Hidden metabolic crisis in lean youth: Waist-to-height ratio uncovers endoplasmic reticulum stress-driven insulin resistance in platelets

Abstract

The rising prevalence of insulin resistance (IR) among metabolically unhealthy normal-weight youth represents a critical public health challenge, often missed by conventional body mass index (BMI)-based screening. Numerous studies have suggested that the waist-to-height ratio (WtHR) may be more effective than BMI for the early prediction of IR. This superiority likely stems from WtHR’s capacity to more accurately reflect abdominal fat accumulation. However, the precise underlying molecular mechanisms remain to be fully elucidated. In this article, we synthesize evidence linking elevated WtHR to visceral adiposity, systemic inflammation, and endoplasmic reticulum stress, a convergent molecular mechanism driving IR across liver, adipose, and vascular tissues. Notably, emerging data implicate platelets as accessible biosensors of endoplasmic reticulum stress, offering a novel window for early detection. Supported by epidemiological, clinical, and molecular studies - including recent findings in young adults with normal BMI but high WtHR - this paradigm shift underscores the urgency of integrating WtHR into routine youth health assessments to enable precision prevention and mitigate lifelong metabolic disease burden.

Key Words: Insulin resistance; Waist-to-height ratio; Endoplasmic reticulum stress; Platelet; Body mass index; Adiposity; Metabolic syndrome; Adolescent

Core Tip: The waist-to-height ratio, with a threshold of > 0.5, offers a simple yet powerful anthropometric tool to unmask hidden insulin resistance and cardiometabolic risk in metabolically unhealthy normal-weight youth - a population routinely missed by body mass index-centric screening. This paradigm shift is grounded in mechanistic links among visceral adiposity, systemic inflammation, and endoplasmic reticulum stress, with emerging evidence positioning platelets as accessible biosensors of endoplasmic reticulum stress. Integrating waist-to-height ratio into routine youth health assessments enables early, precise intervention and holds potential to curb the lifelong burden of metabolic disease.

INTRODUCTION

The contemporary landscape of metabolic health is undergoing a profound and unsettling transformation, particularly among younger generations. For decades, insulin resistance (IR) has been predominantly framed as a consequence of advanced age or overt obesity, manifesting only after prolonged metabolic strain[1,2]. This paradigm, however, is increasingly challenged by compelling epidemiological evidence revealing a silent epidemic: A significant proportion of lean and normal-weight adolescents and young adults exhibit early signs of metabolic dysfunction, often overlooked by conventional clinical screening[3,4]. This phenomenon is captured by the term “metabolically obese normal-weight”, to describe individuals who, despite a normal body mass index (BMI), exhibit metabolic profiles - such as hyperinsulinemia and dyslipidemia - typical of persons with overt obesity. Our evolving understanding of metabolically obese normal-weight has shifted from viewing it as a rare clinical anomaly to recognizing it as a significant, often undetected, public health crisis among younger generations. The historical oversight of this group stems from a “BMI-centric” clinical bias that equates a slender appearance with metabolic health, thereby creating a critical blind spot in preventive medicine[5,6].

Recent analyses from the National Health and Nutrition Examination Survey (NHANES) indicate that nearly 40% of young adults aged 18-25 demonstrate IR based on homeostatic model assessment (HOMA-IR > 3.0), with approximately half of these individuals maintaining a BMI within the clinically defined normal range (18.5-24.9 kg/m²)[7,8]. The high prevalence reported in NHANES data must be interpreted with caution regarding potential confounding variables, such as ethnic variations in muscle mass, dietary patterns, and physical activity levels, which significantly influence HOMA-IR scores beyond simple adiposity. Furthermore, relying on a single fasting insulin measurement may introduce sampling bias, as it does not capture the dynamic nature of insulin secretion compared to a glucose tolerance test. Despite these limitations, the consistency of this trend across multiple NHANES cohorts suggests that the dissonance between anthropometric appearance and underlying metabolic pathology is a robust clinical finding. This dissonance underscores a fundamental limitation of the current diagnostic framework, which is characterized by an excessive reliance on BMI as the primary screening tool. While BMI provides a convenient population-level estimate of total adiposity, it fails to capture the nuanced distribution of adipose tissue, particularly the insidious accumulation of visceral fat[9,10]. In lean youth, metabolic dysfunction often occurs not due to total body mass, but because of “ectopic” fat deposition in the liver and around organs, which acts as a key driver of systemic inflammation and lipotoxicity. It is within this context that emerging studies have revealed the waist-to-height ratio (WtHR) to be a potentially superior indicator for IR screening[11-13]. In recent years, endoplasmic reticulum (ER) stress has been recognized as a key factor in the progression of IR[14,15]. Interestingly, young individuals with a high WtHR are more prone to exhibiting a state of ER stress, further underscoring the importance of WtHR as an early marker of metabolic dysfunction[16,17]. However, the precise mechanisms linking WtHR, ER stress, and IR remain elusive. Further exploration is warranted to clarify this triad, thereby optimizing the clinical utility of WtHR as a screening indicator.

