Sleep Scientists Clash Over How Much of Sleep Apnea Is Truly Caused by Fat

A terse exchange in the Journal of Clinical Sleep Medicine has ignited one of the more consequential debates in sleep science: when obesity and obstructive sleep apnea appear together in the same patient, how much of the breathing disorder is actually caused by the excess weight, and how much is merely coincidence? The dispute began when a team led by Maizura and colleagues published a commentary titled “From coexistence to causation: defining adiposity-attributable obstructive sleep apnea,” arguing that the field needs a rigorous framework for separating the sleep apnea that obesity drives from the sleep apnea that would exist anyway. Now Christopher N. Schmickl and Atul Malhotra of the University of California, San Diego, have responded in a formal reply, defending the framing they laid out in their own earlier review of sleep and obesity and pushing back on what they see as an overly narrow reading of the evidence.

The stakes of this seemingly academic quarrel are enormous. Obstructive sleep apnea affects roughly a billion people worldwide by some estimates, and its overlap with obesity is so pervasive that the two conditions are often treated as two faces of the same metabolic syndrome. Yet the causal architecture linking them remains surprisingly murky. If clinicians cannot quantify how much of a given patient’s apnea is adiposity-attributable, they cannot accurately predict how much that patient will benefit from weight loss, whether through lifestyle intervention, pharmacotherapy such as the new generation of incretin-based drugs, or bariatric surgery. That uncertainty ripples directly into treatment decisions, insurance coverage, and the design of clinical trials.

At the heart of the exchange is a concept that sleep researchers call endotypes, the distinct physiological mechanisms that produce the shared clinical picture of repetitive upper airway collapse during sleep. Schmickl, Malhotra and their colleagues had previously argued, in their review “Sleep and obesity: known interactions and open questions,” that obesity acts on sleep apnea through multiple parallel pathways: it narrows the pharyngeal airway through fat deposition around the neck and tongue, it reduces lung volume and thereby diminishes the tracheal tug that stiffens the upper airway, it destabilizes respiratory control by increasing loop gain, and it alters arousal thresholds and fluid shifts that further compromise airway patency during sleep. Weight loss, in this view, does not simply shrink a pipe; it reconfigures an entire physiological system.

The Maizura commentary, in turn, pressed the field to go further and define what fraction of apnea severity is genuinely attributable to adiposity rather than merely coexisting with it. The distinction matters because correlation between body mass index and apnea severity, while real, is notoriously imperfect. Many patients with severe obesity never develop sleep apnea, and many lean patients do. Craniofacial anatomy, upper airway muscle responsiveness, genetic predisposition, and age all modulate the relationship. The commentators argued that without a formal definition of adiposity-attributable disease, the field risks overestimating the benefits of weight-targeted therapies and underdiagnosing the apnea that persists after the pounds come off.

Schmickl and Malhotra’s reply, accepted by the journal in September 2026, engages this critique directly. While the published reply is brief, its positioning within the citation trail reveals its argumentative center of gravity: the authors point to recent work by Beatty and colleagues, published in Chest, that measured how weight loss reshapes the physiological endotypes of obstructive sleep apnea. That study provides some of the most direct evidence to date that reducing adiposity produces measurable, mechanistically specific changes in airway collapsibility, loop gain, and arousal threshold, rather than a diffuse or nonspecific improvement. In other words, the causal pathways that obesity exploits are identifiable, quantifiable, and reversible, which is precisely the kind of evidence needed to move from coexistence to causation.

The reply also draws on a foundational observation from Peppard, Ward and Morrell, who showed more than a decade and a half ago that obesity amplifies oxygen desaturation during sleep-disordered breathing independent of its effect on airway collapse itself. Fat tissue, particularly around the abdomen and chest wall, mechanically loads the respiratory system, reducing the oxygen reserves available when breathing pauses occur. Two patients with identical degrees of airway obstruction can therefore experience dramatically different falls in blood oxygen depending on their body habitus. This finding complicates any attempt to define adiposity-attributable apnea using the apnea-hypopnea index alone, because the index counts events but says nothing about their physiological consequences.

