Lustig’s confidence comes from the unique study, described in Obesity, of 43 Hispanic or African-American children aged eight to 18 years old. He collected detailed food questionnaires from each of the adolescents to get an idea of the average amount of calories they ate per day, then designed a special menu for each of them for nine days that matched the total numbers of calories they would normally eat. The only difference in the nine-day diet was that most of the sugar the children ate was replaced by starch — the overall number of calories remained the same. The children weighed themselves daily, and if they were losing weight, they were told to eat more of the provided food in order to keep their weight the same throughout the study.

“Everything got better,” says Lustig. Some of the children went from being insulin resistant, a precursor state to developing diabetes, in which the body’s insulin levels can no longer keep up with the pace of breaking down sugar that’s coming in from the diet, to insulin sensitive.