Provided herein are peptides, that in combination with metformin, are effective for the treatment of type I diabetes, Type II diabetes, metabolic syndrome, or obesity, or of appetite suppression, or for mitigating insulin resistance, or for reducing an undesirably high fasting serum glucose level, or for reducing an undesirably high peak serum glucose level, or for reducing an undesirably high peak serum insulin level, or for reducing an undesirably large response to a glucose tolerance test in synergistic combination with metformin. The peptides are selected from sequences SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, and SEQ ID NO: 24.