Here's the thing. At 600 calories you're not eating enough to physically keep your heart beating, to carry on breathing or any kind of function the body is required to do. Therefore the body takes the energy needed from itself. First from fat and muscle and eventually from the organs themselves, which leads to heart attacks as the heart wall weakens.
VLCD are recommended for people with high BMI's and nearly always with medical supervision and a set timescale and with nutritionally complete food packs. From the sounds of it, not only are you doing a VLCD but also restricting the types of food you can eat? Which means you’re unlikely to be getting everything the body needs in terms of vitamins and minerals.
At 600 calories your body will be holding on to every single calorie you consume because it will think it’s in a time of starvation and store accordingly (known as starvation mode), this is why 1200 is many peoples lowest range, because at that you’re consuming at least as many calories as required to keep yourself alive, therefore all the extras you do like, sitting up and moving around are taken from fat stores.
Work out your BMR (basic metabolic rate – basically what you’d need to consume if you spent all day in bed) and always try and eat close to or slightly above that in calories.
BMR Calculator.
Trust me, it seems REALLY weird BUT in this case if you eat more you’ll actually lose more and probably faster.