Hormones of the gastrointestinal tract are difficult to study because they are not produced by discrete groups of cells organized into glands. Instead, they are released from single cells scattered along the digestive tract. Furthermore, as several of these peptides are found both in specific endocrine cells and in neurons and their nerve terminals it is difficult to establish which effects are dependent on peptide release from nerve terminals and which represent endocrine activity. The gastrointestinal peptides act locally on various organs and processes within the gastrointestinal system. There is a correlation between carbohydrate absorption, plasma concentration of glucose and release of gastrointestinal tract hormones.