The benefits derived earlier in life could be extended in later life with the use of HRT, he says.

He described the study, which was published in Alzheimer’s Research & Therapy, as “one of the most exciting bits of research I’ve been involved in”.

HRT was found to be most effective when introduced early in menopause, according to lead author Professor Anne-Marie Minihane, director of the UK’s Norwich Institute of Healthy Ageing at the University of East Anglia.

Brain cells are most responsive when the oestrogen receptors in the brain are in full working order, Minihane says. And the benefits are likely to be most evident before any age-related decline in the function of the neuronal cells, which are key to the brain’s messaging system.

The current hypothesis is that HRT may improve cerebrovascular function, including blood flow to the brain, small vessel function and blood-brain barrier function, an important determinant of brain health.

Oestrogen is also involved in glucose metabolism, Minihane says, and “95 per cent of brain energy comes from glucose” as well as omega-3 fatty acid metabolism.

A 2021 study published in Alzheimer’s & Dementia, which canvassed 400,000 women in America over a period of 10 years, delivered similarly encouraging results – suggesting that those who used HRT had a significantly reduced risk of all neurodegenerative disease.

Those women who are on HRT often have to advocate for the treatment, need to know what’s available to them, are often body-aware and wellness-conscious, and are usually well educated. All of these factors in and of themselves have been linked to a lower dementia risk.

Not having enjoyed the benefits of a good education and failing to look after your physical health are listed among the dozen independent risk factors for dementia that the Lancet Commission cites.

Dr Andrea Kwakowsky at the University of Galway in Ireland studies the molecular and cellular basis of brain function in neurodegenerative disease – including Alzheimer’s. She recently led research that dug deep into the question of whether HRT posed a risk or offered protection in the case of Alzheimer’s.

The results were published in February in the International Journal of Molecular Sciences.

HRT can be prescribed as both oestrogen-only, usually for women who have had a hysterectomy, and oestrogen with progesterone, for those who still have a womb. The progesterone helps reduce cancer risks associated with a woman’s reproductive organs, Kwakowsky says.

If HRT starts after a critical window – after the age of 65 – the cells in the brain will have already been deprived of oestrogen for many years, she says. This means there has likely been a decline in neuronal connections and changes in neuronal activity – the brain’s messaging system.

It’s possible, she says, that during that “critical” period, within five years of the onset of menopause, HRT could have a restorative effect, compensate for changes, and maintain neuronal health, because the cells are still responsive to HRT.

Oestrogen also helps support the brain’s vascular structure – the arteries that provide many paths for blood to supply needed oxygen and nutrients and remove waste.

“The fine regulation of hormonal levels and cell survival and death processes is key to maintaining a healthy neuronal environment.”

There are many studies that support the notion HRT could be neuroprotective – and there are many more under way – but we need more evidence from large scale human trials, she adds.

“Future research needs to identify the required dosage, formulation and duration of treatment to gain maximum benefit from HRT and avoid possible side effects.”

Minihane stresses that the benefits of HRT for most women – including lowering the risk for dementia – far outweigh the risk, and that the risk associated with HRT use is minor and associated with long-term use in certain subgroups.

Most women on HRT are on a combination of oestrogen and progesterone.