That limitation has driven a broader movement in the field toward measures such as the hypoxic burden, championed by Azarbarzin, Sands and colleagues in analyses of the Osteoporotic Fractures in Men Study and the Sleep Heart Health Study. Their work demonstrated that the total burden of nocturnal oxygen deprivation predicts cardiovascular mortality better than the conventional event index, suggesting that the downstream harm of sleep apnea flows substantially through intermittent hypoxia. If adiposity disproportionately worsens hypoxic burden, then the fraction of apnea-related cardiovascular risk that is adiposity-attributable may exceed the fraction of apnea events that weight loss eliminates. Schmickl and Malhotra’s reply implicitly invokes this distinction, arguing that any definition of adiposity-attributable disease must grapple with severity measures that capture physiological consequence, not just event frequency.

The exchange also touches on a practical question that clinicians confront daily: what happens to sleep apnea after weight loss? Bariatric surgery cohorts and pharmacological trials consistently show substantial reductions in apnea severity, but remission is far from universal, and severity often recurs even when weight loss is maintained. The Beatty endotype study helps explain why. Weight loss preferentially improves certain mechanisms, such as airway collapsibility and lung volume effects, while leaving others, such as inherently narrow craniofacial anatomy or high loop gain driven by ventilatory control instability, largely untouched. A patient whose apnea was mostly anatomical may remain apneic after losing significant weight, while a patient whose apnea was mostly weight-driven may remit completely. Defining adiposity-attributable apnea, in this light, is less a semantic exercise than a prerequisite for personalized therapy.

Neither side of the dispute disputes the fundamental biology. Obesity is the single strongest modifiable risk factor for obstructive sleep apnea, and the epidemic of adiposity has been a principal engine of the global rise in sleep-disordered breathing. The disagreement is about epistemics: how confidently the field can assign causal fractions to a multifactorial disease when the contributing mechanisms interact, overlap, and compensate for one another. Schmickl and Malhotra, whose review emphasized the bidirectional nature of the relationship, note that the arrow also points the other way. Sleep apnea fragments sleep, promotes daytime sleepiness that reduces physical activity, and dysregulates hormones such as leptin and ghrelin that govern appetite, thereby promoting weight gain in a self-reinforcing loop. Any causal accounting that treats obesity as the sole upstream actor risks missing this feedback structure entirely.

The reply, published as volume 22, article 174 of the Journal of Clinical Sleep Medicine, is unlikely to settle the debate on its own, but it clarifies what a resolution would require: longitudinal studies that measure endotype-specific responses to defined amounts of weight loss, severity metrics that capture hypoxic and cardiovascular consequence, and analytical frameworks that can partition variance across interacting mechanisms. As incretin-based weight-loss therapies reshape the treatment landscape and millions of patients begin losing weight while still wearing their CPAP machines, the question of how much apnea the fat was actually causing will move from the pages of academic journals into the examination room. The Schmickl-Malhotra reply signals that the field’s leading sleep physiologists intend to answer that question with mechanistic precision rather than assumption, and the exchange it concludes may well be remembered as an early milestone in the effort to make sleep medicine genuinely causal in its reasoning.

Subject of Research: The causal relationship between adiposity and obstructive sleep apnea and its physiological endotypes

Article Title: Reply to “From coexistence to causation: defining adiposity-attributable obstructive sleep apnea”

Article References: Reply to “From coexistence to causation: defining adiposity-attributable obstructive sleep apnea”. (n.d.). https://doi.org/10.1007/s44470-026-00197-2

Image Credits: AI Generated

DOI: 10.1007/s44470-026-00197-2

Keywords: obstructive sleep apnea, obesity, adiposity, endotypes, weight loss, hypoxic burden, loop gain, airway collapsibility, bariatric surgery, sleep medicine, CPAP, causation