Last fall, Alexandra Duarte, who is now 16, went to see her endocrinologist at Texas Children’s Hospital, outside Houston. From age 10, she had been living with polycystic ovary syndrome and, more recently, prediabetes. After Alexandra described her recent quinceañera, the doctor brought up an operation that might benefit her, one that might help her lose weight and, as a result, improve these obesity-related problems. Alexandra, who smiles shyly and speaks softly but confidently, says she was “a little skeptical at first because, like, it’s a surgery.” But her mother, Gabriela Velez, suggested that her daughter consider it. “Ever since I was a toddler, my mom knew that I was struggling with obesity,” Alexandra says. The teasing started in fifth grade. Alexandra couldn’t eat without her classmates staring at and judging her. Though she sought counseling for her sadness and anxiety, these troubles still caused her to leave school for a month. The bullying finally stopped after she...