Elevated blood glucose (HbA1c), long duration of diabetes, and the presence of established macro- and microvascular complications are some of the risk factors associated with development of gastroparesis [6]. Women have been found to have a higher risk of developing these complications than men [4]. This can be partly explained by the fact that females in general tend to have higher rates of GI symptoms and functional disorders irrespective of whether they have diabetes [7]. They also tend to seek health care more frequently than the men. Female patients with diabetes, in particular, have an increased incidence of eating disturbances [8]. The GI transit time is significantly prolonged during the luteal phase of the menstrual cycle when progesterone levels are increased compared with the follicular phase [9]. However, the exact role of ovarian hormones on gastric emptying is still unclear [10].

The enteric nervous system (ENS) is an independent network of neurons and glial cells that spread from the esophagus up to the internal anal sphincter. Structured as two major plexuses, myenteric (Auerbach’s) and the submucous (Meissner’s) plexus, the ENS regulates GI tract functions including motility, secretion, and participation in immune regulation [12, 13]. GI complications and their symptoms in patients with diabetes arise secondary to both abnormalities of gastric function (sensory and motor modality), as well as impairment of GI hormonal secretion [14], but these abnormalities are complex and incompletely understood. Over the last several years, knowledge of the mechanisms of DM-induced changes in GI tract has expanded. It has been known for a long time that diabetic autonomic neuropathy (i.e., dysfunction of the neurons supplying the ENS) leads to abnormalities in the GI motility, sensation, secretion, and absorption, serving as the main pathogenic mechanism underlying GI complications.

Recently, evidence has emerged to suggest that other processes might also play a role. Loss of the pacemaker interstitial cells of Cajal, impairment of the inhibitory nitric oxide-containing nerves, abnormal myenteric neurotransmission, smooth muscle dysfunction, and imbalances in the number of excitatory and inhibitory enteric neurons can drastically alter complex motor functions causing dysfunction of the enteric system [7, 11, 15, 16]. This dysfunction can further lead to the development of dysphagia and reflux esophagitis in the esophagus, gastroparesis, and dyspepsia in the stomach, pseudo-obstruction of the small intestine, and constipation, diarrhea, and incontinence in the colon.

In animal models of DM (i.e., streptozocin-induced DM in rats), defective tropic signaling of neurotransmitters (vasoactive intestinal peptide, acetylcholine, substance P, nucleotides), paracrine agents (serotonin), anti-inflammatory agents (prostaglandins, leukotrienes), histamine, and loss of adrenergic enteric innervation can also cause abnormalities in epithelial function and development, resulting in enhanced nutrient transport and abnormalities in salt and water transport [17, 18]. Compromised intestinal vascular flow arising due to ischemia and hypoxia from microvascular disease of the GI tract can also cause abdominal pain, bleeding, and mucosal dysfunction.

Mitochondrial dysfunction has been implicated in the pathogenesis of gastric neuropathy. It involves the degeneration of dorsal root ganglion neurons in peripheral nerves; dorsal root ganglion mitochondria are particularly effected [19]. Formation of irreversible advanced glycation end products (AGE) can cause qualitative and quantitative changes in extracellular matrix components such as type IV collagen, laminin, and vitronectin. This can affect cell adhesion, growth, and matrix accumulation. AGE-modified proteins also alter cell function by interacting with specific receptors on macrophages and endothelial cells, inducing changes that promote matrix overproduction, focal thrombosis, and vasoconstriction [20].

Motility alterations can cause overgrowth of the small bowel microflora and induce bloating, diarrhea, abdominal pain, and malabsorption [21]. However, there is some evidence to suggest that the diarrhea might actually be due to colonic bacterial metabolism of carbohydrate secondary to rapid small bowel transit rather than small bowel bacterial overgrowth [22].

Acute and chronic hyperglycemia or hypoglycemia which can alter intestinal function by affecting the metabolic and signaling pathway of the enteric neurons and effect gut motility [16, 23–26]. Acute (insulin induced) hypoglycemia accelerates gastric emptying [27]. Thus GI motor function is highly sensitive to fluctuations in glycemic state. Hyperglycemia can also cause vagal inhibition leading to acute GI symptoms [15]. Another possible association between DM and the gastrointestinal tract can be infrequent autoimmune diseases associated with type I DM like autoimmune chronic pancreatitis, celiac disease (2–11 %), and autoimmune gastropathy (2 % prevalence in general population and three- to fivefold increase in patients with type 1 DM) [28, 29].