This article synthesizes current evidence to argue that metabolic dysfunction in lean youth is not an anomaly but a systemic crisis demanding reevaluation of screening protocols, mechanistic understanding, and early intervention strategies (Figure 1). By integrating insights from epidemiology, molecular physiology, and translational research, we elucidate how a simple anthropometric metric - WtHR - can unmask a cascade of cellular stress responses with profound implications for lifelong cardiometabolic health.

Figure 1 The hidden metabolic crisis in lean youth. Waist-to-height ratio > 0.5 identifies high visceral adiposity risk, even in normal-weight individuals. Platelets serve as early biosensors of endoplasmic reticulum stress-driven insulin resistance, enabling precision prevention through routine screening, lifestyle intervention, and future targeted therapies. WtHR: Waist-to-height ratio; BMI: Body mass index; ER: Endoplasmic reticulum; FFAs: Free fatty acids; p-PERK: Phosphorylated protein kinase R-like endoplasmic reticulum kinase; p-JNK: Phosphorylated c-Jun N-terminal kinase; SERCA: Sarco-endoplasmic reticulum Ca-ATPase.

WTHR: EARLY METABOLIC RISK STRATIFICATION

The inadequacy of BMI in identifying metabolically vulnerable youth has spurred rigorous investigation into alternative anthropometric indices. Among these, WtHR has garnered substantial empirical support for its superior sensitivity in detecting central adiposity and associated metabolic risk[18,19]. Unlike BMI, which conflates lean mass, bone density, and fat distribution into a single metric, WtHR directly quantifies abdominal obesity relative to stature - a critical distinction given that visceral adipose tissue is metabolically active, secreting pro-inflammatory cytokines and free fatty acids (FFAs) that disrupt systemic insulin signaling[20,21].

A WtHR exceeding 0.5 has been consistently validated across diverse ethnic cohorts as a robust threshold for elevated cardiometabolic risk, correlating strongly with dyslipidemia, hypertension, hepatic steatosis, and IR[22-24]. Ashwell and colleagues’ seminal meta-analysis, encompassing over 300000 participants globally, demonstrated that WtHR outperformed both BMI and waist circumference in predicting type 2 diabetes, cardiovascular events, and all-cause mortality, with a ratio > 0.5 conferring a 2.5-fold increased risk of metabolic syndrome in young adults[22]. This superiority stems from WtHR’s inherent normalization for height, mitigating ethnic and sex-based variations in body proportions that often confound waist circumference interpretation. In recent studies, young adults (18-25 years) with WtHR > 0.5 - despite normal BMI - exhibited significantly elevated fasting insulin, HOMA-IR scores, triglyceride levels, and reduced HDL cholesterol compared to peers with lower ratios[25,26]. Critically, these alterations occurred in the absence of clinical obesity, highlighting WtHR’s capacity to identify a “pre-disease” metabolic state. This finding resonates with longitudinal data from the Bogalusa Heart Study, where elevated WtHR in adolescence predicted subclinical atherosclerosis two decades later, independent of BMI trajectory, and with United Kingdom Biobank data showing WtHR as a stronger predictor of ischemic cardiovascular disease than body fat percentage[27].

The practicality of WtHR further enhances its translational appeal: It requires only a tape measure and basic arithmetic, making it feasible for integration into school health screenings, primary care visits, and community health initiatives worldwide. By shifting focus from total body weight to abdominal adiposity distribution, WtHR reframes metabolic risk assessment toward precision and accessibility, offering a scalable tool to intercept pathology before irreversible organ damage ensues. Its adoption represents not merely a methodological refinement but a philosophical pivot - from reactive disease management to proactive metabolic stewardship in youth.

MOLECULAR PATHOGENESIS: ER STRESS LINKS ADIPOSITY TO IR

Beneath the anthropometric surface of elevated WtHR lies a sophisticated molecular cascade wherein visceral adiposity initiates ER stress, triggering a self-perpetuating cycle of IR.

