Lab Literacy / Why GLP-1 Trials Separate Obesity From Diabetes Arms
9 min readSemaglutide is one molecule. It was tested in two separate trial programs with different names, different populations, different primary endpoints, and different lab panels. The diabetes program was called SUSTAIN. The obesity program was called STEP. Tirzepatide followed the same pattern: its obesity program was called SURMOUNT.
The line separating the diabetes arm from the obesity arm is a single lab value: HbA1c.
If your HbA1c was 7% or higher — the threshold for type 2 diabetes diagnosis — you entered the diabetes trials. If your HbA1c was below the diabetes threshold and your BMI was 30 or above, you entered the obesity trials. Same drug. Two completely different research questions, measured against different endpoints, gated by different labs.
The separation is not a regulatory formality. It reflects a genuine difference in what the trials were designed to prove.
HbA1c measures your average blood glucose over roughly 90 days. The standard ranges:
| Range | HbA1c | Interpretation |
|---|---|---|
| Normal | < 5.7% | No diabetes |
| Prediabetes | 5.7 – 6.4% | Elevated risk |
| Diabetes | ≥ 6.5% | Diagnostic threshold (confirmed with a second test) |
SUSTAIN — the semaglutide diabetes program — enrolled people with established type 2 diabetes. The entry requirement: HbA1c between 7% and 10%. You had to have the disease, and it had to be inadequately controlled. The primary endpoint was HbA1c reduction. The question the trial asked: can this drug lower blood sugar in people with diabetes?
SURMOUNT — the tirzepatide obesity program — enrolled people based on BMI: 30 or above, or 27 or above with a weight-related comorbidity. The obesity arms excluded undiagnosed diabetes. The primary endpoint was percent body-weight reduction. The question: can this drug produce significant weight loss in people with obesity?
One molecule, two trial programs, separated by whether your HbA1c sat above or below the diabetes line.
Because the trials asked different questions, they needed different enrollment lab panels. Each gate exists for a reason tied to the specific intervention and population.
SUSTAIN (diabetes arm) key gates:
| Lab | Threshold | Why it exists |
|---|---|---|
| HbA1c | ≥ 7% (enrolled 7–10%) | Defines the population — the trial is about diabetes |
| Fasting glucose | Measured at baseline and multiple timepoints | Primary endpoint context; confirms diabetes status |
| Fasting insulin | Tracked throughout | Enables HOMA-IR calculation — insulin resistance is the mechanism |
SURMOUNT (obesity arm) key gates:
| Lab | Threshold | Why it exists |
|---|---|---|
| HbA1c | Below diabetes threshold (undirected diabetes excluded) | Keeps the obesity population clean — the question is weight, not glucose |
| eGFR | Excluded < 30 | GLP-1 drugs are cleared renally; severe kidney impairment changes the risk profile |
| ALT | Excluded > 3× upper limit of normal | Liver stress marker — GLP-1 class carries pancreatitis risk, liver function is a safety gate |
| TSH | Excluded outside 0.4–6.0 mIU/L | Thyroid dysfunction is both a safety concern and a confounder for metabolic outcomes |
| Calcitonin | ≥ 20 ng/L if eGFR ≥ 60; ≥ 35 if < 60 | Medullary thyroid cancer surveillance — a class-specific theoretical risk for GLP-1 agonists based on rodent data |
| Blood pressure | Excluded ≥ 160/100 | Uncontrolled hypertension is a safety exclusion standard across metabolic trials |
Notice what is present in one but not the other. SUSTAIN tracked fasting insulin and HOMA-IR because insulin resistance is the disease mechanism in type 2 diabetes. SURMOUNT did not need that — the obesity trial was not primarily about insulin resistance. SURMOUNT added calcitonin and tighter renal gates because the obesity indication meant longer treatment duration in a population without diabetes, which shifts the risk calculus.
A single trial mixing diabetic and non-diabetic participants would create an unresolvable confound. If the drug lowers HbA1c by 1.5% and also produces 15% weight loss, and both groups are combined, you cannot tell whether the glucose effect is secondary to weight loss or is an independent action of the drug. The FDA requires a clean answer to each question separately.
The separation also matters for the safety profile. A person with type 2 diabetes on a GLP-1 agonist has a different risk landscape than someone with obesity but normal glucose: the diabetic participant may be on multiple other medications that interact, may have established end-organ effects, and may be more susceptible to hypoglycemia if the drug's glucose-lowering effect combines with their existing regimen. The obesity participant without diabetes does not carry those specific risks.
Regulators require the safety data for each population independently. You cannot extrapolate the diabetes-trial safety profile to the obesity population, because the comorbidities, concomitant medications, and baseline physiology are different.
SUSTAIN titrated semaglutide in fixed quarterly steps: 0.25 mg, then 0.5 mg, then 1.0 mg, each step held for four weeks. The obesity program (STEP) continued the escalation up to 2.4 mg — a higher ceiling, because the obesity indication tolerated and required more aggressive dosing than the diabetes starting point.
SURMOUNT used the same fixed quarterly step structure for tirzepatide, beginning at 2.5 mg. The schedule was predetermined — every four weeks, increase the dose — regardless of what any lab showed. The only reason to pause or reduce was gastrointestinal intolerance, not a lab value.
This is a point that connects directly to the next article in this series: in the GLP-1 class, labs do not drive the dose. The schedule does. That is a deliberate design choice, and it is the opposite of how growth-hormone-axis peptides are managed.
When you see a GLP-1 trial result, the first question is: which program was this — diabetes or obesity? The answer changes what the primary endpoint means, what the enrollment criteria were, and what population the safety data cover.
A trial showing "HbA1c reduction of 1.5%" is a diabetes-arm result. A trial showing "15% body-weight reduction" is an obesity-arm result. The same drug produced both, but each number is valid only for the population it was tested in. Applying the obesity-trial weight-loss numbers to a person with type 2 diabetes — or the diabetes-trial HbA1c numbers to someone without diabetes — is using data outside the population it was generated in.
The HbA1c line is not a technicality. It is the boundary between two different research questions, and the trial data on each side answer only the question they were designed to answer.
This article is for educational purposes and is not medical advice. It describes how GLP-1 clinical trials were structured and why. It does not recommend any peptide, test, or protocol. Decisions about treatment belong with a qualified clinician who knows your medical history.
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