Optimizing early enteral nutrition in severe stroke (OPENS): protocol for a multicentre randomized controlled trial oleh Fang Yuan, Fang Yang, Wei Zhang, Yi Jia, Yaling Ma, Yongcai Qu, Xinglai Wang, Kang Huo, Chengkai Wang, Xiangjun Yuan, Chaohui Song, Bei Zhang, Wen Jiang, on behalf of the OPENS study group