Lipids and organic acids in three gut locations affect feed efficiency of commercial pigs as revealed by LC–MS-based metabolomics oleh Yong Ye, Jie Wu, Jianping Quan, Rongrong Ding, Ming Yang, Xingwang Wang, Shenping Zhou, Zhanwei Zhuang, Sixiu Huang, Ting Gu, Lingjun Hong, Enqin Zheng, Zhenfang Wu, Jie Yang