Glucose-Dependent Insulinotropic Peptide Explained

Glucose-dependent insulinotropic peptide (GIP) is a gut hormone that plays a central role in regulating blood glucose, fat metabolism, and energy balance. Released from the small intestine after eating, GIP stimulates insulin secretion only when blood glucose is elevated, making it a key component of the body's postprandial response. Once considered a poor therapeutic target due to reduced activity in type 2 diabetes, GIP has attracted renewed clinical interest following evidence that activating its receptor alongside the GLP-1 receptor produces greater metabolic benefits than either pathway alone, driving the development of a new class of dual-action medicines.

What is glucose-dependent insulinotropic peptide and what does it do?

GIP is an incretin hormone released from the small intestine after eating that stimulates insulin secretion only when blood glucose is elevated, and also influences fat storage, bone metabolism, and energy balance via receptors across multiple organ systems.

Glucose-dependent insulinotropic peptide (GIP) is a hormone produced by K-cells in the lining of the small intestine, released in response to eating, and it plays an important role in regulating blood glucose and fat metabolism.^[1] GIP belongs to a family of incretin hormones, which are gut-derived signals that communicate with the pancreas, brain, and other organs after a meal. Its name reflects its most studied action: stimulating insulin secretion from the pancreatic beta cells, but only when blood glucose is already elevated, which means it does not cause insulin release when glucose levels are normal.^[2][3]

GIP's effect on glucagon is context-dependent. At elevated glucose concentrations, GIP may contribute to glucagon suppression; however, at normal or low glucose levels, GIP can stimulate glucagon secretion, which distinguishes it from GLP-1 in this respect.^[5] GIP also promotes fat storage in adipose tissue and may influence bone metabolism and energy balance. Receptors for GIP (known as GIPR) are found not only in the pancreas but also in the brain, adipose tissue, bone, and the cardiovascular system, suggesting that GIP has a broader physiological role than was initially appreciated.^[5]

In people with type 2 diabetes, the insulinotropic response to GIP is significantly blunted, a phenomenon sometimes described as GIP resistance. This has historically led researchers to question whether targeting GIP would be therapeutically useful. However, more recent evidence suggests that restoring or amplifying GIP signalling, particularly in combination with other hormonal pathways, can produce meaningful metabolic benefits, which has renewed scientific and clinical interest in this hormone.

How does GIP work alongside GLP-1 in the body?

GIP and GLP-1 are complementary incretin hormones that together account for most of the postprandial insulin response; dual activation of both receptors by tirzepatide produces greater improvements in glycaemic control and body weight than GLP-1 agonism alone.

GIP and glucagon-like peptide-1 (GLP-1) are the two principal incretin hormones, and they work in a complementary but distinct fashion to regulate blood glucose after meals. GLP-1 is released from L-cells in the lower gut and exerts strong effects on insulin secretion, glucagon suppression, gastric emptying, and appetite. GIP, released earlier from K-cells in the upper gut, acts more rapidly after eating and contributes to the initial insulin response. Together, the two hormones account for the majority of the so-called incretin effect, which is the amplification of insulin secretion that occurs when glucose is taken orally rather than intravenously.^[6][7]

The two hormones also differ in their effects on appetite and body weight. GLP-1 receptor activation is well established as a driver of reduced appetite and slower gastric emptying, contributing to weight loss. GIP's role in energy balance is more complex and less well characterised in humans; animal model data suggest that GIPR agonism may reduce food intake and body weight, but these findings do not translate directly to human physiology and the mechanisms are not fully understood.

Clinical trial data from the SURPASS and SURMOUNT programmes indicate that when GIP and GLP-1 receptors are activated simultaneously by tirzepatide, the combined effect on insulin secretion, body weight, and glycaemic control appears to be greater than that seen with GLP-1 receptor agonism alone. This synergy has driven the development of dual receptor agonist medicines and offers a pharmacological strategy that addresses multiple aspects of metabolic dysfunction, including postprandial glucose control, fasting hyperglycaemia, and excess body weight.

