In a controlled laboratory in Shenzhen, twenty-two adults stopped eating for fifteen days while researchers watched what happened to the mind when the body is stripped of its ordinary fuel. The brain adapted — shifting from glucose to ketones — but attention wavered in waves, and those who exercised at moderate intensity paid a hidden cognitive price, making more errors in learning tasks than those who simply rested. The study asks an ancient question in a modern setting: when survival demands both movement and deprivation, what does the thinking self sacrifice first?