We analyzed the available evidence and found that 37 studies support the idea that reduced glucose metabolism in the posterior cingulate cortex and precuneus appears early in people who are beginning to forget things, before they develop full Alzheimer’s dementia. No studies in our review contradicted this. These two brain areas are involved in memory and self-awareness, and they use less energy—measured by glucose uptake—when Alzheimer’s is starting to take hold. This change shows up in brain scans before memory problems become severe enough for a dementia diagnosis. What we’ve found so far leans toward this being one of the earliest detectable signs in the brain’s energy use, even when someone still seems mostly normal in daily life. The evidence doesn’t say this change causes Alzheimer’s, but it does suggest it’s linked to the earliest stages of the process. We don’t yet know if this pattern happens in everyone who later develops dementia, or if it’s seen in other conditions too. But based on what we’ve reviewed so far, this drop in brain energy use in those two regions is consistently tied to the very early memory changes that come before dementia. If you or someone you know is noticing mild forgetfulness, this research doesn’t mean a diagnosis is coming—but it does show that the brain may be changing in ways we can now measure, even before symptoms get worse.