Why are doctors and researchers interested in GIP for treating type 2 diabetes?

Activating the GIP receptor alongside the GLP-1 receptor produces greater reductions in HbA1c and body weight than GLP-1 agonism alone, as demonstrated in the SURPASS and SURMOUNT clinical trial programmes with tirzepatide.

Doctors and researchers are interested in GIP because activating the GIP receptor alongside the GLP-1 receptor produces greater improvements in blood glucose control and body weight than GLP-1 receptor agonism alone, offering a more comprehensive approach to managing type 2 diabetes and obesity. For many years, GIP was considered a poor therapeutic target because its insulinotropic effect is diminished in people with type 2 diabetes. However, research has shown that this resistance can be partially overcome by sustained pharmacological stimulation of the GIPR, particularly when combined with GLP-1 receptor activation.

Data from the SURPASS clinical trial programme demonstrated that tirzepatide, a dual GIP and GLP-1 receptor agonist, produced substantial reductions in HbA1c and significant weight loss in adults with type 2 diabetes, including in a head-to-head comparison with semaglutide 1 mg (SURPASS-2). The SURMOUNT programme similarly showed clinically meaningful weight reduction in adults with obesity or overweight. The weight loss observed with dual agonism is particularly relevant because excess body weight is both a driver and a complication of type 2 diabetes, and addressing it can improve insulin sensitivity, blood pressure, and cardiovascular risk factors.

Researchers are also exploring whether GIP receptor signalling may have protective effects on beta cell function, potentially slowing the progressive loss of insulin-producing cells that characterises type 2 diabetes over time; however, this remains a hypothesis-generating area and further clinical evidence is needed. The cardiovascular and renal implications of GIP-based therapies are under active investigation. The broader distribution of GIP receptors across multiple organ systems suggests that the therapeutic potential of this pathway may extend beyond glucose regulation, making it an active and evolving area of clinical research.

Which medicines available in the UK target the GIP pathway?

Tirzepatide (Mounjaro) is the only UK-authorised medicine targeting the GIP pathway, approved by the MHRA as a once-weekly injection for type 2 diabetes and chronic weight management in eligible adults.

Tirzepatide (brand name Mounjaro) is currently the only medicine available in the UK that directly targets the GIP pathway, acting as a dual agonist at both the GIP receptor and the GLP-1 receptor. It is administered as a once-weekly subcutaneous injection using a multiple-dose KwikPen, with each pen containing four weekly doses. The MHRA has authorised tirzepatide in the UK for two indications: the management of type 2 diabetes in adults, and as an adjunct to a reduced-calorie diet and increased physical activity for chronic weight management in adults with obesity or overweight with at least one weight-related comorbidity.

Tirzepatide is available at several dose strengths, starting at 2.5 mg weekly and escalating gradually, allowing the body to adjust and helping to minimise gastrointestinal side effects during the initiation period. The dose escalation schedule is set out in the UK Summary of Product Characteristics (SmPC) and should be followed under clinical supervision.

For patients considering whether tirzepatide or another treatment might be appropriate for them, a clinical assessment is essential. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where tirzepatide is one option a prescriber may consider if it is clinically appropriate. Other GLP-1 receptor agonists available in the UK, such as semaglutide (Wegovy or Ozempic) and liraglutide (Saxenda), act on the GLP-1 pathway only and do not target the GIP receptor, making tirzepatide pharmacologically distinct within the current UK treatment landscape.

What side effects and safety considerations should patients be aware of?

The most common side effects of tirzepatide are gastrointestinal, including nausea, vomiting, and diarrhoea, which are most pronounced during dose escalation; serious risks include pancreatitis, gallbladder disease, and worsening diabetic retinopathy.

The most commonly reported side effects of tirzepatide, the UK's only available GIP-targeting medicine, are gastrointestinal in nature, including nausea, vomiting, diarrhoea, constipation, and reduced appetite. These effects are most pronounced during dose escalation and typically improve over time. Starting at the lowest dose and increasing gradually, as directed in the UK SmPC, is the recommended approach to minimising these symptoms.

