The Emerging Role of Serum Resistin as a Biomarker in Inflammation and Diabetes

Chronic low-grade inflammation and metabolic dysregulation are central drivers of type 2 diabetes, cardiovascular disease, and non‑alcoholic fatty liver disease. Identifying reliable biomarkers that reflect both inflammatory status and insulin resistance is essential for early risk stratification, monitoring disease progression, and guiding therapy. Among the many adipokines studied over the past two decades, resistin has attracted sustained attention for its dual role in inflammation and glucose homeostasis. Serum resistin levels are consistently elevated in individuals with obesity, metabolic syndrome, and type 2 diabetes, and mounting evidence implicates resistin as a pro‑inflammatory mediator that worsens insulin sensitivity. This article provides a detailed overview of resistin biology, its measurement and clinical associations, the underlying inflammatory mechanisms, and the current state of evidence supporting its use as a biomarker. We also highlight the challenges that remain before resistin can be integrated into routine clinical practice.

Resistin: Discovery, Structure, and Sources

Resistin is a 12.5 kDa cysteine‑rich protein belonging to the resistin‑like molecule (RELM) family. It was first identified in 2001 by Steppan et al. during a screen for genes that are down‑regulated by thiazolidinediones (insulin‑sensitizing drugs) in mouse adipocytes. The name “resistin” was coined because the factor appeared to induce insulin resistance. In rodents, resistin is almost exclusively secreted by adipocytes, and its expression is negatively regulated by rosiglitazone. In humans, however, the story is more complex: the primary source of resistin is not adipose tissue but rather mononuclear cells—specifically macrophages and monocytes. Human resistin shares only about 60% amino‑acid identity with mouse resistin, and its regulation differs substantially. This species difference has important implications for translating animal findings to human pathophysiology.

The human resistin gene (RETN) is located on chromosome 19p13.3. It encodes a pre‑protein of 108 amino acids; the mature secreted protein is 92 amino acids in length. Resistin circulates as a disulfide‑linked dimer, though higher‑order oligomers have also been observed. The functional significance of these oligomeric forms remains an active area of investigation, with some evidence suggesting that the dimer might be the bioactive entity. Resistin expression in macrophages is strongly induced by pro‑inflammatory stimuli, including lipopolysaccharide (LPS), tumor necrosis factor‑alpha (TNF‑α), and interleukin‑6 (IL‑6). This observation provided an early clue that resistin is intimately linked to the inflammatory cascade.

Numerous studies have measured resistin in human serum and plasma using enzyme‑linked immunosorbent assays (ELISA) or multiplex bead‑based assays. Reported normal values vary considerably due to differences in assay formats, antibodies used, and study populations. Typical concentrations in healthy lean individuals range from 4 to 20 ng/mL, with higher levels in obese or diabetic subjects. Standardization of resistin measurement remains a major hurdle; without a certified reference material, direct comparison across studies is difficult. Nevertheless, the relative differences between healthy and disease groups are remarkably consistent.

Resistin and Inflammation: Mechanisms and Pathways

A large body of evidence demonstrates that resistin acts as a pro‑inflammatory cytokine. In vitro, recombinant human resistin stimulates the expression of TNF‑α, IL‑6, IL‑1β, and monocyte chemoattractant protein‑1 (MCP‑1) in human macrophages and endothelial cells. This induction occurs through activation of the nuclear factor‑kappa B (NF‑κB) pathway, a master regulator of inflammation. Resistin binds to a putative receptor (possibly TLR4, though the data are not fully settled) and triggers downstream signaling via p38 MAPK and JNK. The result is a feed‑forward loop: resistin released by macrophages promotes further macrophage recruitment and activation, perpetuating a state of low‑grade chronic inflammation.

In human endothelial cells, resistin up‑regulates adhesion molecules such as ICAM‑1 and VCAM‑1, facilitating leukocyte adhesion and transmigration. This pro‑inflammatory and pro‑atherogenic effect links resistin directly to vascular dysfunction. Moreover, resistin has been shown to induce the production of reactive oxygen species (ROS) in multiple cell types, contributing to oxidative stress that damages cellular components and worsens metabolic health. Several cross‑sectional and prospective studies have found strong positive correlations between serum resistin and high‑sensitivity C‑reactive protein (hs‑CRP), an established inflammatory marker. For example, a meta‑analysis of over 30 cohorts reported a significant positive association between resistin and CRP levels, independent of body mass index. This consistent observation supports the concept that resistin is a marker of systemic inflammation.

