Leptin, a protein occurring naturally in the blood, appears to regulate how much fat the body carries by speeding up the metabolism and decreasing the appetite when the body has too much fat. Mice that do not naturally produce leptin have more fat than other mice, but lose fat rapidly when they are given leptin injections. Unfortunately, however, leptin cannot be used as a dietary supplement to control fat, since__. Which of the following most logically completes the passage? ['mice whose bodies did produce leptin also lost some of their body fat when given leptin injections', 'the mice that do not naturally produce leptin were from a specially bred strain of mice', 'the digestive system breaks down proteins before they can enter the bloodstream', 'people with unusually low levels of leptin in their blood tend to have a high percentage of body fat']