HIROSAKI, July 2 -- An older adult running low on vitamin C may be losing something less visible than energy or immune resistance: the physical substance of the brain itself, and the coherence of the network that holds memory and identity together.

A study published this month in PLOS One by researchers at Hirosaki University found that among 2,044 Japanese adults over the age of 64, those with lower plasma vitamin C levels had measurably reduced gray matter volume and weaker connectivity within the default mode network, a circuit the brain depends on for autobiographical memory and self-directed thought.

The research team drew blood plasma samples and conducted MRI brain scans on all participants, then statistically controlled for conf...