Being taller is associated with a shorter life span, for reasons that are far from fully explored. The role of growth hormone in longevity is no doubt close to the roots of this correlation, but there are plenty of questions remaining, such as why lung disease plays a sizable role in greater mortality for taller people in later life. As illustrated by the research here, there is a bright side to being taller, which is that epidemiological studies show taller people to have a lesser incidence of dementia. Again, why exactly this is the case is far from fully explored. These and other natural variations between people are interesting, but we should expect them to vanish with the introduction of the first rejuvenation therapies in the near future, swamped by the benefits that might be achieved by directly addressing the causes of aging.

This study examined the relationship between body height and dementia and explored the impact of intelligence level, educational attainment, early life environment, and familial factors. A total of 666,333 men, 70,608 brothers, and 7388 twin brothers born 1939-1959 and examined at the conscript board were followed in Danish nationwide registers (1969-2016). Cox regression models were applied to analyze the association between body height and dementia. The findings of this current study provide substantial support to previous evidence of a link between body height and dementia. All previous studies had accounted for educational level and other socioeconomic indicators, yet none of these studies had adjusted for intelligence level earlier in life. Intelligence level has been suggested to be a stronger marker of brain and cognitive reserve than educational level. Intelligence level is furthermore correlated with body height and by itself associated with dementia.

In contrast to previous studies, we also investigated the impact of other potential early-life familial factors including genetics and socioeconomic resources in the family that may influence both body height and later risk of dementia. Body height has been shown to have a strong genetic component with around 80% of the variation in populations being explained by genetic differences between individuals. The genetic component of height has furthermore been found to be consistent across countries independent of living standards. The genetic and environmental variation influencing body height, but not risk of dementia, is smaller within brothers than between men in general, which may weaken the association between body height and dementia in the latter compared to the former group. Through this mechanism, the finding of a stronger association within brothers may be explained by less dilution of the effects of different harmful exposures early in life influencing both body growth and risk of dementia. These findings furthermore suggests that genetics has a minor role in the association of body height and dementia.

In conclusion, taller body height at the entry to adulthood, supposed to be a marker of early-life environment, is associated with lower risk of dementia diagnosis later in life. The association persisted when adjusted for educational level and intelligence test scores in young adulthood, suggesting that height is not just acting as an indicator of cognitive reserve.