米格尔·赫南:吸烟是全球最为严重的健康问题之一
MIGUEL HERNAN: Cigarette smoking is one of the most important global health problems.
据世界卫生组织估计
The World Health Organization estimates that
每年由烟草所导致的死亡人数约为六百万
tabacco kills about six million people each year.
吸烟会导致很多种疾病
Cigarette smoking has a causal effect on many cancers
如癌症 心脏病 中风 呼吸道疾病等等
and heart disease, stroke, respiratory diseases, you name it.
但这些大家都知道 对吧?
But everybody knows this, right?
然而几十年来 香烟广告上都是健康而强壮的人在抽烟
And yet for decades cigarette ads showed tough, healthy men smoking.
但事实却是无可辩驳的
But reality is something hard to argue against.
至少四名“万宝路人”死于吸烟相关疾病
At least four Marlboro men died of smoking related diseases.
为香烟打广告的还有著名的好莱坞明星
And some well-known Hollywood actors also advertised cigarettes.
有一位死于癌症 另一位死于失智症
One died of cancer, the other of dementia,
这就引发出了一个问题:吸烟是否
which begs the question: does smoking have a causal effect
增加患失智症的风险?
on the risk of dementia.
吸烟之所以增加患失智症的风险
There are biological reasons why smoking
背后是存在生物学依据的
increase the risk of at least some forms of dementia.
但是 我们绝不会对吸烟进行随机实验
However, we will never have a randomized trial of cigarette smoking.
幸运的是 这种不道德的实验是不被允许的
Fortinately, such unethical trial is not allowed.
这就意味着我们只能依赖于观测数据
That means that we’ll have to rely on observational data only.
事实上 一些关于抽烟和失智症的前瞻性观测研究
In fact, several observational prospective studies of smoking and dementia
早在二十世纪九十年代就开始了
have been conducted since the 1990s.
但是 这些观测性研究所得出的结果却非常迥异
However, the findings of these observational studies vary dramatically.
有些研究发现吸烟者比非吸烟者更易患上失智症
Some studies found a higher risk of dementia in smokers than in nonsmokers.
其它研究的结果却刚好相反
Other studies found the opposite.
我们能够将这些关联解释为
Can we interpret any of these associations
吸烟对失智症的影响吗?
as the effect of smoking on dementia?
如果真是这样 那答案又是什么呢?
And if so, what’s the answer?
吸烟究竟是会导致还是防止失智症的患病呢?
Does smoking cause or prevent dementia?
而且最重要的是 为什么不同的研究获得的结果不同?
And most importantly, why do results differ across studies?
有可能是因为在研究中存在着不可控的混淆
One possibility is uncontrolled confounding in some of the studies,
例如 酒精
for example, by alcohol.
成为烟民的人也更倾向于喝酒
People who become cigarette smokers also tend to drink more alcohol.
如果酒精会增加患失智症的风险
If alcohol increases the risk of dementia,
我们就会发现吸烟与失智症相关
then we would expect to find an association between smoking and dementia,
即使吸烟对失智症没有影响
even if smoking had no effect on dementia.
但许多对于吸烟和失智症的观测性研究
But it looks like many observational studies of smoking
似乎已经为最重要的干扰因子做出了调整
and dementia did adjust for the most important confounders.
那么 为什么在某些研究中 烟民患失智症的风险更高
Then why did the smokers have a higher risk of dementia in some studies
而在其它研究中却又更低呢?
and a lower risk of dementia in other studies?
让我来给你个小提示
Let me give you a hint.
这种现象也许跟实验所选择的个体
Perhaps it has to do with the selection of individuals
有所关系
in each of these studies.
我们会在这堂课的最后再回到这个问题上来
We will return to this problem at the end of this lesson.
到那时 你就能够解释这是怎么回事儿了
And by then you will be able to explain what’s going on here.

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