Introduction: fat is not a passive store

Adipose tissue was long regarded as inert lipid storage — a metabolic pantry opened during caloric shortfall. Contemporary biology paints a strikingly different picture: adipose tissue is a highly dynamic endocrine organ, secreting dozens of adipokines (adiponectin, leptin, resistin, chemerin), modulating systemic insulin sensitivity, regulating inflammation through immune cell populations within its stromal-vascular fraction, and communicating bidirectionally with the hypothalamus, pancreas, and liver. The metabolic consequences of obesity are determined not merely by total fat mass but by the depot distribution, adipocyte size distribution, and the inflammatory state of adipose tissue. Modern metabolic peptides — spanning GLP-1R/GIPR agonists, triple agonists, GH axis peptides, and lipolytic fragments — each interact with adipose tissue through distinct molecular mechanisms, producing patterns of fat redistribution that cannot be explained by caloric restriction alone.

Lipolysis vs lipogenesis: the fundamental balance

Within a mature white adipocyte, lipid homeostasis is governed by the opposing rates of lipolysis and lipogenesis.

Lipolysis is catalysed sequentially by adipose triglyceride lipase (ATGL, the rate-limiting first step), hormone-sensitive lipase (HSL, activated by PKA-mediated phosphorylation under cAMP stimulation), and monoacylglycerol lipase (MGL). The end products — free fatty acids (FFAs) and glycerol — are released into the circulation. HSL is activated by catecholamines (via β-adrenoceptor/cAMP) and by glucagon/GH/GIPR (also via cAMP) while being potently inhibited by insulin (via PI3K/Akt/PDE3B-mediated cAMP degradation).

Lipogenesis encompasses both de novo fatty-acid synthesis (via fatty-acid synthase, FASN, driven by acetyl-CoA carboxylase, ACC) and re-esterification of circulating FFAs into triglyceride (via GPAT, AGPAT, lipin, DGAT). Post-prandially, insulin stimulates lipogenesis; fasting or cAMP-elevating signals suppress it.

The ratio of lipolytic to lipogenic flux determines net adipocyte size over time. Metabolic peptides with cAMP-elevating receptor targets (GcgR, GIPR, β3-AR, GHR) shift this balance towards lipolysis; they simultaneously suppress ACC activity, reducing de novo lipogenesis. This dual effect — accelerate outflow, restrict inflow — produces the net fat-mass reduction observed with chronic administration.

Depot heterogeneity: visceral versus subcutaneous adipose

Visceral adipose tissue (VAT — omental, mesenteric, retroperitoneal depots) and subcutaneous adipose tissue (SAT) are biologically distinct despite both being white adipose. VAT adipocytes are larger on average, more metabolically active, more lipolytically responsive to catecholamines, and drain directly into the portal circulation — meaning FFAs liberated by VAT lipolysis reach the liver directly, driving hepatic insulin resistance, steatosis, and VLDL overproduction. VAT is also more immunologically active, with higher macrophage infiltration and pro-inflammatory cytokine secretion than SAT.

From a receptor distribution perspective:

GIPR is expressed at higher density in visceral than subcutaneous adipocytes, consistent with the preferential VAT reduction observed with tirzepatide in SURMOUNT body composition sub-studies.
GH receptor (GHR) density is also higher in visceral adipocytes, providing the mechanistic basis for tesamorelin's selective VAT reduction in HIV-associated lipodystrophy.
GLP-1R is more highly expressed in subcutaneous adipocytes and the adipose stromal-vascular fraction; GLP-1R agonism monotherapy tends to reduce SAT and VAT proportionally rather than preferentially targeting the visceral depot.