Visceral adipose tissue releases excessive FFAs and inflammatory mediators that overwhelm metabolic capacity in both classic insulin-sensitive tissues and platelets, disrupting ER proteostasis and calcium homeostasis[28-30]. This triggers the unfolded protein response; specifically, chronic protein kinase R-like endoplasmic reticulum kinase (PERK) phosphorylation attenuates global translation while activating the c-Jun N-terminal kinase (JNK)[31]. Activated JNK then phosphorylates IR substrate-1 on inhibitory serine residues, uncoupling insulin receptors from downstream phosphatidylinositol 3-kinase-protein kinase B signaling and impairing glucose metabolism[32,33]. Additionally, FFAs directly inhibit sarco-endoplasmic reticulum Ca-ATPase (SERCA) activity, depleting ER calcium stores and further exacerbating protein misfolding[34,35].

Intriguingly, Casillas et al[12] observed elevated SERCA expression in platelets of high-WtHR youth - a finding aligning with Gustavo Vazquez-Jimenez et al’s work[36] showing transient SERCA upregulation as a compensatory response to acute lipotoxic stress in endothelial cells. This adaptive surge likely represents an early attempt to restore calcium homeostasis before chronic stress leads to SERCA dysfunction and irreversible ER damage[37,38]. The elegance of this mechanism lies in its universality: ER stress pathways activated in adipose tissue propagate systemically via circulating FFAs and cytokines, creating a feed-forward loop where IR begets further metabolic strain. Animal models corroborate this; Ozcan et al[39] demonstrated that chemical chaperones alleviating ER stress restored insulin sensitivity in obese mice, confirming ER stress as a causal driver - not merely a correlate - of IR. Human studies further show that adipose tissue from obese, insulin-resistant individuals exhibits elevated markers of ER stress (stress such as glucose-regulated protein, C/EBP homologous protein and X-box binding protein 1 splicing) compared to metabolically healthy counterparts[39].

While the link between visceral adiposity and ER stress is strong, the relationship between WtHR and IR is modulated by several non-adipose factors[40-42]. Genetic predisposition potentially causing ectopic fat deposition even at low WtHR levels. Additionally, dietary factors can independently trigger ER stress, while physical activity can alleviate molecular stress markers like phosphorylated (p)-PERK regardless of changes in waist circumference[43,44]. Acknowledging these confounders is essential, as WtHR’s predictive value should be contextualized within an individual’s genetic and lifestyle framework.

PLATELETS AND WTHR: YOUTH METABOLIC DETECTION

Platelets, once considered only hemostatic cells, are now recognized as dynamic sentinels of systemic metabolic health[45,46]. They retain intact insulin signaling machinery, circulate in direct contact with metabolic substrates, and can be easily sampled via routine venipuncture. Critically, platelets from lean young adults with WtHR > 0.5 exhibit molecular hallmarks of ER stress - including elevated SERCA, phosphorylated PERK, and activated JNK - mirroring changes seen in liver and adipose tissue[47]. Their short lifespan ensures these markers reflect recent metabolic status, offering a minimally invasive “liquid biopsy” for early IR and thrombotic risk.

The integration of WtHR screening with platelet-based molecular profiling enables a paradigm shift in early intervention. For high-risk individuals, lifestyle interventions - such as resistance training and Mediterranean-style diets - can reduce visceral fat and ER stress within weeks[39,48], while emerging therapies like chemical chaperones offer promise for precision prevention guided by platelet biomarkers[49,50]. Notably, platelet hyperactivity in IR states is linked to increased cardiovascular risk, further underscoring their dual role as metabolic and thrombotic sensors[51,52].

Admittedly, platelet ER stress markers have certain limitations; their assay reproducibility, intra-individual variability, and feasibility for routine clinical practice remain in the investigational stage. Scaling this strategy requires coordinated action across clinical, public health, and policy domains. School-based WtHR education, validated digital tools for self-monitoring, and urban planning that supports active living can amplify reach. Crucially, cutoffs must be ethnicity-specific, as populations like South Asians develop metabolic dysfunction at lower WtHR thresholds[53,54]. Together, WtHR and platelet phenotyping form a scalable, multi-level framework to detect, monitor, and mitigate metabolic risk long before clinical disease manifests.