Resistin in Autoimmune and Infectious Diseases

Elevated resistin has been reported in rheumatoid arthritis, systemic lupus erythematosus, and inflammatory bowel disease, where it correlates with disease activity scores. In rheumatoid arthritis, synovial fluid resistin levels are higher than in serum, suggesting local production by infiltrating macrophages. Resistin also appears to play a role in the acute‑phase response: circulating levels rise rapidly after LPS administration in human volunteers, indicating that resistin is part of the innate immune response to infection. While these observations underscore resistin’s broad involvement in inflammation, they also raise a note of caution: resistin is not specific to metabolic disorders. Its elevation in diverse inflammatory states means that any interpretation of resistin as a diabetes biomarker must account for concurrent acute or chronic inflammatory conditions.

The original association between resistin and insulin resistance was drawn from rodent studies. In mice, administration of recombinant resistin impairs glucose tolerance, while resistin knockout mice have improved insulin sensitivity and lower fasting glucose. Mechanistically, resistin reduces insulin signaling in skeletal muscle and liver by decreasing phosphorylation of Akt and increasing expression of suppressor of cytokine signaling 3 (SOCS‑3). This leads to impaired glucose uptake and increased hepatic gluconeogenesis. In humans, however, the picture is more nuanced. Early studies failed to find a consistent relationship between resistin and insulin resistance after adjusting for adiposity. More recent and larger studies, using better assays and homogenous populations, have re‑established a connection.

A meta‑analysis of 46 studies (covering more than 12,000 subjects) found that serum resistin levels were significantly higher in individuals with type 2 diabetes than in controls, with a weighted mean difference of approximately 2.6 ng/mL. Furthermore, resistin levels were positively correlated with homeostatic model assessment for insulin resistance (HOMA‑IR) and fasting insulin, even after controlling for BMI. The magnitude of the association was modest but robust. Importantly, prospective studies have shown that higher resistin levels predict future development of type 2 diabetes over 5–10 years, independent of traditional risk factors such as obesity, family history, and physical activity. This suggests that resistin may play a causal role in the progression from normoglycemia to diabetes, rather than being merely a consequence of hyperglycemia.

Resistin and Beta‑Cell Function

Resistin’s effects are not limited to peripheral insulin resistance. There is emerging evidence that resistin directly impairs pancreatic beta‑cell function. In vitro, resistin reduces glucose‑stimulated insulin secretion from human islet cells and induces beta‑cell apoptosis. In vivo, transgenic mice overexpressing human resistin in macrophages develop reduced beta‑cell mass and glucose intolerance. These findings are consistent with the idea that resistin contributes to the two key defects of type 2 diabetes: insulin resistance and beta‑cell dysfunction. If confirmed in humans, this would strengthen the case for targeting resistin therapeutically.

Resistin as a Biomarker for Metabolic Disease

Given its links to inflammation, insulin resistance, and beta‑cell health, resistin holds promise as a biomarker for early detection and risk stratification. The main potential applications include:

  • Screening for pre‑diabetes: In normoglycemic individuals, elevated resistin may signal a heightened risk of progressing to impaired fasting glucose or impaired glucose tolerance. Several large cohort studies have reported odds ratios of 1.3–1.6 per standard deviation increase in resistin for incident diabetes, after adjustment for confounders.
  • Monitoring response to lifestyle or pharmacological interventions: Weight loss, physical activity, and insulin‑sensitizing drugs such as metformin and thiazolidinediones have been shown to lower resistin levels in some studies. Serial measurement might help gauge the effectiveness of treatment.
  • Differentiating diabetes subtypes: Resistin levels are markedly elevated in type 2 diabetes but not in type 1 diabetes, consistent with the inflammatory etiology of the former. In cases where the diabetes phenotype is ambiguous, resistin measurement could provide supportive information.
  • Cardiovascular risk assessment: Because resistin promotes atherosclerosis directly, elevated levels may identify individuals at high risk for cardiovascular events beyond that predicted by traditional lipid panels. A meta‑analysis of prospective studies found that higher resistin levels were associated with a 40% increased risk of cardiovascular disease.

