What happened

The study followed more than 8,200 adults aged over 50 for 24 years. A higher BMI was associated with faster decline in memory, general cognition and executive function; the effect was strongest in people over 65.

Lead author Suhang Song said controlling weight could slow the decline noticeably within about two years.

Why it matters

Weight is one of the few modifiable risk factors for cognitive decline, and 24 years of follow-up is unusually long. But BMI is a crude measure: it does not distinguish muscle from fat, or where the fat sits.

What comes next

The practical question is whether losing weight in later life actually slows decline — that needs a trial. This study says where to look, not what to prescribe.