You might drop 7kg in a week from being dehydrated... you know, like the woman who wants to "fit into a dress for the week-end party"... but you can't lose that much "weight". Your weight can fluctuate ~ 5% between being "extremely bloated" and "dehydrated"... that's just a temporary water condition. Most diets that claim "rapid weight loss" rely upon diuretics to temporarily dehydrate your body. Others claim to "rev up your metabolism to burn fat". That can be done chemically so a small degree... like 200-300 calories per day.... but it may come with undesirable side effects like increased blood pressure or the jitters like you might get from too much caffeine. Better to exercise more to "rev up" rather than trying to do it chemically. When you lose "weight", you generally lose about a pound of water along with a pound of fat.... at least during the early stages. To put on 1 pound of fat, you have to consume an excess of ~4,000 calories. To lose that pound, you need to run a deficit of 4,000 calories. Nowhere in that formula is the potential to "lose 7kg in a week".