I-7. Systemic consequences of obesity

肥満の全身性影響

Lipodystrophy & the “Metabolic Sink”

A key role of adipose tissue is to protect the body from harmful lipids by sequestering them (the “metabolic sink”).
Lipodystrophy patients lack subcutaneous adipose tissue: they look skinny/healthy but have terrible metabolic health — insulin resistant, early-onset diabetes — because fat cannot be stored subcutaneously and is deposited ectopically.

Ectopic Fat & Lipotoxicity

When the subcutaneous depot is full, absent, or insulin-resistant, fatty-acid leakage → fat accumulates in tissues not designed to store it (lipotoxicity).
Lipotoxicity → lipid droplets in parenchymal cells (steatosis) → impaired insulin-receptor signaling → insulin resistance via:
- ↑Mitochondrial FFA oxidation → mitochondrial stress → ROS → dysfunction/lysis.
- ROS-induced lipid peroxidation.
- Ceramide & DAG production (side pathways).
These cause insulin resistance through inflammation, apoptosis, and fibrosis — a self-amplifying cycle (insulin resistance → less subcutaneous storage → more ectopic fat → more resistance).

Ectopic Fat Depots

Predominantly systemic effects

Visceral adipose tissue (vWAT): associated with higher metabolic risk; an ectopic depot reflecting sWAT’s inability to store excess fat.
Liver: → NAFLD / NASH (lipotoxicity, insulin resistance, hyperinsulinemia, inflammation, glycation, RAAS); chronic inflammation → cirrhosis → hepatocellular carcinoma.
Pancreas: lipotoxicity → β-cell dysfunction/apoptosis → T2DM.
Muscle: lipotoxicity → insulin resistance.
⇒ Ultimately leads to systemic insulin resistance → diabetes and its complications.

Predominantly local effects

Perivascular adipose tissue (PVAT): healthy → secretes adiponectin/NO, scaffolding, BAT-like insulation; in obesity → ↑vascular tone (constrictors), promotes atherosclerosis (pro-inflammatory cytokines, leptin).
Peri-/epicardial fat: surrounds coronary arteries; acts against cardiac contraction/relaxation.
Myocardium: intramyocardial fat → cardiomyocyte apoptosis, heart failure.
Kidney: renal sinus fat compresses vein/inner medulla → ↑interstitial pressure → compresses vasa recta & loop of Henle → ↑tubular transit → ↑NaCl/water reabsorption → ↑GFR + renin.

Systemic Low-Grade Inflammation

Hypertrophic adipose tissue shifts its secretion toward pro-inflammatory cytokines → systemic low-grade inflammation.
Linked to insulin resistance, endothelial dysfunction, atherosclerosis, thrombosis, heart failure, kidney failure, and malignancies.

Sympathetic activation

↑Leptin (via melanocortin system): selective leptin resistance — loses appetite-suppressing effect but keeps SYM activation → renal SYM outflow → renin → Na⁺ retention → hypertension.
↓Adiponectin, ↓ghrelin, baroreflex dysfunction.
Sleep apnea (neck compression → nocturnal hypoxia → SYM overactivation).

RAAS

Systemic: ↑circulating RAAS components; SYM → ↑renin; endothelial dysfunction → ↑renin; leptin → ↑aldosterone; adipocytes secrete renin/angiotensinogen/aldosterone (up to ~30% of circulating angiotensinogen).
Local: adipocyte ANGII → local inflammation, vascular injury, atherosclerosis; vasoconstriction → adipose hypoxia → fibrosis.

Mechanical Consequences

Renal compression → renin, hypertension.
Pharyngeal/thoracic → obstructive sleep apnea, respiratory distress.
Joint load → osteoarthritis.
Intra-abdominal pressure → GERD → adenocarcinoma.

Metabolic Syndrome

Abdominal obesity → insulin resistance → type 2 diabetes, often with dyslipidemia and hypertension (plus endothelial dysfunction, inflammation) → ↑cardiovascular disease risk.
Diagnosis (≥3 criteria): ↑BP, ↑fasting glucose, ↑waist circumference, ↑TAG, ↓HDL (or being treated for any). The main risk factor is obesity.

一問一答

▶What is the "metabolic sink" function of adipose tissue?