Despite these promising applications, resistin has not yet entered clinical use. The reasons include lack of standardized assays, variable reference ranges across populations, and the absence of large‑scale interventional trials showing that resistin‑guided management improves outcomes. Additionally, resistin levels are influenced by age, sex, ethnicity, renal function, and smoking status, complicating interpretation. For example, African‑American individuals have higher resistin levels than Caucasians, even after adjusting for adiposity. These factors must be accounted for in any clinical algorithm.

Resistin in Obesity, Metabolic Syndrome, and Fatty Liver

Obesity is associated with a state of chronic inflammation driven by macrophage infiltration into adipose tissue. Since resistin is primarily produced by macrophages, it is not surprising that obese individuals have higher serum resistin levels than lean controls. Weight loss—whether through bariatric surgery, caloric restriction, or exercise—consistently reduces resistin concentrations. This reduction correlates with improvements in inflammatory markers and insulin sensitivity. Resistin has also been linked to the metabolic syndrome, a cluster of risk factors including abdominal obesity, hypertension, dyslipidemia, and hyperglycemia. Several cross‑sectional studies report that resistin levels increase across the number of metabolic syndrome components, suggesting that resistin reflects the cumulative burden of metabolic dysregulation.

Non‑alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome. Emerging data indicate that resistin is associated with both steatosis and non‑alcoholic steatohepatitis (NASH). In patients with biopsy‑proven NAFLD, serum resistin correlates with histological severity, including hepatocyte ballooning and fibrosis. Resistin is thought to promote hepatic inflammation by activating Kupffer cells and stellate cells, driving fibrosis. A few studies have suggested that resistin might help non‑invasively distinguish simple steatosis from NASH, though the diagnostic accuracy is not yet sufficient to replace liver biopsy. These findings underscore the broad metabolic impact of this adipokine.

Challenges and Future Directions

While resistin is a compelling biomarker candidate, several obstacles must be overcome before it can be adopted clinically. First, assay standardization is critical. Currently, ELISAs from different manufacturers yield discordant results. An international calibration standard would enable establishment of universal cut‑points. Second, longitudinal studies are needed to determine whether changes in resistin over time track with changes in disease activity and whether intervention‑induced reductions in resistin predict better outcomes. Third, the causal role of resistin in human disease remains debated. Human genetic studies show that common variants in the RETN locus are associated with resistin levels, but whether they are associated with diabetes is inconsistent. Mendelian randomization studies have not yet provided clear evidence for a causal effect of resistin on type 2 diabetes. It is possible that resistin is primarily a marker—rather than a mediator—of the inflammatory state that drives metabolic disease.

Future research should focus on the following areas:

  • Development of a harmonized ELISA or mass‑spectrometry‑based assay with traceability to a common standard.
  • Large, multi‑ethnic prospective studies that examine resistin as a predictor of incident diabetes, cardiovascular events, and NASH, with careful adjustment for confounders.
  • Clinical trials of drugs that lower resistin (e.g., statins, PPAR‑γ agonists, anti‑inflammatory agents) to evaluate whether resistin reduction is linked to improved outcomes.
  • Investigation of resistin isoforms and binding partners: post‑translational modifications may affect bioactivity and assay recognition.
  • Exploration of resistin in other inflammatory conditions such as atherosclerosis, cancer, and chronic kidney disease.

Conclusion

Serum resistin is an adipokine with dual roles in inflammation and glucose metabolism. It is produced by macrophages, is upregulated by inflammatory stimuli, and promotes insulin resistance and beta‑cell dysfunction. Clinical studies consistently show that circulating resistin levels are elevated in obesity, type 2 diabetes, metabolic syndrome, and NAFLD, and that higher levels predict future disease and adverse outcomes. However, the lack of standardized assays, uncertainty about causality, and the influence of non‑metabolic factors limit its current clinical utility. With continued research and assay harmonization, resistin may become a valuable component of a multi‑biomarker panel for metabolic and inflammatory risk assessment. In the meantime, its measurement remains a powerful research tool that has deepened our understanding of the interface between immune activation and metabolic regulation.