Why Add GIP?

When researchers first discovered GIP, it appeared to function primarily as an insulin-stimulating hormone released after meals.

Like GLP-1, GIP enhances glucose-dependent insulin secretion, helping the pancreas respond appropriately to rising blood glucose concentrations. Because of this overlap, many investigators initially assumed that GIP offered little therapeutic advantage beyond what GLP-1 receptor agonists already provided.

However, advances in molecular biology and neuroscience have dramatically changed this perspective.

Today, GIP receptors are known to be expressed throughout the body, including in:

• The pancreas

• Adipose tissue

• Skeletal muscle

• Bone

• The central nervous system

• Components of the cardiovascular system

Research suggests that GIP participates in much more than insulin secretion. It appears to influence appetite regulation, nutrient partitioning, lipid metabolism, insulin sensitivity, skeletal muscle physiology, and energy balance. Although many aspects of GIP biology remain under investigation, it has become increasingly clear that GIP provides metabolic signals that complement rather than duplicate those of GLP-1.

How Tirzepatide Works

Tirzepatide is often described as a dual incretin receptor agonist because it activates both GLP-1 and GIP receptors with a single engineered peptide molecule.

Rather than producing two separate medications in one injection, tirzepatide was designed from the ground up as a single peptide capable of engaging both receptor systems.

GLP-1 receptor activation contributes to:

• Reduced hunger

• Increased satiety

• Slower gastric emptying

• Improved glucose-dependent insulin secretion

• Reduced glucagon release during hyperglycemia

GIP receptor activation appears to contribute to:

• Enhanced insulin responsiveness

• Improved metabolic flexibility

• Better nutrient partitioning

• Favorable effects on adipose tissue

• Complementary appetite regulation

• Potential preservation of lean tissue during weight loss (an area of ongoing research)

Together, these effects create a broader metabolic response than stimulation of the GLP-1 receptor alone.

Importantly, the precise contribution of GIP to the overall clinical effects of tirzepatide continues to be actively studied. While clinical outcomes consistently demonstrate superior weight loss compared with GLP-1 receptor agonism alone, the relative importance of individual mechanisms remains an area of ongoing investigation.

Beyond Appetite Suppression

Many people assume tirzepatide works simply by making people eat less.

Reduced caloric intake is certainly an important contributor to weight loss, but current evidence suggests the medication's effects extend well beyond appetite regulation.

Clinical studies have demonstrated improvements in:

• Insulin sensitivity

• Fasting glucose

• Hemoglobin A1c

• Blood pressure

• Triglycerides

• HDL cholesterol

• Hepatic fat accumulation

• Inflammatory biomarkers

• Waist circumference

Many patients also report a reduction in compulsive eating behaviors and food preoccupation, often referred to as "food noise." While these experiences are subjective and vary among individuals, they likely reflect altered communication between peripheral metabolic signals and central appetite-regulating pathways.

The combination of improved glycemic control, enhanced satiety, and favorable metabolic adaptations contributes to the substantial weight reductions observed in clinical trials.

Landmark Clinical Evidence

Tirzepatide has been evaluated through the SURMOUNT clinical trial program for obesity and the SURPASS program for type 2 diabetes.

These randomized, controlled studies represent one of the most comprehensive bodies of evidence available for any metabolic medication.

SURMOUNT-1

Published in 2022, SURMOUNT-1 enrolled adults with obesity or overweight who did not have diabetes.

After 72 weeks of treatment, participants receiving the highest approved dose achieved average weight reductions exceeding 20% of baseline body weight, with many individuals losing substantially more.

Additional findings included significant improvements in:

• Waist circumference

• Blood pressure

• Lipid profiles

• Glycemic measures

• Quality-of-life assessments

These results represented the greatest average weight loss reported for an FDA-approved medication at the time of publication.

SURPASS Program

The SURPASS trials evaluated tirzepatide in adults with type 2 diabetes.

Across multiple studies, tirzepatide demonstrated:

• Superior reductions in Hemoglobin A1c compared with several established diabetes therapies

• Greater weight loss than comparator medications, including semaglutide in head-to-head trials using the doses studied at that time

• Improvements in multiple cardiometabolic risk factors

These findings established tirzepatide as an important therapeutic option for both diabetes management and obesity treatment.

Tirzepatide vs. Semaglutide

Both semaglutide and tirzepatide are highly effective medications with substantial clinical evidence supporting their use. Rather than viewing one as universally "better" than the other, it is more appropriate to recognize that they differ in mechanism, potency, and clinical considerations.

Feature Comparison

Primary mechanism -

Semiglutide - GLP-1 receptor agonist

Tirzepitide - Dual GLP-1/GIP receptor agonist

FDA approval for obesity - Yes, Both

Typical average weight loss in pivotal obesity trials

Semiglutide - ~15%

Tirzepitide - ~21%

Weekly dosing - Yes, Both

Cardiovascular outcomes evidence

Semiglutide - Robust

Tirzepitide - Expanding

Diabetes evidence - Extensive, Both

Individual response varies considerably. Some patients achieve excellent results with semaglutide, while others experience greater benefits with tirzepatide. Factors such as tolerability, metabolic health, previous treatment history, insurance coverage, and personal preferences all influence treatment selection.

Safety and Tolerability

The adverse-effect profile of tirzepatide is broadly similar to that of other incretin-based therapies.

The most frequently reported side effects include:

• Nausea

• Vomiting

• Diarrhea

• Constipation

• Abdominal discomfort

• Reduced appetite

These symptoms are generally most pronounced during dose escalation and often improve as treatment continues.

As with all GLP-1–based therapies, gradual dose titration is intended to improve tolerability by allowing the gastrointestinal system time to adapt.

Although serious adverse events are uncommon, treatment decisions should always be individualized after consideration of medical history, current medications, contraindications, and ongoing clinical monitoring.

Looking Ahead

Tirzepatide demonstrated that activating two incretin receptors simultaneously could substantially improve weight-loss outcomes beyond those achieved through GLP-1 receptor agonism alone.

This success naturally raised another question:

If two metabolic pathways are beneficial, could activation of a third produce even greater improvements?

Researchers answered this question by incorporating glucagon receptor activation into a single molecule, leading to the development of retatrutide—the first triple hormone receptor agonist currently being investigated for obesity and metabolic disease.

→ Continue to: Retatrutide — Triple Agonist Therapy

Key References

1. Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1).New England Journal of Medicine. 2022.

2. Frias JP, et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2).New England Journal of Medicine. 2021.

3. Campbell JE, Drucker DJ. Pharmacology, Physiology, and Mechanisms of Incretin Hormones.Cell Metabolism.2020.

4. Nauck MA, Meier JJ. Incretin Hormones: Their Role in Health and Disease.Diabetologia. 2018.

5. Finan B, Müller TD, DiMarchi RD. Next-Generation Multi-Receptor Agonists for Obesity.Nature Reviews Drug Discovery. 2021.

6. Coskun T, et al. Dual and Triple Hormone Receptor Agonists in Metabolic Disease.Cell Metabolism. 2022.