CLINICAL TRANSLATION: WTHR AS A WINDOW TO PLATELET AND ER STRESS

Recent evidence highlights a complex pathological crosstalk among central adiposity, ER stress, and circulating platelets in the progression of IR[55-57]. The WtHR is now widely recognized as a robust surrogate marker for visceral fat accumulation, a state characterized by chronic low-grade inflammation and lipotoxicity. Interestingly, this systemic metabolic derangement profoundly impacts circulating cells, particularly platelets[58,59]. In individuals with elevated WtHR, the lipotoxic and pro-inflammatory microenvironment triggers pronounced ER stress within platelets (Figure 2).

Figure 2 Endoplasmic reticulum stress acts as the molecular bridge linking visceral adiposity to systemic insulin resistance. Visceral fat releases free fatty acids and cytokines into circulation, inducing endoplasmic reticulum dysfunction in liver, muscle, and platelets. This activates the unfolded protein response pathway, leading to c-Jun N-terminal kinase-mediated insulin-receptor substrate 1 inhibition and impaired insulin signaling, forming a self-perpetuating feed-forward loop. Endoplasmic reticulum; FFAs: Free fatty acids; TNF-α: Tumor necrosis factor α; IL: Interleukin; PERK: Protein kinase R-like endoplasmic reticulum kinase; p-PERK: Phosphorylated protein kinase R-like endoplasmic reticulum kinase; p-JNK: Phosphorylated c-Jun N-terminal kinase; IRS-1: Insulin-receptor substrate 1; UPR: Unfolded protein response; ATF4: Activating transcription factor 4; CHOP: C/EBP homologous protein; PI3K: Phosphoinositide 3-kinases; Akt: Protein kinase B; SERCA: Sarco-endoplasmic reticulum Ca-ATPase; p-eIF2α: Phosphorylated eukaryotic translation initiation factor 2 alpha.

Crucially, this ER stress is not merely an epiphenomenon but a potent driver of pathogenesis. ER stress within platelets impairs local insulin signaling, leading to “platelet IR”, and disrupts cytosolic calcium homeostasis, thereby inducing platelet hyperreactivity[60,61]. Furthermore, these hyperreactive, ER-stressed platelets secrete pro-inflammatory microvesicles and bioactive mediators that exacerbate endothelial dysfunction and systemic IR. Therefore, measuring WtHR provides a valuable, non-invasive clinical window to detect this ER stress-driven thrombo-inflammatory cascade long before the onset of overt metabolic syndrome.

LIMITATIONS AND FUTURE PERSPECTIVES

Current evidence linking WtHR to platelet ER stress markers in lean youth is primarily cross-sectional, limiting causal inference about whether central adiposity drives cellular stress or vice versa. Most data - including key biomarker findings - derive from relatively small, ethnically homogeneous cohorts, raising concerns about generalizability across diverse genetic, dietary, and socioeconomic backgrounds. Additionally, while platelets offer a minimally invasive window into systemic metabolic stress, their anucleate nature restricts assessment of transcriptional regulation, necessitating complementary approaches (circulating extracellular vesicles or immune cells) for fuller mechanistic insight. Future research must prioritize longitudinal studies tracking WtHR, platelet ER stress markers (p-PERK, SERCA), and hard clinical endpoints from adolescence into adulthood. Large, multi-ethnic trials are needed to validate ethnicity-specific WtHR thresholds and integrate omics profiling (metabolomics, epigenetics) to uncover upstream drivers. Specifically, randomized controlled trials are needed to test whether targeted interventions, such as resistance training or chemical chaperones, can effectively “reset” platelet ER stress signatures (p-PERK/p-JNK) and improve insulin sensitivity. Developing point-of-care assays for platelet ER stress markers would further enable real-world implementation in primary care and public health settings.

CONCLUSION

Metabolic dysfunction in normal-weight youth is neither rare nor benign - it is a hidden crisis. Current evidence suggests that WtHR > 0.5 may offer greater sensitivity than BMI in identifying occult cardiometabolic risk, which is closely linked at the molecular level to ER stress activation in platelets. While existing data are primarily derived from cross-sectional studies, the integration of WtHR with BMI holds significant clinical screening potential. By combining simple anthropometry with accessible biospecimens like platelets, we can enable timely, personalized interventions before irreversible organ damage occurs. Specifically, a tiered clinical screening pathway should be established, prioritizing universal WtHR screening regardless of the patient's BMI. The proactive adoption of WtHR in clinical and public health practice offers a scalable pathway toward preserving metabolic health across generations.