Hadn't heard of Smart BMI before I read the recent posts above, but I just found it and put in all the info they asked for, so as to be given the comments.
Had put that I'm low-carbing. This is what it said in that section of the comments:
Your low-carb diet
If you are currently dieting: With this diet you can achieve a fast weight loss by tricking your body. In the long run this may be unhealthy, as your body will try hard to compensate for the carbohydrate shortage by storing more fat. This will most likely result in a yo-yo effect. If you want to stop dieting, do it step by step.
I'm not low-carbing as a 'diet' in the sense that I think they mean it, although I'm wondering what they mean about the yo-yo and compensatory fat storage....