A sensible weight loss target is 1kg (2lb) a week - and this is based on very sound scientific and medical data. For every 1kg of weight lost, about 75% is fat and 25% lean tissue (including water). In terms of energy this is equal to about 7,000 calories! So, to lose 1kg of weight in a week, you need to use up 1,000 more calories of energy a day than you eat - and you need to keep this up for the full seven days.
The same maths also apply the other way round, when we put on weight. The problem is we don’t notice weight creeping on, but we expect it to disappear like magic as soon as we start dieting.