"Why does Alzheimer's disease strike women more often than it does men?" Well, actually, it doesn't. When you hear that an illness affects one group more than another, your first question should be, "Is that based on incidence or prevalence?" Although we assume the report is about incidence (how frequently an event occurs for the first time within a population over a set time period ("four new cases in 100,000 people per year"), it may be describing something very different, the number of people with the illness at a particular point in time ("70 percent of the citizens of Toronto now have..."). The incidence gives you a rate of occurrence; the prevalence combines new and old ongoing cases.
Your second question might be about whether it can be a recurrent illness, happening to the same group member frequently (two women, five times each vs. 10 different women). The incidence if two women are the new cases but the illness is severe for those two women if it strikes five times per year.
Dementia, by definition, has one onset and continues to decline over a fairly long course of illness (20 years). This means that prevalence of dementia may look very different from incidence of dementia. And when people say that Alzheimer's disease (AD) affects women more than men, the length of the illness and the long survival of women over men with the disease lead to higher prevalence of AD. Yet the incidence of AD is the same for men and women. I was recently asked to give a talk on this topic, and it gave me incentive to check the literature to see if the male:female incidence ratio has changed. It has not.
Whether from Taiwan (1,2), New York (3,4), Spain (5), or France (6), the prevalence of AD is higher in women but NOT the incidence. Only one paper, which combined data from the U.S., Europe, Japan and China in 1998, found that women had an odds ratio of 1.56 compared to men for AD incidence. One could argue that these are not bad odds for women, and that we should be more concerned if women had as much as double the risk of men to be newly diagnosed with AD, but the conclusion was never replicated, and many of us would consider the case closed. (7)
If anything, any increased AD risk for women has to do with some cultural barriers to women against achieving at least a junior high school level of education, as reported in Northern Israel and Taiwan.( 2,8) This illustrates an intersection of culture, gender and cognitive reserve factors.
Of interest, seeking further sex differences in AD beyond incidence or prevalence, reveals that the deaths of older women is more commonly attributed to AD than in men. (9) This means that the causes of death in men are reported as something other than dementia although they may have been diagnosed with AD before the time they died. Unfortunately, this error bias in reporting significant medical problems for men at the time of death may help to perpetuate the myth that more women than men get AD.
Previously, scientists had remained open to the idea that familial cases of AD were transmitted to children through something from their mothers. A small but vital component of your body's cells is the mitochondrion, and the DNA for your mitochondria (plural of mitochondrion) came from your mother and not your father. There is some evidence that mitochondria falling apart are related to the onset of AD, and there was speculation that if the mitochondrial problems led to AD (a causal relationship, as opposed to a coincidence), maybe AD patients should suspect their mothers. This Mother's Day, please let's put that myth to rest also. (I can't resist citing here that bumper sticker that says, "Insanity is inherited. You get it from your kids!") A 2012 study compared dementia family histories in 196 AD cases and 200 normal controls, expecting the cases to report AD diagnosed in more of their mothers than would the controls. But the findings were the opposite, with an odds ratio showing that controls were much more than twice likely to have moms with AD than controls. (10) Therefore, it's hard to support that mitochondrial hypothesis any longer.
The bottom line is that women do not get AD more frequently than men. More women are touched by AD at any given time because our increased prevalence and because many more of us than men become caregivers to patients with AD.
(1) Prevalence and subtypes of dementia in southern Taiwan: impact of age, sex, education, and urbanization. Lin et al. J Neurol Sci. 1998 Sep 18;160(1):67-75.
(2) Prevalence and subtypes of dementia in southern Taiwan: impact of age, sex, education, and urbanization. Lin et al. J Neurol Sci. 1998 Sep 18;160(1):67-75.
(3) Age-specific and sex-specific prevalence and incidence of mild cognitive impairment, dementia, and Alzheimer dementia in blacks and whites: a report from the Einstein Aging Study. Katz et al. Alzheimer Dis Assoc Disord. 2012 Oct-Dec;26(4):335-43.
(4) Prevalence of dementia in the United States: The Aging, Demographics and Memory Study Plassman, BL; Langa, KM; Fisher, GG; Heeringa, SG; Weir, DR;. Ofstedal, MB; et al. Neuroepidemiology 2007;29:125-132.
(5) Clinical and demographic characteristics of the cases of dementia diagnosed in the Health District of Girona throughout the period 2007-2010: data from the Girona Dementia Registry (ReDeGi)]. Calvo-Perxas et al. Rev Neurol. 2012 Apr 1;54(7):399-406.
(6) Early onset dementia: characteristics in a large cohort from academic memory clinics. Picard et al. Alzheimer Dis Assoc Disord. 2011 Jul-Sep;25(3):203-5.
(7) The Relationships Between Age, Sex, and the Incidence of Dementia and Alzheimer Disease: A Meta-analysis. Sujuan Gao, Hugh C. Hendrie, Kathleen S. Hall, Siu Hui. Arch Gen Psychiatry. 1998;55(9):809-815.
(8) High prevalence of mild cognitive impairment and Alzheimer's disease in arabic villages in northern Israel: impact of gender and education. Afgin et al. J Alzheimers Dis. 2012;29(2):431-9.
Maternal transmission of Alzheimer disease. Heggeli et al. Alzheimer Dis Assoc Disord. 2012 Oct-Dec;26(4):364-6.
(9) Burden of Alzheimer's disease-related mortality in the United States, 1999-2008. Moschetti K, Cummings PL, Sorvillo F, Kuo T. J Am Geriatr Soc. 2012 Aug;60(8):1509-14.
(10) Maternal transmission of Alzheimer disease. Heggeli et al. Alzheimer Dis Assoc Disord. 2012 Oct-Dec;26(4):364-6.