Incidence rates of Alzheimer’s disease

January 5, 2009 by Walter Schneider.

Edmonton Journal

2009 01 05; A1

Aging population means dementia could double

[Canadiann] Alzheimer society warning based on new study

The article states that,

The number of Canadians living with dementia is expected to double over the next 25 years, according to the Alzheimer Society of Canada.

From about 500,000 Canadians today, the country could see between one million and 1.3 million people with dementia in a quarter century, the society says.

That announcement is based on a yet-to-be-released report: Rising Tide: The Impact of Dementia on Canadian Society.

A major finding was that 72 percent of those with Alzheimer’s are women, while other forms of dementia tend to affect mostly men….(Full Story)

That last statement in the article is seriously misleading and needs to be set straight.

Quite naturally, it can be expected that as the years go by we will see an increase in the number of elderly, as, quite properly identified by the article, the number of the elederly will swell by those of the baby boomers who now add to the age group comprised by the elderly.

Although the article quite properly states that 72 percent of those with Alzheimer’s are women, it does not identify that to be a consequence of the vastly disproportionate distribution of the sexes in the elderly population sector.

Women live much longer lives than men do; about six more years in the US, and about seven more years in Canada.

Men are on average vastly less likely than women are to live long enough to be able to contract Alzheimer’s.  Roughly two thirds of the elderly over age 65 are women.  If the indident rates of contracting Alzheimer’s were equal for both sexes, and if the incident rates would remain unchanged with advancing age, we could expect to see that in the age group 65 and over women would comprise two thirds of all Alzheimer victims.

However, the study announced by the Alzheimer Society of Canada found that women comprise 72 percent of Alzheimer victims.  That shows that the incident rate for Alzheimer’s disease in women is not about twice as high as that for men but only about five percent higher.  However, even that is still misleading.

The risk of contracting Alzheimers increases with advancing age, so that it is quite normal to see increasing numbers of Alzheimer victims the older the age group that is being studied.  However, the older an age group is that is being studied, the more pronounced the disproportionate prevalence of women in the older age groups becomes.  For example, women comprise about 80 percent of those in the age group 85 and older.  Moreover, those women then also present much larger numbers of Alzheimer victims than the vastly lower portion of men does in that age group.

The reason why 72 percent of those with Alzheimer’s are women is simply that women live on average much longer lives than men do and not because the incidence rate of Alzheimer’s disease in women is higher than that in men.

That is born out by the latest studies examining Alzheimer’s disease.  Those studies found that, although men were nearly twice as likely to develop [non-Alzheimer-specific] mild cognitive impairment as women [1], “the age-specific incidence rates were similar in men and women, continued to increase after age 84 years, and remained stable over time for both dementia and AD [Alzheimer’s disease],”[2] and that the “consistency of findings across study designs suggests that sex or sex-related exposures do not consistently play a major role in AD [Alzheimer’s disease] causation in American populations.” [3]

To summarize: The incidence rates of Alzheimer disease in men and women do not differ.  The reason why there are almost three times as many women than men with Alzheimer’s disease is nothing other than that men are vastly more likely to die before they run the risk of contracting Alzheimer’s disease.  The higher number of women with Alzheimer’s disease is not a consequence of discrimination, but it is the price women pay for being allowed to live longer lives than men do.
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1. Mayo Clinic Finds More Seniors With Mild Cognitive Impairment Than Assumed (2008)
2. Incidence of Dementia and Alzheimer’s Disease (1998)
3. Dementia and Alzheimer disease incidence rates do not vary by sex in Rochester, Minn. (2002)

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