Menopause changes the brain’s supply.

Estrogen opens the brain’s vessels. It raises nitric oxide, relaxes the small vessels, and supports the brain’s glucose use and mitochondrial output.¹ Across most imaging, women carry higher resting brain blood flow than men: the oldest and most replicated figure is about 11 to 15 percent higher,² and modern ASL-MRI reports gaps as large as 40 percent in young adults.³ That biggest number is partly an artifact of women’s blood composition: a lower red-cell fraction changes how the scan reads and quantifies flow, so correcting for it shrinks the gap.⁴ A real gap remains, on the order of 15 to 20 percent by the methods least fooled by that.

Androgens act on blood vessels, but they push both ways. At normal levels they mostly open them. Much of testosterone’s protective effect in the male brain is estrogen, converted from testosterone on site by the enzyme aromatase.⁵ Chronic or very high exposure flips the other way, toward stiffer, more clot-prone arteries, and low testosterone tracks with vessel dysfunction. The pattern is U-shaped: too little and too much are both bad. The clinical example is hormone-blocking therapy for prostate cancer, tied to a 14 to 21 percent higher dementia risk, more with longer treatment.⁶ The male brain also starts from a lower baseline flow, which leaves less to spare once perfusion is threatened.

As estrogen falls through perimenopause, the brain signal is energy, not flow. Brain glucose metabolism dips in the regions Alzheimer’s later targets, stepwise with menopause stage.⁷ Blood flow is more contested: some studies show a drop, a meta-analysis found no clear change, and one careful study saw flow rise in places, perhaps to compensate.⁸ The metabolic decline is real; the perfusion direction is unsettled.

This is the biology under perimenopausal brain fog. It hits verbal memory hardest, and for many women it lifts again once the brain adjusts.⁹ Hot flushes and night sweats run on the same wiring, autonomic surges from the brain’s thermostat, and women with more night-time flushes show more small-vessel wear on brain scans.¹⁰ Estrogen loss is one leading reason roughly two in three Alzheimer’s patients are women, alongside longer female lifespan and immune and metabolic differences.¹¹

The picture is more reassuring than the 2002 headlines. The Women’s Health Initiative tested hormones started late, in women 65 and older, and combined therapy roughly doubled dementia risk.¹² It never enrolled women starting near 50. Long-term follow-up now finds a favourable benefit-to-risk balance for women under 60 treating menopausal symptoms.¹³ But the hope that early hormone therapy protects the brain is unproven: the dedicated early-start trials found no cognitive benefit.¹⁴ It treats symptoms; it does not prevent dementia.