SURPASS-3 Trial: Tirzepatide vs Insulin Degludec — Superior Glucose Control With Weight Loss Instead of Gain

SURPASS-3: Rewriting the Insulin Paradigm

For decades, insulin has been the cornerstone of type 2 diabetes treatment when oral medications fail. The SURPASS-3 trial, published in The Lancet in October 2021 by Ludvik et al., challenged this paradigm by demonstrating that tirzepatide not only matched but dramatically exceeded insulin degludec in glycemic control — while producing massive weight loss instead of the weight gain that plagues insulin therapy [1].

Study Design

SURPASS-3 was a 52-week, open-label, randomized Phase 3 trial conducted at 122 sites across 13 countries. It enrolled 1,444 adults with type 2 diabetes inadequately controlled on metformin with or without an SGLT2 inhibitor.

Randomization (1:1:1:1):

• Tirzepatide 5 mg weekly
• Tirzepatide 10 mg weekly
• Tirzepatide 15 mg weekly
• Insulin degludec (titrated to fasting glucose <90 mg/dL)

Insulin degludec (Tresiba) was chosen as the comparator because it represents a modern, long-acting basal insulin with a favorable safety profile and low hypoglycemia risk — making it a strong comparator [1].

Glycemic Control: Tirzepatide Dominates

Mean HbA1c Change from Baseline (~8.2%):

• Tirzepatide 5 mg: -1.93%
• Tirzepatide 10 mg: -2.20%
• Tirzepatide 15 mg: -2.37%
• Insulin degludec: -1.34%

Treatment Difference vs. Insulin (15 mg): -1.04% (P<0.001)

HbA1c Target Achievement:

• HbA1c <7.0%: 82% (5 mg), 90% (10 mg), 93% (15 mg) vs. 61% (insulin)
• HbA1c <5.7%: 26% (5 mg), 39% (10 mg), 48% (15 mg) vs. 5% (insulin)

The fact that tirzepatide achieved nearly a full percentage point more HbA1c reduction than optimally titrated insulin is remarkable. Insulin has long been considered the most potent glucose-lowering therapy, yet tirzepatide surpassed it decisively [1].

The Weight Story: Loss vs. Gain

The weight data represents the most dramatic contrast:

Mean Body Weight Change:

• Tirzepatide 5 mg: -7.5 kg (-7.5%)
• Tirzepatide 10 mg: -10.7 kg (-10.7%)
• Tirzepatide 15 mg: -12.9 kg (-12.9%)
• Insulin degludec: +2.3 kg (+2.4%)

Total Weight Difference (15 mg vs. insulin): 15.2 kg (33.5 lbs)

This 15.2 kg difference is clinically enormous. A patient choosing tirzepatide over insulin would weigh approximately 33 pounds less after one year — while achieving better glucose control [1].

Why Insulin Causes Weight Gain

Understanding why insulin promotes weight gain helps explain tirzepatide's advantage:

1. Anabolic effects: Insulin promotes fat storage and inhibits fat breakdown
2. Hypoglycemia avoidance: Patients eat defensively to prevent low blood sugar
3. Reduced glucosuria: Better glucose control means less glucose lost in urine (fewer "free" calories excreted)
4. Appetite stimulation: Insulin can increase hunger through central nervous system effects

Tirzepatide avoids all of these mechanisms while providing superior glucose lowering through appetite reduction, improved insulin sensitivity, and glucose-dependent insulin secretion [2].

Safety Comparison

Hypoglycemia (<54 mg/dL):

• Tirzepatide: 0.4-1.1%
• Insulin degludec: 7.5%

The dramatically lower hypoglycemia rate with tirzepatide is a major safety advantage. Hypoglycemia is the most feared complication of insulin therapy, limiting dose titration and causing significant anxiety.

Gastrointestinal Events:

• Nausea: 12-24% (tirzepatide) vs. 2% (insulin)
• Diarrhea: 15-17% vs. 4%
• These were the main adverse events more common with tirzepatide

Discontinuation due to AEs:

• Tirzepatide: 3-7%
• Insulin: 1% [1]

Liver Fat Reduction

A notable secondary finding was tirzepatide's effect on liver fat:

• Tirzepatide reduced liver fat content by 33-37% (measured by MRI)
• Insulin degludec reduced liver fat by only 8%
• This has implications for non-alcoholic fatty liver disease (NAFLD), which affects the majority of patients with type 2 diabetes

Clinical Implications

SURPASS-3 has several practice-changing implications:

1. Insulin delay: Tirzepatide provides a powerful alternative before starting insulin
2. Insulin replacement: Some patients currently on insulin may benefit from switching to tirzepatide
3. Weight management: Eliminates the weight gain dilemma of insulin therapy
4. Simplified regimen: Once-weekly injection vs. daily insulin (often multiple daily injections)
5. Hypoglycemia reduction: Dramatically lower risk of dangerous low blood sugar

References

1. Ludvik B, Giorgino F, Jódar E, et al. "Once-weekly tirzepatide versus once-daily insulin degludec as add-on to metformin with or without SGLT2 inhibitors in patients with type 2 diabetes (SURPASS-3): a randomised, open-label, parallel-group, phase 3 trial." The Lancet. 2021;398(10300):583-598. PubMed: 34370970

2. Nauck MA, D'Alessio DA. "Tirzepatide, a dual GIP/GLP-1 receptor co-agonist for the treatment of type 2 diabetes with unmatched effectiveness." Cardiovascular Diabetology. 2022;21:169. PubMed: 36050763

3. Heerspink HJL, Sattar N, Pavo I, et al. "Effects of tirzepatide versus insulin glargine on kidney outcomes in type 2 diabetes in the SURPASS-4 trial." The Lancet Diabetes & Endocrinology. 2022;10(11):774-785. PubMed: 36152639