We can loose weight in one week but only thing is we need to loose it in a healthier manner. So to loose weight in a healthier manner we can take care of our diet and our exercise pattern. One pound of fat contains 3500 calories so targeting a deficit of 600 calories a day either by taking less calories in diet or by burning more calories in exercises. Crash diet is not a healthy diet as it will make a insufficiency of nutrients which leads to loosing of lean muscle mass. So its better to loose slowly but healthier way. Exercises includes resistance exercises and cardio exercises . Cardio exercises includes Swimming, Jogging, Running, Skipping, etc. And resistance exercises includes Squats, Lunges, Push ups, Rowing, Back strengthening exercises ,etc. We need to do these exercises atleast for 45 mi 5/week to get a better results.