A Simple Blood Test Could Identify the Most Effective Obesity Medication

Key Takeaways: 

• A hypothesis-generating pilot study suggests that fasting blood levels of two incretin hormones – GLP-1 and GIP – may help predict how well people with severe obesity respond to semaglutide and tirzepatide.
  
• Low fasting GIP was linked to an optimal response to tirzepatide, while low GLP-1 combined with intermediate-to-high GIP was linked to an optimal response to semaglutide.
  
• The researchers stress that the findings are preliminary and should not guide prescribing until confirmed in larger, adequately powered randomised trials.
  
  
  

A step towards matching patients with the right medication

A straightforward fasting blood test may one day help clinicians decide which obesity medication is most likely to work for an individual patient. That is the tentative conclusion of a new hypothesis-generating pilot study published in the journal Diagnostics, which reports that fasting blood levels of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) may help predict the therapeutic response to semaglutide and tirzepatide in people living with severe obesity.

Why obesity medications produce different responses

Obesity, characterised by excessive fat accumulation in the body, has become a global epidemic, affecting more than 650 million adults worldwide. The condition is associated with a significantly increased risk of cardiovascular disease, type 2 diabetes, certain cancers, and all-cause mortality.

Among the pharmacological options, glucagon-like peptide-1 receptor agonists (GLP-1RAs) such as semaglutide, and dual GIP/GLP-1 receptor co-agonists such as tirzepatide, have shown considerable promise in addressing this challenge. One major limitation of these medicines, however, is the marked variability in response between individuals, which has prompted interest in identifying the biological mechanisms that drive this variation.

GLP-1 and GIP are the two principal incretin hormones, secreted by intestinal cells after food is eaten. GLP-1 suppresses appetite and promotes satiety through central nervous system pathways, whereas GIP regulates adipose tissue metabolism and energy expenditure. Acting synergistically, these hormones help to regulate glucose metabolism and appetite, giving them a major role in the management of obesity and type 2 diabetes.

Because the incretin system is frequently dysregulated in obesity, researchers at the University of Catania and MEDISAN, both based in Italy, designed the study to investigate whether fasting blood levels of GLP-1 and GIP could help identify people more likely to respond to semaglutide and tirzepatide.

How the pilot study was designed

The study enrolled 90 adults with a BMI greater than 40 kg/m² (class III obesity). Fasting blood samples were collected to measure each participant’s GLP-1 and GIP levels.

Each hormone was independently divided into low, intermediate, and high tertiles – a statistical division of a dataset into three equal parts – based on its distribution across the study population. Combining the GLP-1 and GIP tertiles produced nine distinct hormone profiles, each containing 10 participants. Within every profile, participants were randomly assigned to receive either semaglutide or tirzepatide, with five people allocated to each medicine per profile.

Response to treatment was assessed at six months. A reduction in body weight of less than 5% was classed as a low response, a reduction of 5–15% as an intermediate response, and a reduction of more than 15% as an optimal response.

Low incretin levels shaped treatment outcomes

The analysis showed that participants in the three profiles characterised by the low GIP tertile achieved an optimal response to tirzepatide, regardless of their GLP-1 levels. This suggests that low fasting GIP was associated with greater responsiveness to exogenous GIP receptor agonists such as tirzepatide.

For semaglutide, participants in two profiles – those characterised by a low GLP-1 tertile combined with an intermediate-to-high GIP tertile – were the only ones to achieve an optimal response. This may indicate that low endogenous GLP-1 availability leaves more GLP-1 receptors free for activation by exogenous semaglutide. The intermediate-to-high levels of endogenous GIP, meanwhile, may point to intact or compensatory incretin secretory capacity that does not interfere with the efficacy of a GLP-1 receptor agonist.

Participants in the profile characterised by high GLP-1 and high GIP tertiles achieved only a low response to both medicines. The authors suggest this may reflect a dysregulated incretin system that was not overcome by pharmacological doses within six months. They note, however, that fasting hormone measurements alone cannot distinguish between incretin secretory deficiency and receptor resistance, making this interpretation speculative.

On the clinical side, participants who achieved an optimal response to either medicine experienced significant reductions in waist circumference and improvements in insulin sensitivity. These changes paralleled the weight-loss patterns observed across the response groups, indicating clinically meaningful improvements in central adiposity and metabolic health.

How receptor occupancy may explain the findings

The observed variation in response may be explained through incretin receptor occupancy. Tirzepatide, as a dual GIP/GLP-1 receptor co-agonist, activates both receptor systems simultaneously. When GIP is present at low abundance (low fasting levels), it cannot fully occupy its receptor, potentially leaving that receptor available for exogenous tirzepatide. Once bound and activated, tirzepatide may then exert greater therapeutic effects by regulating adipose tissue metabolism, energy expenditure, and potentially central appetite regulation.

Semaglutide, which binds and activates the GLP-1 receptor exclusively, may exert its greatest effects when GLP-1 receptors are relatively unoccupied because of low levels of endogenous GLP-1. In these conditions, semaglutide may more effectively restore GLP-1 receptor signalling and deliver its anorectic, insulinotropic, and metabolic effects.

What this could mean for personalised prescribing

Taken together, the study suggests that fasting blood levels of GLP-1 and GIP were associated with the therapeutic response to semaglutide and tirzepatide in people with severe obesity, and may help identify those more likely to respond to treatment. Specifically, low GIP levels were associated with an optimal tirzepatide response, whereas low GLP-1 levels combined with intermediate-to-high GIP levels were associated with an optimal semaglutide response.

Because a single-timepoint measurement of GLP-1 and GIP cannot reveal receptor resistance, the researchers recommend treating these observations as hypothesis-generating, and highlight the need for mechanistic validation through dynamic measurements of incretin levels and receptor activity. Overall, the findings offer preliminary clinical evidence for incretin-guided, personalised pharmacotherapy that could improve treatment outcomes in obesity management.

Limitations and next steps

Several important caveats apply. This was a small, single-centre, open-label pilot study, with only five participants per treatment arm within each hormone profile. In addition, fasting hormone measurements cannot distinguish incretin secretory deficiency from receptor resistance. The authors therefore emphasise that the findings are preliminary and should not guide clinical practice until they are confirmed in larger, adequately powered randomised trials.