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I have enough content from Harrison's 22E to provide a comprehensive answer on Diabetes Mellitus. Let me now synthesize all the information gathered.

Diabetes Mellitus (DM)

Source: Harrison's Principles of Internal Medicine, 22nd Edition (2025)

Definition

Diabetes mellitus is a group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both. Chronic hyperglycemia is associated with long-term damage, dysfunction, and failure of various organs - particularly the eyes, kidneys, nerves, heart, and blood vessels.

Classification

Type	Description
Type 1 DM	Immune-mediated or idiopathic destruction of pancreatic beta cells, leading to absolute insulin deficiency
Type 2 DM	Progressive loss of adequate beta-cell insulin secretion on a background of insulin resistance
Gestational DM (GDM)	Diabetes diagnosed in the second or third trimester of pregnancy (not pre-existing)
Pancreatic DM (Type 3c)	Due to pancreatic exocrine disease (pancreatitis, pancreatic cancer, cystic fibrosis)
Immune checkpoint-related DM	Rapid-onset insulin-deficient DM triggered by anti-PD-1 / anti-PD-L1 therapy
Other specific types	Monogenic DM (MODY), drug-induced, endocrinopathies

Diagnostic Criteria (ADA 2024)

Test	Diagnostic Threshold
Fasting plasma glucose (FPG)	≥ 126 mg/dL (7.0 mmol/L)
2-hour glucose on OGTT	≥ 200 mg/dL (11.1 mmol/L)
HbA1c	≥ 6.5% (48 mmol/mol)
Random glucose + symptoms	≥ 200 mg/dL (11.1 mmol/L)

Prediabetes (high risk):

IFG: FPG 100-125 mg/dL
IGT: 2-hr glucose 140-199 mg/dL
HbA1c: 5.7-6.4%

Screening: ADA recommends screening all individuals aged >35 years every 3 years, and earlier if overweight with additional risk factors.

Pathophysiology

Glucose Homeostasis

Glucose homeostasis is a balance between:

Dietary energy intake
Hepatic glucose production (gluconeogenesis + glycogenolysis)
Peripheral tissue glucose uptake (mainly skeletal muscle)

Insulin is the key anabolic regulator. In the fasting state, low insulin + increased glucagon promote hepatic glucose production. Postprandially, insulin rises and glucagon falls, driving glucose uptake into skeletal muscle.

Type 1 DM Pathogenesis

Autoimmune destruction of beta cells
Mediated by autoreactive T lymphocytes
Autoantibodies: islet cell antibodies (ICA), anti-GAD65, anti-IA-2, anti-insulin, anti-ZnT8
Results in absolute insulin deficiency
Susceptibility linked to HLA-DR3/DR4 haplotypes

Type 2 DM Pathogenesis

Insulin resistance in peripheral tissues (muscle, fat, liver)
Progressive beta-cell failure - beta cells initially compensate by hypersecretion, eventually failing
Contributed to by obesity, physical inactivity, genetic predisposition, aging
Glucagon levels are paradoxically elevated, worsening hepatic glucose overproduction

Clinical Features

Symptoms of Acute Hyperglycemia

Polyuria (osmotic diuresis)
Polydipsia
Polyphagia with weight loss
Fatigue and weakness
Blurred vision (lens osmolarity changes)
Frequent infections (vaginitis, fungal skin infections)
Slow wound healing

Chronic Complications

Microvascular:

Complication	Details
Diabetic retinopathy	Leading cause of new blindness in working-age adults
Diabetic nephropathy	Leading cause of end-stage renal disease (ESRD)
Diabetic neuropathy	Distal symmetric polyneuropathy, autonomic neuropathy

Macrovascular:

Complication	Details
Coronary artery disease	2-4x increased risk
Cerebrovascular disease	Stroke risk increased
Peripheral arterial disease	Foot ulcers, gangrene, amputation

Management

Glycemic Targets (General)

HbA1c < 7.0% for most non-pregnant adults
Targets individualized based on age, comorbidities, risk of hypoglycemia

Type 1 DM

Insulin therapy is mandatory (MDI or insulin pump)
Basal-bolus regimen: long-acting insulin (basal) + rapid-acting at meals (bolus)
Carbohydrate counting + continuous glucose monitoring (CGM)

Type 2 DM - Pharmacological Approach

Drug Class	Examples	Key Benefit
Metformin	Metformin	First-line; reduces hepatic glucose
GLP-1 receptor agonists	Semaglutide, liraglutide	Weight loss, CV protection
SGLT-2 inhibitors	Empagliflozin, dapagliflozin	CV + renal protection
DPP-4 inhibitors	Sitagliptin	Weight neutral
Sulfonylureas	Glipizide, glyburide	Inexpensive; hypoglycemia risk
Insulin	Various	Any stage; mandatory in late disease

Lifestyle Modifications

Dietary changes (reduced caloric intake, Mediterranean or low-carb diet)
Physical activity (≥150 min/week moderate-intensity)
Weight loss (even 5-10% improves glycemia significantly)
Bariatric surgery for eligible patients (BMI ≥35 with DM): 68% achieve complete remission at 5 years

Acute Complications

Complication	Type	Key Features
DKA (Diabetic ketoacidosis)	Mainly Type 1	Hyperglycemia, ketonemia, metabolic acidosis; treat with IV fluids + insulin
HHS (Hyperosmolar hyperglycemic state)	Mainly Type 2	Severe hyperglycemia (>600 mg/dL), no ketoacidosis, very high osmolality
Hypoglycemia	Both	BG <70 mg/dL; can be life-threatening

Monitoring & Follow-up

HbA1c: Every 3 months until target, then every 6 months
Self-monitoring blood glucose (SMBG) or CGM
Annual screening for complications: retinal exam, urine albumin-to-creatinine ratio, foot exam, lipid panel, blood pressure

Recent Evidence (2025-2026)

Recent high-quality meta-analyses are shaping DM care:

GLP-1 receptor agonists show significant cardiovascular benefit - a 2025 meta-analysis of 99,599 patients in the Journal of the American College of Cardiology (PMID: 40892610) confirmed favorable CV effects of GLP-1 RAs.
Cancer risk with GLP-1 agonists remains under study - a 2026 systematic review in Annals of Internal Medicine (PMID: 41359966) assessed cancer risk with GLP-1 RA and dual agonists.
Nurse-led self-management education significantly improves Type 2 DM outcomes per a 2025 meta-analysis (PMID: 40860544).

Harrison's Principles of Internal Medicine, 22E (2025), pp. 3227-3292

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