The study was carried out by researchers at Weill Cornell Medicine - the biomedical research unit and medical school of Cornell University - in New York City, NY, in collaboration with scientists at the University of Arizona Health Sciences in Tucson.
Dr. Lisa Mosconi, from Weill Cornell Medicine, is the lead author of the study, and the findings were published in the journal PLoS One.
As Dr. Mosconi and her colleagues explain, after old age, being female is the second most major risk factor for developing Alzheimer's disease.
In fact, two thirds of those with Alzheimer's disease in the United States are women, according to 2017 estimates.
Although the mechanisms responsible for this increased risk are not yet known, previous research has hinted at the transition to menopause as a potential key.
As the authors explain, this transition also involves neurological and metabolical changes. So, Dr. Mosconi and team decided to investigate these transformations.
Low glucose brain metabolism found
The team used positron emission tomography imaging technology to examine how the brains of 43 women metabolize glucose, which is the main source of energy for brain cells.
In fact, previous research that Medical News Today reported on has shown that low levels of the sugar precede and may even trigger the development of Alzheimer's disease.
In the current study, the women participating were aged between 40 and 60. Of them, 15 were premenopausal, 14 were perimenopausal - or transitioning to menopause - and 14 were postmenopausal.
The study found that the perimenopausal and postmenopausal women had significantly lower glucose metabolism levels than those who were premenopausal.
In addition to this so-called hypometabolism, the researchers found signs of mitochondrial dysfunction, which means that the brain cells were not as efficient at processing energy.
A key metabolic enzyme called "mitochondrial cytochrome oxidase" was found to be less abundant among menopausal and perimenopausal women. These women also scored significantly lower in memory tests.
The findings support previous research by the same team, which showed how menopause is linked with an increase in the beta-amyloid protein, a biomarker of Alzheimer's disease, in the brain.
The same study found a reduction in gray matter and white matter in brain areas affected by the neurodegenerative condition.
Loss of estrogen may impact brain cells
Cognitive decline is known to be associated with menopause, and the authors suggest that the estrogen deficiency that characterizes menopause may also be responsible for the neurodegeneration that occurs in Alzheimer's.
Brain cells have estrogen receptors, they explain, and a drop in estrogen levels may cause a "starvation reaction" in these cells. Such a metabolic state can lead to brain cell dysfunction.
"Our findings show that the loss of estrogen 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 aging and Alzheimer's disease."
Dr. Lisa Mosconi
"We urgently need to address these problems because, currently, 850 million women worldwide are entering or have entered menopause," she warns.
"Our studies demonstrate," Dr. Mosconi says, "that women need medical attention in their 40s, well in advance of any endocrine or neurological symptoms."
There may be a "critical window of opportunity when women are in their 40s and 50s, to detect metabolic signs of higher Alzheimer's risk and apply strategies to reduce that risk."
She also offers examples of such potential strategies, saying that "women may need antioxidants to protect their brain activity and mitochondria in combination with strategies to maintain estrogen levels."
Antioxidant-rich foods, such as flaxseeds, and exercise may help to maintain healthy estrogen levels, she suggests.
Regarding hormone replacement therapies, Dr. Mosconi explains, "We believe that more research is needed to test [their] efficacy and safety [...] at the very early stages of menopause, and to correlate hormonal changes with [the] risk of Alzheimer's."