Adipose tissue protects the body from harmful lipids by sequestering them; failure of this function leads to ectopic fat deposition.

▶Why do lipodystrophy patients have poor metabolic health despite being thin?

They lack subcutaneous adipose tissue, so fat cannot be stored subcutaneously and is deposited ectopically → insulin resistance and early-onset diabetes.

▶What is lipotoxicity and how does it cause insulin resistance?

Ectopic lipid droplets in parenchymal cells (steatosis) impair insulin-receptor signaling via mitochondrial FFA oxidation → ROS, lipid peroxidation, and ceramide/DAG production → inflammation, apoptosis, fibrosis.

▶Describe the self-amplifying cycle of ectopic fat and insulin resistance.

Insulin resistance → less subcutaneous storage → more ectopic fat → more resistance — a self-amplifying loop.

▶How does ectopic fat in the liver and pancreas cause diabetes?

Liver → NAFLD/NASH (→ cirrhosis → HCC); pancreas → β-cell dysfunction/apoptosis → T2DM; both contribute to systemic insulin resistance.

▶What are the local effects of perivascular adipose tissue (PVAT) in obesity?

Healthy PVAT secretes adiponectin/NO; in obesity it increases vascular tone (constrictors) and promotes atherosclerosis via pro-inflammatory cytokines and leptin.

▶How does renal sinus fat raise blood pressure in obesity?

It compresses the renal vein and inner medulla → ↑interstitial pressure → compresses vasa recta and loop of Henle → ↑tubular transit → ↑NaCl/water reabsorption → ↑GFR and renin.

▶How does obesity cause systemic low-grade inflammation?

Hypertrophic adipose tissue shifts secretion toward pro-inflammatory cytokines, driving insulin resistance, endothelial dysfunction, atherosclerosis, thrombosis, and organ failure.

▶How does selective leptin resistance cause hypertension?

Elevated leptin loses its appetite-suppressing effect but retains sympathetic activation → renal sympathetic outflow → renin → Na⁺ retention → hypertension.

▶How does sleep apnea contribute to obesity-related hypertension?

Neck compression causes nocturnal hypoxia → sympathetic overactivation → hypertension.

▶How does adipose tissue activate the RAAS in obesity?

Systemic: ↑circulating RAAS components, SYM → renin, leptin → aldosterone, and adipocytes secrete renin/angiotensinogen/aldosterone (~30% of circulating angiotensinogen). Local: adipocyte ANGII → inflammation, vascular injury, fibrosis.

▶What are the mechanical consequences of obesity?

Renal compression (→ hypertension), pharyngeal/thoracic compression (→ obstructive sleep apnea, respiratory distress), joint load (→ osteoarthritis), and ↑intra-abdominal pressure (→ GERD → adenocarcinoma).

▶What is metabolic syndrome and its diagnostic criteria?

A cluster (≥3 of): ↑BP, ↑fasting glucose, ↑waist circumference, ↑TAG, ↓HDL (or treatment for any); abdominal obesity → insulin resistance → ↑cardiovascular risk.

▶Why is visceral adipose tissue considered an ectopic depot?

It reflects subcutaneous tissue's inability to store excess fat and is associated with higher metabolic risk.

▶How does fat affect the heart directly in obesity?

Peri-/epicardial fat surrounds coronary arteries and acts against cardiac contraction/relaxation; intramyocardial fat causes cardiomyocyte apoptosis and heart failure.

▶By what molecular side-pathways does lipotoxicity impair insulin signaling?

Accumulation of ceramide and DAG (along with mitochondrial stress/ROS and lipid peroxidation) interferes with insulin-receptor signaling.

▶What is the main risk factor for metabolic syndrome?

Obesity.

▶When does fatty-acid leakage and ectopic deposition occur?

When the subcutaneous depot is full, absent, or insulin-resistant, fat accumulates in tissues not designed to store it (lipotoxicity).

▶How does NAFLD progress to hepatocellular carcinoma?

Hepatic lipotoxicity, insulin resistance, hyperinsulinemia, inflammation, glycation, and RAAS drive NAFLD/NASH → chronic inflammation → cirrhosis → hepatocellular carcinoma.

▶What is the overall endpoint of ectopic fat in liver, pancreas, and muscle?

Systemic insulin resistance → diabetes and its complications.