You would first feel very weak (seeing black when you stand up and possibly feinting). If the diet is sustained, you will eventually feel chronically sick in the stomach, followed by being very cold all the time, probable constipation and pain, and after that you may lose your hair and your nails may turn blue from oxygen starvation. Losing weight very quickly typically causes gallstones, and they're excruciatingly painful.
The diet you mention is seriously lacking in essential nutrients including fibre, antioxidants, and pretty much every mineral and vitamin known to man. 800 calories a day is starvation, basic subsistance level. Less than this and your body will begin to destroy both the fat AND the muscle tissue in your body in order to feed your brain. This is what happens to people with severe eating disorders, and the muscles involved include the heart - that's right, your fitness regime will cause heart damage if you do not feed the machine (body) correctly.
The main issue is that a starvation level of calorie intake will send your body into starvation mode itself. Every gram you eat will be kept. Your metabolism will slow down and the weight will be extremely hard to shift.
No diet will allow you to lose more than 2 pounds or 1 kilo per week unless you are merely losing fluid. The people on the diet reality shows losing more are under strict guidelines from nutritionists, are eating far more than you, and are working out 8-10 hours a day.
The best solution for weight loss is to increase fresh fruit and vegetables by replacing some of your meals with them. Ensure you are still getting daily dairy and meat (or a vegetarian alternative like lentils or soy). Instead of three big meals, change to 6 light meals and start your exercise. This boosts metabolism, you'll feel more energetic and the weight will be faster to lose.
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