Brown adipose tissue and the thermogenic dimension

Brown adipose tissue (BAT) dissipates chemical energy as heat via uncoupling protein 1 (UCP1), which short-circuits the mitochondrial proton gradient. Adult humans retain metabolically active BAT predominantly in supraclavicular, paravertebral, and perirenal depots; BAT activity is inversely correlated with BMI and insulin resistance. In addition to classical BAT, white adipose depots can undergo "browning" to generate beige (or "brite") adipocytes expressing UCP1 and related thermogenic proteins in response to cold exposure, β3-adrenoceptor stimulation, and various endocrine signals.

Multiple metabolic peptides influence BAT and browning:

GLP-1R agonism: Central GLP-1R activation in the sympathetic nervous system increases adrenergic outflow to adipose tissue, enhancing β3-adrenoceptor stimulation and UCP1 expression. Animal data consistently demonstrate GLP-1R-mediated BAT activation; human data are more equivocal but positive in PET-FDG studies.
GcgR agonism: Glucagon receptor activation directly upregulates UCP1 expression in brown and beige adipocytes via cAMP/PKA signalling — contributing to the metabolic-rate elevation from GcgR co-agonism in triple agonists and explaining part of the resting energy expenditure benefit of retatrutide.
GH/GHRH axis: Growth hormone promotes BAT thermogenic activity and WAT-to-beige transdifferentiation through JAK2/STAT5-mediated upregulation of thermogenic gene programs, providing a mechanistic link between GH axis restoration and improved body composition.

The GIP–adipose axis in depth

The GIP receptor biology guide covers GIPR pharmacology in detail; here we focus on the adipose-specific consequences. GIPR activation on adipocytes generates a cAMP signal that has nutrient-state-dependent effects: in the fed/post-prandial state (high insulin), the dominant outcome is facilitation of fatty-acid re-esterification and lipid storage; in the fasted state (low insulin), the same cAMP signal drives HSL-mediated lipolysis. This apparent paradox — GIPR both promotes storage and drives lipolysis — reflects the insulin co-signalling context rather than constituting contradictory biology.

Chronic GIPR agonism in the context of caloric restriction (as occurs with dual agonist use alongside reduced food intake) may also favour a shift in adipocyte size distribution: rather than a few hypertrophic adipocytes, the adipose depot is repopulated with more numerous, smaller adipocytes. This distribution shift is associated with improved adiponectin secretion per unit fat mass and reduced hypoxia-driven inflammation within the adipose stroma.

Lipolytic fragment mechanisms: AOD-9604 and HGH 176-191

AOD-9604 (also designated hGH 177-191) and the closely related HGH 176-191 are C-terminal fragments of the human growth hormone sequence corresponding to the lipolytic domain of GH. Unlike intact rhGH, these fragments do not substantially activate GHR-mediated JAK2/STAT5 signalling (and therefore do not raise IGF-1 or produce insulin resistance); their adipose effects are mediated instead through β3-adrenoceptor interaction and lipid raft-associated signalling in the adipocyte membrane.

Proposed mechanisms include:

β3-adrenoceptor stimulation: Like catecholamines, C-terminal GH fragments appear to activate β3-AR on adipocytes, raising cAMP and phosphorylating HSL — producing lipolytic output without IGF-1 axis activation.
ACC inhibition: In vitro and rodent data suggest AOD-9604 inhibits acetyl-CoA carboxylase (ACC), reducing the malonyl-CoA substrate for de novo fatty-acid synthesis and simultaneously relieving malonyl-CoA inhibition of CPT1A, thereby increasing mitochondrial fatty-acid import and β-oxidation.

Human clinical data for AOD-9604 are limited: a Phase 2 trial (MetAb-001) demonstrated statistically significant weight loss versus placebo at 12 weeks in obese adults without diabetes, but the programme was not pursued to Phase 3. The mechanistic interest persists because a pure lipolytic effect without IGF-1 axis activation would address a therapeutic gap in patients where GH axis activation is contraindicated.

