Glucagon-like peptide-1 receptor agonists (GLP‑1RAs) have moved from specialist tools for type 2 diabetes to mainstream therapies for obesity and cardiometabolic risk reduction. Alongside the familiar gastrointestinal profile — nausea, vomiting, diarrhoea — a growing body of observational data highlights less visible phenomena: persistent cold sensitivity, fatigue-like states, and changes in skin sensation or aesthetics. [1] [2] [3] [4]
This is a research-oriented narrative review. It synthesises mechanistic hypotheses and emerging literature on these phenomena during GLP‑1RA exposure and sustained energy deficit. The core framing: GLP‑1RA exposure creates a coordinated state of energy deficit and weight loss [5] [6] in which coldness, fatigue, and skin sensitivity are better understood as adaptive outputs of the whole system — not isolated, mysterious adverse events. Mechanistic work on GLP‑1, thermogenesis, neuroendocrine signalling, and skin biology gives sufficient structure to propose testable pathways and research priorities. [7] [8] [9]
A Shared Lens: GLP‑1RA Exposure + Energy Deficit
Across drug classes, chronic energy deficit and weight loss produce predictable adaptive phenomena: reductions in resting energy expenditure, increased movement efficiency, shifts in autonomic tone, and changes in tissue composition and microcirculation. [5] [6] GLP‑1RAs layer additional effects on top: satiety and slower gastric emptying that reduce caloric intake and alter nutrient timing [1] [10]; central GLP‑1 signalling in hypothalamic, reward, and stress circuits that modulates motivation and fatigue perception [11]; and rapid fat loss — sometimes with imperfect lean mass preservation — that affects insulation, mechanical load, and skin tension. [6] [12]
Viewed through this lens, coldness, fatigue, and cutaneous changes are three outputs of the same system — "GLP‑1RA exposure + energy deficit" — with different tissues broadcasting the organism's adaptive state.
Coldness: Thermoregulation in a Weight‑Loss State
Cold sensitivity is rarely listed as a formal adverse event in prescribing information, but multiple pharmacy guides and patient reports now describe persistent "feeling cold" on semaglutide, tirzepatide, and related agents. [13] [14] [15] [16] [17] A recent synthesis explicitly frames this as a weight-loss phenomenon rather than direct drug toxicity. [17]
Loss of subcutaneous fat. Subcutaneous adipose tissue acts as a physical insulator. As fat mass decreases, the thermal gradient from core to skin becomes easier to dissipate — consistent with reports of cold extremities as an early symptom. [6]
Adaptive thermogenesis. GLP‑1RAs reliably reduce caloric intake via appetite suppression and delayed gastric emptying. [1] [10] Over time, the body responds with reductions in resting metabolic rate and non-exercise activity, shifting fuel use toward conservation over "wasteful" heat production. [5] [6]
GLP‑1, glucagon, and brown adipose tissue. GLP‑1 pathways intersect with glucagon and hypothalamic AMPK signalling, which can stimulate brown adipose tissue and white fat "browning" in some contexts. [7] However, during real-world prolonged energy deficit, the net effect appears dominated by energy conservation rather than sustained hyper-thermogenesis.
Autonomic tone and peripheral blood flow. Cold extremities often track sympathetic-driven vasoconstriction and centralised blood redistribution. GLP‑1RA-induced changes in autonomic balance and vascular tone may amplify this in some subjects. [7] [18] Pharmacy resources increasingly note that feeling cold is a consequence of fat loss, reduced intake, and metabolic adaptation — with the drug's role being the creation of the energy deficit and body-composition change. [15] [19]
Fatigue‑Like States: Energy Deficit, GI Effects, and CNS Signalling
Fatigue, low energy, and tiredness are among the most commonly reported complaints in observational data and social-media case series, even though formal trials foreground GI events. [1] [2] [3] [4]
Energy deficit and GI physiology. Appetite suppression reduces meal frequency and portion size. If total intake falls below activity requirements, less substrate is available for peripheral tissues and the brain. [2] GI symptoms fragment nutrient delivery, amplifying energy fluctuations even when daily caloric totals are similar. [1] [10] Vomiting, diarrhoea, and reduced fluid intake can produce volume contraction and electrolyte shifts — mechanisms flagged in safety analyses as contributing to "washed out" states. [20]
Neuroendocrine components. GLP‑1 receptors are expressed in brain regions involved in appetite, reward, and stress. Mechanistic work highlights reduced hedonic drive — experienced by some as "flatness" rather than sedation [11] — and possible effects on sleep quality via GI discomfort and altered feeding patterns. [2] Emerging pharmacovigilance analyses are also examining mood signals and affect changes. [21]
Modifiable variables. Slower dose titration reduces peak GI burden and may indirectly lower fatigue. [1] [10] Proactive hydration and electrolyte replacement when GI symptoms are present is consistently recommended. Maintaining adequate protein and key micronutrients — B-vitamins, iron, magnesium — becomes harder when food volume falls but is critical for energy metabolism. [6]
Skin Sensitivity and Cutaneous Changes
Dermatology and endocrinology reviews now describe GLP‑1RAs as having bidirectional cutaneous effects: potential benefits in inflammatory conditions alongside hypersensitivity reactions, aesthetic changes, and altered touch perception. [8] [9] [22] [23] Popular discourse has coined "Ozempic face" to describe the perceived aesthetic impact of rapid fat loss. [12] [24]
Rapid fat loss and skin tension. As subcutaneous fat decreases, the mechanical support for dermis changes, altering mechanoreceptor loading and subjective tightness or laxity. [12] [6]
Nutrient insufficiency and barrier function. Sustained energy deficit can reduce intake of zinc, essential fatty acids, and vitamins A, D, and B-complex — all critical for barrier integrity and sebum production. [25] [24]
Dehydration and microcirculation. Fluid loss and lower intake reduce skin turgor and perfusion, contributing to dryness and increased sensitivity. [24] GLP‑1RAs also exert immunomodulatory and neuroendocrine effects that may alter somatosensory thresholds indirectly, though this area remains mechanistically under-mapped. [9] [23]
An Integrative Framework
Coldness, fatigue-like states, and cutaneous changes can be conceptualised as three branches from a central node — GLP‑1RA exposure + sustained energy deficit: thermoregulation disruption via insulation loss, adaptive thermogenesis, and autonomic redistribution [7]; impaired energy availability via lower intake, fragmented nutrient timing, and central GLP‑1 signalling [1] [2] [21]; and altered skin physiology via body-composition change, nutrient status, microcirculatory shifts, and neuroendocrine modulation. [8] [9] [12]
Future Directions
Key research priorities include: separating drug-specific from weight-loss-mediated effects using non-pharmacologic comparator arms and body-composition adjustment [5] [26]; systematically varying deficit depth, protein density, hydration strategy, and resistance-training volume rather than treating them as background noise [10]; combining indirect calorimetry, skin-temperature imaging, autonomic measures, and quantitative sensory testing with validated symptom scales [7] [23]; and systematically mining patient-reported outcomes including "hidden" phenomena — chills, brain fog, mood shifts — and reconciling them with randomised-trial safety tables. [4] [2] [3]
A more precise mechanistic understanding of how GLP‑1RAs interact with the physiology of energy deficit, thermoregulation, and tissue composition will help design mitigation strategies that preserve research validity while characterising the full adaptive-state profile of these compounds.
Sources: PMC5397288 · PMC12270588 · PMC6678955 · PMC11940170 · PMC11404059 · BMJ 2025 · JCI 2025