Rhamnan sulfate (RS) is a sulfate polysaccharide composed of L-rhamnose and sulfated L-rhamnose found in the green algae such as Monostroma nitidum. It has been reported to have anticoagulant and antiviral effects in the last decade. In 2015, our group discovered the anti-obesity effect of RS using diet-induced obesity model of zebrafish (Zang L, et al. J Funct Foods. 2015;17:364-370). In this study, we first administrated RS (0.25 mg/g food volume) orally to high-fat diet treated mice for 4 weeks. RS increased fecal volume and calorie excretion with suppression of body weight increase, and decreased plasma lipids and fasting blood glucose levels, which was consistent with the results of our zebrafish study. Noting the increased excretion by RS, we administrated RS to patients with chronic constipation for 2 weeks according to a double-blind placebo-controlled manner. As a result, RS administration significantly increased frequency of dejection without any side effects, although there was no effect on body weight and blood lipids. In addition, we performed 16s rRNA-seq analysis for gut microbiota on these patients. Metagenomics profiling using PICRUSt revealed functional alternation of the KEGG pathways which would be involved in the therapeutic effect of RS for constipation.