The number of people with dementia clearly seems to be increasing. Rarely these days do you hear of families not having a loved one affected by dementia, and public awareness, especially for Alzheimer’s, is rising. But while there are more people affected by dementia, our individual risk is actually going down. A lot. How can that be?
Your individual risk of dementia has been declining by almost 30% per decade!
A study was quietly published in 2011 by Walter Rocca and colleagues in the journal Alzheimer’s & Dementia. In it they described their observation of three large groups of participants 65 years or older enrolled at different time points – one group first enrolled in 1975, another in 1985, and the last one in 1990. They then compared how many individuals enrolling in 1975 developed dementia several years later compared to those enrolling in 1985, all being at the same age an enrolment. The conclusion was dramatic, although it received relatively little press at the time: If you were 80 years old in 1985, you had a whopping 30% lower risk of getting dementia compared to being 80 in 1975! If a drug showed a 30% reduction in dementia risk, it would be headlined in every newspaper all over the world. This one went by fairly quietly.
But the data kept coming. Multiple investigations of large cohorts of people who were followed over long periods of time have shown the same surprising result: in 2023 we clearly have a substantially reduced risk of dementia than our parents did, and thus far every decade seems to afford further risk reduction. While the type of dementia where the risk has been reduced the most is not fully clear, one study compared brain amyloid plaque over the past several decades (instead of only a clinical diagnosis of dementia). In line with a reduced dementia risk, the study indicated that if the same person lived simultaneously in 1972 and 2006, there would be substantially less brain amyloid in 2006 than in 1972. Pretty amazing.
But the number of persons with dementia keeps rising. Why?
This piece of great news contrasts with concerning statistical reports showing a significant increase in the number of individuals with dementia worldwide. According to WHO, 55 million people have dementia worldwide today, with nearly 10 million new cases every year, and much higher than 20 years ago. So why the discrepancy? How can the number of people getting dementia be increasing while the individual risk is decreasing?
It is all about the number of people at risk. Age is by far the biggest risk factor for dementia. If you are 20 you don’t get Alzheimer’s, but in your 60s or 70s you might, and our life expectancy continues to increase, with a greater number of people living well past 70. So imagine you live in a village where there are 100 individuals who have reached the age of 75. If the risk of dementia is, say, 30%, then 30 will have dementia. Fast forward 20 years. People are living longer and now 200 individuals are 75 years or older. The individual risk has been dramatically reduced to 20%, but even with this great news, 40 individuals will have dementia, which is higher than when the 75+ population was only 100. So the more individuals reaching an older age, the higher the total number of people with dementia, even if the overall risk is lower.
Why is the risk of dementia decreasing?
The most interesting part of the finding of reduced dementia risk in recent decades is understanding why. Answering this question could unlock a pathway to a therapy or a universal prevention strategy. But finding the underlying mechanism driving the reduced dementia risk has not been easy. Better cardiovascular health, better education, better quality of life, and healthier lifestyle continue to be explored, and while all likely contribute to some degree to a reduction in dementia risk, it is not yet fully clear what is driving this amazing trend.
Lifestyle and modifiable risk factors, such as preventing hearing loss and increasing social engagement, have emerged as a top areas for dementia prevention, in part because of the studies described here. It is thought that up to 40% of all cases of dementia might be prevented through lifestyle interventions, although this remains a hypothesis that needs to be tested through research. But for now, the good news is that something is headed in the right direction even though we don’t fully understand why.