Patients and clinicians should be aware of the following important safety considerations:

• Pancreatitis: Acute pancreatitis has been reported in association with tirzepatide, including post-marketing reports of necrotising pancreatitis and cases with a fatal outcome.^[19][15] If pancreatitis is suspected, tirzepatide should be stopped promptly and medical attention sought. It should not be restarted if pancreatitis is confirmed. Patients should seek prompt medical attention if they develop persistent, severe abdominal pain, particularly if it radiates to the back.

• Gallbladder disease: Cholelithiasis (gallstones) and cholecystitis have been reported. Patients with symptoms such as upper abdominal pain, nausea, or jaundice should contact their GP.

• Dehydration and kidney function: Severe gastrointestinal side effects can lead to dehydration, which may affect kidney function. Patients should maintain adequate fluid intake and seek medical advice if they are unable to keep fluids down or develop signs of dehydration.

• Diabetic retinopathy: Rapid improvement in glycaemic control has been associated with worsening of diabetic retinopathy in some patients.^[15] Those with pre-existing retinopathy, particularly pre-proliferative or proliferative disease, should ensure they are under appropriate ophthalmological or retinal screening follow-up and inform their care team when starting tirzepatide.

• Heart rate: A modest increase in resting heart rate has been observed; this should be monitored in patients with pre-existing cardiac conditions.^[15][22]

• Hypoglycaemia: When used in combination with insulin or a sulphonylurea, the risk of low blood glucose is increased. Dose adjustments of the concomitant medicine may be needed.

• Pregnancy and contraception: Tirzepatide should not be used during pregnancy. Women of childbearing potential should use effective contraception. The UK SmPC advises using an additional non-oral contraceptive method for four weeks after starting tirzepatide and for four weeks after each dose increase, as oral contraceptive absorption may be affected by changes in gastric emptying.^[15][20]

• Breastfeeding: Tirzepatide should not be used while breastfeeding.

Suspected side effects can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk. For urgent symptoms, contact NHS 111; for life-threatening emergencies, call 999.

What does current NICE guidance say about GIP-based treatments?

NICE has recommended tirzepatide for NHS use in specific patient groups with type 2 diabetes and obesity, subject to HbA1c, BMI, and comorbidity thresholds defined in its technology appraisals, though local ICB commissioning arrangements may affect access.

NICE has recommended tirzepatide, the only GIP-targeting medicine currently authorised in the UK, for use within the NHS for specific patient groups, subject to defined eligibility criteria set out in its technology appraisals. NICE does not approve medicines; it recommends them for NHS use following an assessment of clinical and cost effectiveness, and local Integrated Care Boards (ICBs) are responsible for commissioning decisions within their areas.

For type 2 diabetes, NICE has evaluated tirzepatide as an add-on therapy for adults whose blood glucose is not adequately controlled on existing treatments. The specific eligibility criteria, including HbA1c thresholds and prior therapy requirements, are set out in the relevant NICE technology appraisal, which patients and clinicians should consult directly for the most current and precise criteria. For chronic weight management, NICE guidance addresses the use of tirzepatide in adults with obesity or overweight, typically within a specialist weight management service pathway, with specific BMI and comorbidity thresholds defined in the relevant technology appraisal. Guidance is updated periodically and local commissioning arrangements may affect availability.

It is important to note that NHS availability does not follow automatically from a NICE recommendation; ICBs must implement guidance within a defined timeframe, but practical access can vary and may depend on local specialist weight-management pathways. Patients who do not meet NHS eligibility criteria, or who are seeking treatment through private routes, should ensure they are assessed by a qualified prescriber and that any treatment is supplied through a regulated pharmacy.

For those managing type 2 diabetes, NICE also recommends a broader approach that includes lifestyle modification, structured education, and stepwise pharmacological therapy. GIP-based treatment with tirzepatide represents one option within this framework, particularly for patients who would benefit from both improved glycaemic control and weight reduction.

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