calories a day, though, as the activity creates a huge energy deficit (known as the after-burn effect).
After increasing your daily calorie intake back to a sensible level, in order to ensure that you’re eating to support weight loss and your level of training, aim to consume around 1g of protein per lb of body weight, whilst obtaining your carbohydrate intake from wholegrains, legumes and veg (avoiding as many simple, refined carbs as possible). Don’t deny yourself the odd treat, though.
As for lipids, aim to obtain it from oils, seeds and nuts, keeping your saturated fat intake (with the exception of coconut oil) to a minimum.
Lastly (yes, I’m nearly finished), as you begin to lose weight, re-assess your active BMR in order to ensure that you avoid the dreaded plateau, as each stone lost will lead to a reduction in your resting BMR, thus affecting your active BMR.
In the immediate interim, don’t worry too much if your active BMR (once you’ve introduced a daily 500 Kcal deficit) is less slightly than your resting BMR, as the excess fat you currently carry will help to provide your body’s energy systems with the fuel needed to keep your metabolism ticking over nicely.
Moreover, I hope the above helps to explain why there is simply no need to introduce such a severe calorie deficit, as you strive to meet your 2016 weight loss goals.
