While the scale will show a drop, losing 20kg (44 lbs) of fat in 13 days is physiologically impossible for most people. A deficit of ~3,500 calories is needed to lose one pound of fat. To lose 44 lbs of fat requires a deficit of 154,000 calories. On a 600-calorie diet, the total deficit over 13 days is roughly 20,000–25,000 calories—translating to roughly 6–7 lbs of fat. The remaining weight loss is water and waste.
The exact origins of the 13-Day Diet are obscure. It is popularly known as the "Danish Diet," though there is no evidence linking it to the Danish government or a specific Danish medical institution. Similarly, it is sometimes associated with the Royal Danish Hospital, a claim widely regarded as an urban legend. 13 day diet