Adipocyte size distribution and metabolic consequences

Adipocyte size is a stronger predictor of metabolic dysfunction than total fat mass. Hypertrophic adipocytes (diameter greater than 100 µm in VAT) exceed the diffusion limit for oxygen, creating a hypoxic adipose microenvironment that drives HIF-1α activation, macrophage infiltration (predominantly M1 phenotype), and secretion of pro-inflammatory adipokines (MCP-1, TNF-α, IL-6) while suppressing adiponectin. This inflammatory adipose state — metabolic adiposopathy — drives hepatic insulin resistance and systemic low-grade inflammation independent of total adipose mass.

Effective adipose remodelling with metabolic peptides produces a measurable shift in adipocyte size distribution: histological studies from tirzepatide-treated participants in SURMOUNT adipose sub-studies demonstrate increased proportions of small and medium adipocytes (less than 70 µm) with corresponding decreases in hypertrophic cells — a change that precedes and helps explain the anti-inflammatory and insulin-sensitising improvements in the trial.

Trial data: SURMOUNT, STEP and STEP-HFpEF

SURMOUNT-1 (tirzepatide): Mean weight loss of 20.9% at the 15 mg dose over 72 weeks in adults with obesity without diabetes. DXA body composition sub-studies demonstrated that approximately 67% of total weight lost was fat mass (with lean mass comprising the remaining ~33% of weight reduction) — a fat-mass fraction meaningfully higher than typically observed with caloric restriction alone, consistent with direct adipose-tissue-targeting effects via GIPR and GLP-1R. Visceral adipose tissue reduction was disproportionately large relative to total weight loss, consistent with the higher GIPR density in visceral depots.

STEP-1 (semaglutide 2.4 mg): Mean weight loss of 14.9% at 68 weeks. Body composition analyses demonstrated a similar lean-mass-loss proportion (approximately 39% of weight lost from lean tissue), underscoring the challenge of lean mass preservation with caloric restriction-dominant GLP-1R mechanisms alone.

STEP-HFpEF (semaglutide in heart failure with preserved ejection fraction):Semaglutide 2.4 mg produced significant improvements in KCCQ-CSS score and 6-minute walk distance, with mean weight loss of 13.3% versus 2.6% in placebo. Critically, echocardiographic improvements in cardiac remodelling indices (left atrial volume, E/e') were seen despite modest differences in blood pressure — suggesting that adipose tissue remodelling and reduction of epicardial fat (which expresses GLP-1R and was reduced on imaging) contributed directly to cardiac functional improvement, not merely haemodynamic unloading.

The lean mass challenge: where GHRH/GHSR fits

The consistent observation across SURMOUNT and STEP programmes that 25–40% of total weight loss comes from lean tissue (predominantly skeletal muscle) has driven interest in combination approaches that preserve or build lean mass alongside caloric restriction-driven fat loss. The mechanistic case for adding GHRH analogues (tesamorelin) or GHSR agonists (CJC-1295/ipamorelin) to incretin-based protocols is that GH-driven IGF-1 promotes myofibrillar protein synthesis and attenuates the protein catabolism that accounts for lean mass loss during aggressive caloric deficit. Early-phase combination protocols are being investigated, though definitive Phase 2/3 data pairing GH secretagogues with GLP-1R/GIPR agonists are not yet available.

Why this matters for next-generation compound design

The ideal metabolic peptide — or peptide combination — would maximise visceral and hepatic fat loss, preserve or increase lean mass, promote browning of residual subcutaneous fat to increase resting energy expenditure, and shift adipocyte size distribution towards the small, metabolically healthy phenotype. No single current compound achieves all four goals simultaneously. Triple agonists (via GcgR-mediated thermogenics) address the energy expenditure dimension; GHRH/GHSR agonists address lean mass preservation; the GIP-adipose axis addresses visceral depot selectivity. The multi-receptor design space is therefore not merely an incremental extension of GLP-1R pharmacology — it represents a convergent effort to achieve adipose tissue remodelling approaching the comprehensive effect of bariatric surgery without the associated morbidity.

Adipose tissue remodelling research