GI symptoms are often associated with the presence of other diabetic complications, especially autonomic and peripheral neuropathy [2, 30, 31]. In fact, patients with microvascular complications such as retinopathy, nephropathy, or neuropathy should be presumed to have GI abnormalities until proven otherwise. In a large cross-sectional questionnaire study of 1,101 subjects with DM, 57 % of patients reported at least one GI complication [31]. Poor glycemic control has also been found to be associated with increased severity of the upper GI symptoms. There is some discordant data linking diabetic autonomic neuropathy to the duration of diabetes, but the Diabetes Control and Complications Trial suggested that, at least in persons with type 1 DM, neuropathy and other GI complications are associated with poor glycemic control, rather than the duration of diabetes [32].

Establishing a diagnosis of GI autonomic neuropathy is difficult as there are no tests available to evaluate the GI innervation and autonomic tone directly. Moreover, the cardiovascular alterations are of low value in the prediction of motor alterations of the GI tract. The most widely available and standard method for evaluating motility disorders and assessing gastric emptying is scintigraphy. Other alternatives include radiolabeled breath testing and wireless motility capsule testing [33, 34]. Techniques such as ultrasound, single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI) are predominantly research tools used for evaluating gastric volume, contractility, distribution of meals, and emptying.

The fundamental basis of treating gastroparesis is dietary modification, such as eating frequent, small, soft meals with a low fat or fiber content. Beyond dietary changes, the management of gastroparesis includes Food and Drug Administration (FDA)-approved D2 receptor antagonists and 5-HT4 agonist metoclopramide, the long-term use of which is limited by side effects such as restlessness and acute dystonia, including tardive dyskinesia. Another alternative is domperidone, a D2-receptor antagonist, which does not carry the same risk of extrapyramidal side effects, but is it not FDA approved and is not marketed for sale in Europe or the United States. In addition, erythromycin, a prokinetic drug which is administered either orally or parenterally, can improve gastric emptying time and reduce nausea and vomiting by its molecular mimicking of motilin. For refractory cases of gastroparesis, gastric electrical stimulation with endoscopically implanted electrodes has been in use since its approval by FDA in 2000 [35].

Lately, muscarinic receptor antagonists and 5HT4-, D2-, ghrelin, and motilin receptor agonists (without antibiotic action) are being evaluated as newer therapeutic agents to control symptoms due to gastroparesis [36, 37]. Diarrhea secondary to diabetic visceral neuropathy can be a troubling GI complication and has been shown to be treated effectively and safely with loperamide. Constipation can be relieved by laxative use, but in recent years, agents such as lubiprostone have been used to relieve constipation not adequately treated by laxatives. Another newer therapeutic approach for treating constipation is stimulation of epithelial guanylate cyclase-C (GC-C) receptor on intestinal epithelial cells [37]. Linaclotide is approved for treatment of constipation and acts by binding to this specific receptor; it is used with spare use of opiate agents for the same reason. Patients with neuropathy also suffering from chronic abdominal pain may respond to agents such as low-dose tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), and selective serotonin-noradrenaline reuptake inhibitors (SNRIs). It helps reduce opiate agents for the same reason.

GI symptoms are also commonly reported as side effects of oral hypoglycemic agents particularly metformin and alpha-glucosidase inhibitors. However it is hard to prove a causal relationship as it is difficult to distinguish between spontaneous and true drug-related symptoms due to the high incidence of background GI symptoms in these patients. The most frequent symptoms caused by these medications include diarrhea and vomiting, which can lead to poorer quality of life and reduced compliance with treatment. Newer agents targeting the incretin system like glucagon-like peptide (GLP)-1 receptor agonists and dipeptidyl peptidase 4 (DPP-4) inhibitors are increasingly being used to treat type 2 diabetes. GLP-1 agonists increase insulin secretion while inhibiting glucagon release. They also delay gastric emptying and help decrease food intake. Their most common adverse effect includes mild to moderate GI symptoms particularly nausea, vomiting, and diarrhea. But the nausea tends to be transient and can be reduced with dose titration (Table 1.1). Acute pancreatitis has been associated with DPP-4 inhibitors use but causal relationship has not been established [38]. Liver function test needs to be monitored with certain DPP4-inhibitors such as vildagliptin and alogliptin [39].