Obesity affects more than 40 percent of US adults and 1 in 8 people globally — and WHO issued its first GLP-1 treatment guideline in December 2025, recognising pharmacological treatment as a necessary component of obesity care. This complete 2025 guide covers BMI classification with US prevalence rates, biological causes including leptin resistance and adaptive thermogenesis, all 200+ health consequences, a 6-medicine weight loss comparison table (tirzepatide 20 to 22 percent