Gastric restriction device implants and their use in controlling body weight are described. In some embodiments, activation of a shape memory material drives an actuator coupled to an implant, resulting in a conformational change in the implant. In some embodiments latch and ratchet mechanisms operate incrementally to increase or decrease a size of a stomal opening produced by the gastric restriction device. Methods are described by which adjusting the size of the stomal opening is used to restrict the rate at which food passes through the stomach.