Enteric contrast agents are important in gastrointestinal MRI. However, no currently available agent is well established as the standard of care. In this study, in vitro relaxivities of manganese threonine chelate (Mn-Thr), a common nutritional food supplement, were measured at 1.5 T and 3 T with further investigation of its efficacy and safety in vivo as an enteric contrast agent. According to the calculated relaxivities, T1W and T2W TSE sequences of Mn-Thr solutions at different concentrations were acquired, and the optimal concentration for dark lumen imaging on both T1W and T2W images was determined in vitro. To validate the optimal concentration in vivo, eight Sprague-Dawley rats were randomly divided into two groups. Each group received rectal injection of either 2.00 g/L (about 3.80 mM) Mn-Thr or saline as an enteric contrast agent and underwent MRI. After a time interval of one week, the same procedures were repeated with the alternative contrast agent. Animals were sacrificed after the second MRI. Tissue manganese quantification and histopathological examination were obtained. Qualitative MR image quality assessments were performed and compared between Mn-Thr and saline. Measured T-1 and T-2 relaxivities of Mn-Thr were significantly higher than those of MnCl2 in vitro (p < 0.05). At the concentration of 2.00 g/L (about 3.80 mM), Mn-Thr produced a dark lumen on T1W and T2W images both in vitro and in vivo. Compared with saline, Mn-Thr showed significantly more homogenous luminal signal and increased bowel wall conspicuity in image quality assessments. Tissue manganese concentrations were not significantly different between two groups. Histopathological examinations were normal in both groups. Our data suggest that Mn-Thr possesses favorable paramagnetic properties and can create a homogenous dark lumen on T1W and T2W images without obvious side effects in healthy rats. As a commercially available nutritional food supplement, Mn-Thr appears to be a promising enteric contrast agent for MRI.