Hormone replacement therapy (HRT) could help prevent Alzheimer's disease in women at risk of developing the condition, a study suggests.

Researchers from East Anglia and Edinburgh universities said HRT, which helps control symptoms of the menopause, is associated with better memory, cognitive function and larger brain volume in later life in women carrying a gene called APOE4.

Around a quarter of women in the UK are thought to carry the gene, and Alzheimer's is more common in women than men. APOE4 is the strongest risk factor gene for the disease, though inheriting it does not mean someone will definitely develop the condition.

In the study, researchers found HRT was most effective when given during perimenopause, where symptoms build up months or years before periods stop.

Professor Anne-Marie Minihane, from the University of East Anglia's (UEA) Norwich Medical School, who led the study with Professor Craig Ritchie at the University of Edinburgh, said: "We know that 25% of women in the UK are carriers of the APOE4 gene and that almost two-thirds of Alzheimer's patients are women.

"In addition to living longer, the reason behind the higher female prevalence is thought to be related to the effects of menopause and the impact of the APOE4 genetic risk factor being greater in women.

"We wanted to find out whether HRT could prevent cognitive decline in at-risk APOE4 carriers."

The experts studied data from 1,178 women taking part in the European Prevention of Alzheimer's Dementia initiative, which studied participants' brain health over time.

The project, which involved 10 countries, tracked the brains of 1,906 people over 50 who did not have dementia at the start of the study.

In the latest research, experts looked at the results of cognitive tests and brain volumes recorded by MRI scans.

Their results showed APOE4 carriers who also used HRT had better cognition and higher brain volumes than those not on HRT and non-APOE4 carriers.

Professor Minihane said while the team did not look at dementia cases, cognitive performance and lower brain volumes are predictive of future dementia risk.

Dr Rasha Saleh, also from UEA's Norwich Medical School, said: "We found that HRT use is associated with better memory and larger brain volumes among at-risk APOE4 gene carriers.

"The associations were particularly evident when HRT was introduced early - during the transition to menopause, known as perimenopause.

"This is really important because there have been very limited drug options for Alzheimer's disease for 20 years and there is an urgent need for new treatments.

"The effects of HRT in this observation study, if confirmed in an intervention trial, would equate to a brain age that is several years younger."

The study was published in the Alzheimer's Research & Therapy journal.