Making sense of the non-deterministic nature of disease risk factors

Smoking may kill you, but then it may not.

People from mainland China will tell you that their former paramount leaders Mao Zedong and Deng Xiaoping lived to the ripe old age of 83 and 93 respectively. Not bad for a couple of chain smokers.

There is no shortage of anecdotal evidence to support the opening statement. Yet medical science has clearly warned us of the danger of smoking for various cancers and cardiometabolic diseases. So what gives?

Risk factors

Smoking is just 1 risk factor for heart diseases and diabetes. Other risk factors include stress, hypertension, alcohol consumption, obesity, insufficient fruit and vegetables intake, etc .

Note that many of the aforementioned risk factors are by-products of lifestyle choices, therefore modifiable. Having a correct understanding of disease causes will motivate one to make lifestyle decisions conducive to reducing the chance of developing certain diseases.

Dr. Ken Rothman, an American medical researcher. introduced the causal pie model to explain disease causality—the relationship between the cause and effect of a disease. While the model has been criticized by other scientists as being too simplistic, it is a very useful theory to help us understand causality and spur us on to meaningful actions in disease prevention.

Causal pie model

According to Dr. Rothman, a disease, say heart disease, is analogous to a fruit pie. Different fruit pies can consist of different fruit ingredients. One fruit pie may have strawberries, blueberries, and blackberries; another, strawberries, kiwifruit, and raspberries. For a fruit pie to be one, all its fruit components must be present.

The fruits of a fruit pie correspond to a disease’s risk factors, which Dr. Rothman calls component causes. For instance, suppose smoking, obesity, and insufficient fruit and vegetables consumption are the component causes for heart diseases for an individual. For the heart disease to develop for this person, all its component causes must be present. In other words, even if the individual is obese and consumes no fruit and vegetables, unless they smoke, the heart disease will not develop.

Under the causal pie model, the fruit pies are unique to each individual. Mine may be a strawberry-blueberry-blackberry pie while yours may be strawberry-kiwifruit-raspberry. So, smoking may kill a person (given all the other component causes are present), yet spare another.

Lifestyle decisions

We regularly make lifestyle decisions to improve our qualify of life and our general well-being. If we are smokers, should we quit smoking? If we are non-drinkers, can we take up social drinking without negatively affecting our health? Etc, etc.

For Chairman Mao, smoking was most likely not a component cause for him, or perhaps he was fortunate enough because he was not obese and consumed lots of fruit and vegetables. However, we simply don’t know if smoking is a component cause for us. As of now, science cannot accurately determine, prior to disease onset, the component causes applicable to an individual.

In general, a healthier lifestyle reduces the likelihood of completing the fruit pie. With a more solid understanding of disease causality, we can make better informed lifestyle decisions.