Analysis of 36,321 UK Biobank participants reveals an inverted J-shaped relationship between blood glucose levels and brain volume, with optimal brain structure occurring at glucose levels of 4.4-4.7 mmol/L and HbA1c of 33-37 mmol/mol. Beyond these ranges, higher glucose levels correlate with reduced whole brain and grey matter volumes, while evidence for brain volume reduction at very low glucose levels remains less consistent. The relationship between glucose and brain structure shows significant sex differences, with women experiencing steeper declines in brain volume as glucose rises compared to men. White matter hyperintensity volume, a marker of brain aging and vascular damage, increases in a J-shaped pattern with rising HbA1c levels. These findings challenge the traditional view that only clinical diabetes affects brain health, suggesting that glucose-related brain changes occur across the entire glycemic spectrum. The research provides compelling evidence for an optimal glucose range for brain preservation, though being a preprint awaiting peer review, these results require validation. The sex-specific vulnerabilities identified could inform personalized approaches to glucose management for cognitive protection, particularly highlighting the need for tighter glucose control in women to preserve brain structure throughout aging.