Longevity: The Genetic Component
A study of Danish twins gives us some insights
Long lifespans tend to run in families, and, unfortunately, so do short ones. If your parents lived a long time before passing on, chances are you will too. But why? Have you inherited a predisposition towards a longer life from their DNA, or have you just taken on the various diet and lifestyle factors that we know to prolong life? Let's take a look at the nature vs. nurture debate again.
Is our lifespan dictated by nature or nurture?
In short, the answer is both. Genetics play a relatively small component, however; lifestyle and accompanying dietary choices still play the major part in determining how long you're going to live.
How do we know? To examine the genetic component to a group of people's lifespans, we have to be able to remove the genetic factor. The best way to do this is to look at siblings, or, even better, genetically-identical twins. Assuming that they had similar lifestyles, we can reasonably expect that siblings' and twins' lifespans would be similar. Take a look at Kin Narita and Gin Kanie, who died at 107 and 108, respectively, or Sadie and Bessie Delany, who were not identical twins, but still sisters with the same parents, and who lived to the ripe old ages of 109 and 106. But these pairs of sisters also had remarkably similar lifestyles, having even spent a majority of their adult lives together. How much of their remarkable longevity was due to having the same parents, and how much to healthy diets and lifestyles? After all, the Delany sisters themselves attributed their extraordinarily long lives to daily yoga and prayer, cod liver oil, and garlic; neither smoked or drank either.
The study of genetically-identical twins
The most definitive answer to date to this puzzle has come from a study of Danish twins, 2872 pairs born from 1870 to 1900. Some of the twins were identical, having the same genetic foundation, while others were fraternal, having the same parents but a degree of genetic variability common to all non-identical siblings. Using statistical analysis, the researchers were able to derive lifespan heritability (the degree to which a certain factor is genetically inherited) estimates for both men and women: 26% for men, and 23% for women. The remaining approximate three-quarters of our longevity is not heritable; i.e. it is a function of our environment, our lifestyles, and choices we make.
What about that last 25%?
Is it safe to say that the quarter of the factors that determine our longevity is completely out of our hands? Not necessarily. Genetic predispositions towards cancer or heart disease, for example, can be anticipated and dealt with properly. After all, an inherited proclivity towards high cholesterol isn't quite the same death sentence that it was even fifty years ago, as evidenced by falling heart disease and stroke rates (much of the improvement is due to improvements in health care and lifestyle changes). As medical technology and our understanding of the way our bodies work both grow, the extent to which our longevity is just "in the cards" will play a diminishing role in the years and decades to come.
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