Toronto, Dec 5 : Women who have had both ovaries surgically removed before age 50, and carry a variant of the apolipoprotein gene, are at a high risk of late-life Alzheimer's disease, a new study said on Thursday.

The research, published in the Journal of Alzheimer's Disease by a team from University of Toronto in collaboration with colleagues from the University of Alberta, found that a variant of the apolipoprotein gene, the APOE4 allele, out such women at high risk of Alzheimer's and the use of hormone therapy mitigates this risk.

By 2050, Alzheimer's disease is projected to affect 12.7 million individuals age 65 and older, with women comprising two-thirds of that number.

It is still unclear why Alzheimer's disease is more prevalent in women than in men, but it may have to do with early life events, such as ovary removal surgery (oophorectomy).

The researchers analyzed a cohort of 34,603 women from a large dataset, the UK Biobank, and found that in this cohort, women who had had both ovaries surgically removed at the approximate age of 43 showed four times the odds of developing Alzheimer's disease when compared to women who entered natural menopause at a mean age of 54.

'One of our most important findings was the fact that loss of the naturally occurring hormone (endogenous), estradiol, as a result of surgical removal of both ovaries, might interact with the APOE4 allele to further increase Alzheimer's disease risk, placing women with early bilateral oophorectomy and APOE4 in a state of double jeopardy,' said Dr Gillian Einstein from University of Toronto.

APOE4 is a known risk factor for Alzheimer's disease in the general population but presents greater risk in women.

Surprisingly, there was also a modest relationship between body mass index (BMI) and Alzheimer's disease risk, but only for the women with early ovarian removal.

Each additional unit of BMI was associated with a 7 per cent lower risk of developing Alzheimer's disease.

'Higher BMI might be associated with a decreased Alzheimer's disease risk in women with ovary removal surgery because adipose tissue produces estrone (one of the three endogenous estrogens) which, in the absence of estradiol due to oophorectomy, may help maintain cognitive function in early middle age,' said Dr Noelia Calvo from the University of Toronto.

The finding highlights the importance of estrogen-based therapies in decreasing Alzheimer's disease risk for women who have had their ovaries surgically removed before the age of 50.

/IANS