The Nun Study is a longitudinal study of aging and Alzheimer’s disease. It followed 678 sisters of the School Sisters of Notre Dame living in Mankato, Minnesota. The goal of the study is to identify the risk factors for Alzheimer’s and other brain diseases. Some of the results of the study confirm the commonsense notions that healthy lifestyle and active and stimulating intellectual life can help us stay healthy and independent later in life. But the most surprising finding is that certain traits in early 20s can help predict whether someone will have Alzheimer’s disease 60 years later.
The participants in this study are American Roman Catholic sisters who are members of an order called the School Sisters of Notre Dame. When the study began in 1986, the 678 nuns were 75 to 102 years old and the average age was 83 years. Over 85% of the sisters were teachers. In 1990, the study was expanded to include elderly sisters of the same order living in other parts of the United States. This study was under the direction of Dr. David Snowdon (affiliated with the University of Minnesota and later with the University of Kentucky). The study is now under the direction of Kelvin O. Lim, M.D. at the University of Minnesota.
The advantage of studying a religious group such as the sisters from the School Sisters of Notre Dame is that the subjects have homogeneous lifestyles and background. The participants had the same reproductive history and marital status; were non-smokers and did not drink excessive amounts of alcoholic beverages; had similar occupations, income, and socioeconomic status; had similar social activities and support; and had access to similar preventive, nursing, and other medical care services. This means many factors that confound (or confuse) most epidemiologic studies were eliminated or minimized in this study.
What are some of the findings? A college education and an active intellectual life may help stave off Alzheimer’s disease and other forms of dementia. The Nun Study also suggested that strokes and brain trauma might worsen the symptoms of Alzheimer’s disease. The researchers found that among the nuns with physical evidence of Alzheimer’s in the brain, it was more likely that those who had a history of stroke also exhibited the more severe and debilitating symptoms of Alzheimer’s disease.
Another finding is about the potential role folate (folic acid) may play in maintaining the integrity of the brain in late life. The study found that low levels of serum folate are strongly associated with atrophy of the neocortext. Thus folic acid seems to protect the brain’s central language and reasoning regions from shrinkage (folic acid is found in breads, cereals and leafy green vegetables). However, the association did not extend to other nutrients such as antioxidants (vitamin C or E).
The researchers examined the participants through medical exams, blood samples for genetic and nutritional studies, annual assessment of the cognitive and physical function of the participants and post mortem examination of the brain. These investigations produced the core data in the study. The researchers also have full access to the personal and medical records of the participants.
The most surprising result of the Nun Study came not from the core data of the study but instead came from the convent’s archives. Sophisticated language skills turn out to be a powerful predictor of Alzheimer’s disease. The director of the study, Dr. Snowdon, stumbled upon the autobiographical essays written by the sisters when they entered into the order. To the astonishment of Dr. Snowdon and his fellow researchers, the participants who showed signs of Alzheimer’s had consistently authored essays that were lacking in language complexity. To measure language complexity, the notions of idea density (the number of discrete ideas per 10 written words) and grammatical complexity were used.
Most of the participating nuns entered the convent in their early 20s. So the correlation was striking. The more sophisticated the language skills among these nuns in their 20s, the less likely they were to suffer from Alzheimer’s disease several decades later.
Another interesting discovery is that having a extra cushion of mental capacity may help put off the symptoms of Alzheimer’s disease even for those people with Alzheimer like brain damage in the brain. Post mortem brain examination showed that some participating nuns showed brain damage (brain shrinkage and presence of plaques and tangles) typical of Alzheimer’s patients and yet showed no significant mental deterioration prior to death. According to one leading theory, the people who have extra mental capacity (more connections among nerve cells) may have a larger reserve of mental capacity from which to draw upon when the neural network begins to fade.
One disadvantage of this study is that it may be hard to generalize the findings of this study since the participants are from a unique population. Another limitation is that this study is an observational study and not a randomized controlled experiment.
An observational study is a study in which the researchers observe individuals and measure variables of interest but do not attempt to influence the response variable. In an experiment, the researchers deliberately impose some treatment on individuals and then observe the response variable. In the Nun Study, obviously no treatments were applied to its participants. The researchers only followed the subjects and looked back by examining the personal records and medical history of the subjects.
The study indicates that sophisticated language skills in early life may play a role in fending off Alzheimer’s disease later in life. But the findings from the Nun Study do not provide evidence of cause and effect. It is not clear whether strong language skills actually play a role in warding off Alzheimer’s disease or whether some other variables or processes (ones that the researchers still do not know about) are at work. We do not know for certain that the protection from Alzheimer’s disease (the response variable) is due to the strong language skills (the explanatory variable) or some other variables. This is the notion of confounding (when the effect of two variables on the response cannot be distinguished from one another). The other variables that may be confounded with the explanatory variables are called lurking variables.
In the Nun study, it is possible that the effect of highly developed language skills in early life on the status of Alzheimer’s disease in later life is confounded with (mixed up with) some lurking variables (e.g. other characteristics of the participating nuns in the study). For this reason, we cannot conclude that there is a causal link between highly developed language skills and an old age free of Alzheimer’s disease.
In the Nun Study, the correlation between language skills and status of Alzheimer’s disease is real. But the study does not provide evidence for causation. Because of the possibility of the influence of lurking variables, even when an observational study demonstrates a very strong correlation (association) between two variables, we cannot conclude that there is a cause-and-effect link between the variables.
When the goal is to establish cause and effect, experiments are the only way to produce convincing data. In an experiment, the explanatory variable is the only variable that can vary while the other influences on the response are controlled. The basic principles of statistical design of experiments are control, randomization and repetition. The element of control is to prevent the confounding of the explanatory variable (the treatment) with other variables (lurking variables). The elements of randomization and repetition prevent bias (systematic favoritism) in experiments. For more information about these ideas, see .
Of course, in the case of the Nun Study, it is not feasible to perform experiments. The strongest evidence can only be obtained from doing a long term observational study.
Even though the findings in the Nun Study are not definitive, they provide a great deal of insight about aging and Alzheimer’s disease and other forms of brain diseases. The findings of the study, though not cause and effect relationships, are all common sense ideas that can help keep us healthy and active later in life.
If at all possible, prevent head injury (e.g. wearing helmet when riding bicycle, wearing seatbelt in a car). Follow all common sense approaches in promoting cardiovascular health (quitting smoking, exercising regularly, keeping the blood pressure down), thus reducing the risk of a stroke. Most importantly, stay mentally active and become intellectually curious. This is especially important for young adults. Though it is not an absolute protector, education may help build up more connections among the brain’s nerve cells, thus providing a protective cushion of mental capacity beneficial later in life.
- Moore. D. S., McCabe G. P., Craig B. A., Introduction to the Practice of Statistics, 6th ed., W. H. Freeman and Company, New York, 2009