To determine to what extent low-energy CT imaging affects attenuation of gastrointestinal tract (GIT) opacified with positive oral contrast media (OCM). Second, to establish optimal OCM concentrations for low-energy diagnostic CT exams. One hundred patients (38 men and 62 women; age 62±11years; BMI 26±5) with positive OCM-enhanced 120-kVp single-energy CT (SECT), and follow-up 100-kVp acquisitions (group A; n=50), or 40-70-keV reconstructions from rapid kV switching-single-source dual-energy CT (ssDECT) (group B; n=50) were included. Luminal attenuation from different GIT segments was compared between exams. Standard dose of three OCM and diluted solutions (75%, 50%, and 25% concentrations) were introduced serially in a gastrointestinal phantom and scanned using SECT (120, 100, and 80kVp) and DECT (80/140kVp) acquisitions on a ssDECT scanner. Luminal attenuation was obtained on SECT and DECT images (40-70keV), and compared to 120-kVp scans with standard OCM concentrations. Luminal attenuation was higher on 100-kVp (328HU) and on 40-60-keV images (410-924HU) in comparison to 120-kVp scans (298HU) in groups A and B (p<0.05). Phantom: There was an inverse correlation between luminal attenuation and X-ray energy, increasing up to 527HU on low-kVp and 999HU on low-keV images (p<0.05). 25% and 50% diluted OCM solutions provided similar or higher attenuation than 120kVp, at low kVp and keV, respectively. Low-energy CT imaging increases the attenuation of GIT opacified with positive OCM, permitting reduction of 25%-75% OCM concentration.