The invention is directed to methods of using Chlorella protothecoides microalgal biomass in the treatment of individuals having impaired glucose metabolism. In some cases, the patient has impaired fasting glucose, impaired glucose tolerance, or diabetes. In some methods, algal biomass is used to reduce blood glucose and/or body fat in a subject, or to increase the relative abundance of beneficial gut microflora in a subject. In preferred embodiments, the biomass is derived from Chlorella protothecoides cultures grown heterotrophically in which the algal cells comprise at least 15% algal oil by dry weight.