Question: if somebody smokes, or drinks alot of alcohol, how can they live over 100 years old?

Kevin O'Dell answered on 24 Jun 2014:

Simply because we’re not born equal.

I tend to think of there being three things that will determine whether you live to 100 or not.

1: Lifestyle, which includes both the things you mention, drinking and smoking, plus lots of other things. There is absolutely no doubt that smoking and heavy drinking decreases your life expectancy.

2: Genetics: We all have 2 sets of 25000 genes and variation at those genes (different alleles) mean that you’re built more or less healthy. For example you may have a genetic make up that makes you really efficient at repairing DNA errors (which to a certain extent can protect you against cancers, including lung cancer).

3: Luck: How many times have you accidentally crossed the road without looking and luckily a bus wasn’t travelling down the road at that time?

In reality all these factors interact too. But the key is that genetic differences make us differently susceptible to bad lifestyles. Currently we know very little about which varieties of which genes give protection against excessive drinking, smoking or whatever. But the new DNA sequencing technologies that allow us to sequence all 3 billion base pairs of your DNA for £1000 in less than a couple of days, means we’ll soon be able to work this out.