Metabolically normal obesity (BMI ≥30 kg/m² with fewer than two metabolic syndrome criteria) is not associated with an increased risk of dementia in middle-aged adults, with a pooled hazard ratio of 1.10 (95% CI: 0.98–1.24), indicating that obesity alone without metabolic dysfunction does not elevate dementia risk.

When fat tissue becomes unhealthy due to high blood sugar, high triglycerides, or high blood pressure, it releases inflammatory chemicals into the blood. These chemicals cross into the brain, activate immune cells there, and cause ongoing damage to brain cells and connections. This damage accumulates over time and eventually leads to memory loss and dementia. People with obesity but healthy metabolism do not trigger this process, so their dementia risk stays normal.