A recent study funded by the National Institute on Aging (NIA) reveals that among twin pairs, if one sibling is diagnosed with dementia, the other twin may be at a heightened risk of a shorter life expectancy, even if they do not have the disease themselves. The research, published in the journal Alzheimer’s & Dementia, indicates that shared genetic and environmental factors within a family may play a role in this increased mortality risk.
While earlier studies have noted a decreased lifespan in individuals with dementia, the intricate web of genetic and environmental factors that influence both dementia and mortality remains elusive. Twins present a unique opportunity to delve into these factors, as identical twins share all their DNA, and fraternal twins (and full siblings) share about half on average. Twins also often experience similar environmental conditions, including education, physical activity, diet, and exposure to pollution, as they are typically raised in the same home.
Conducted by researchers at the University of Southern California and Sweden’s Karolinska Institute, the study utilized data from the Swedish Twin Registry, which encompasses over 45,000 Swedish twins. The researchers focused on twin pairs where only one sibling was affected by dementia, consisting of 90 pairs of identical twins and 288 pairs of fraternal twins.
In their research, the scientists employed a dual-method analysis: the first compared the life expectancy of twins diagnosed with dementia against unrelated individuals without the condition, while the second focused on the comparative longevity between twins with dementia and their unaffected co-twins. Consistent with past studies, they noted a reduced lifespan in the dementia-affected individuals. Yet, when examining twin pairs with only one diagnosed sibling, the non-dementia twins were found to possess a higher mortality risk. Intriguingly, in identical twin pairs, both siblings showed a comparable duration of survival. For these pairs, a dementia diagnosis in just one twin might be attributed to unique environmental factors such as differing lifestyles, educational paths, or careers. Conversely, among fraternal twins, the non-dementia sibling experienced a marginally reduced life expectancy in contrast to those without any dementia-affected twin sibling.
Further assessment revealed that the non-dementia twins who had a sibling with the condition faced an increased mortality risk compared to twins from pairs without any dementia diagnosis, even after factoring in variables like age, educational level, and self-perceived health. This underscored potential genetic determinants affecting lifespan.
The findings of this investigation suggest that a dementia diagnosis correlates with a lower life expectancy for both the affected twins and their non-diagnosed siblings, influenced by shared genetic and environmental underpinnings. While the study’s concentration on Swedish twins provides a robust and nationally reflective dataset, it may not fully extend its applicability to other demographics. Additionally, the research was limited in its capacity to delve into specific types of dementia owing to the confines of the sample size. Despite these limitations, the insights garnered from this study enhance our comprehension of the nexus between dementia and mortality, which is pertinent for the strategic planning of care and informed decision-making by individuals, families, healthcare practitioners, and policy developers.