London - The menopause triggers changes in the brain that could increase a woman's risk of getting Alzheimer's, research suggests.
The findings could help explain why women are more likely to be diagnosed with the disease than men.
Two thirds of the 850 000 people suffering from Alzheimer's in Britain are women.
Using scans, scientists found that women who were post-menopausal or approaching the menopause had much lower levels of glucose metabolism where sugars from food are converted into energy in key parts of the brain.
A similar result is seen in the brains of people in the early stages of Alzheimer's.
Women produce less oestrogen once they hit the menopause, and the US scientists believe lower levels of the hormone may cause brain cells to go into starvation mode, causing the changes that were seen. Low oestrogen levels have already been linked to other brain-related symptoms including depression, anxiety, insomnia and general memory problems.
The study's lead author Dr Lisa Mosconi, from Weill Cornell Medical College in New York, said: Our findings show that the loss of oestrogen in menopause doesn't just diminish fertility.
It also means the loss of a key neuroprotective element in the female brain and a higher vulnerability to brain ageing and Alzheimer's disease.
We urgently need to address these problems because, currently, 850?million women worldwide are entering or have entered menopause.'
The study of 43 women, published in the journal PLOS One, also found the menopause may prime' the brain for dementia by lowering activity levels for an enzyme called mitochondrial cytochrome oxidase, which plays a key role in cell energy production.
These lower levels coincided with worse performance on memory tests by menopausal women in the study. Dr Rosa Sancho, head of research at Alzheimer's Research UK, said: Going through menopause has widespread impacts on a woman's life and this study shows how it can lead to changes in the brain.
What we don't know is how many women in this study went on to develop Alzheimer's and much larger studies will be needed to determine this.'
Oestrogen receptors are found on cells throughout the brain and evidence suggests that reduced signalling after women stop producing the hormone during their monthly cycle could leave the brain more vulnerable to a range of diseases. The US authors said more research is needed on whether hormone replacement therapy (HRT) to boost oestrogen levels could help.
Dr Louise Walker, research officer at the Alzheimer's Society, said: This study suggests that the menopause could be affecting brain metabolism, but it is too small to be able to draw firm conclusions, and the participants did not have dementia.
There is also currently conflicting evidence as to whether HRT can lower dementia risk.
Anyone considering this therapy should talk options through with their doctor.'