To be as light as possible for a winter marathon I've been monitoring my weight and calorie intake for the past month, using the USDA nutritional database and it matches up perfectly with the weight I've dropped. A 3,500 calorie deficit in one week equals exactly 1lb weight loss of nearly entirely fat.  There's an error of about 1-2kg most days due to natural fluctuations in water balance, but over a course of one month the data doesn't lie. ~14,000 calories burned = 4lbs lost. So the only way they can be making errors is either stupidity or if they give up after one week.