The effect of lifestyle on some non-communicable diseases: A causal mechanism has been proven for some risk factors, but not in others.

The idea

A risk factor is anything that raises the chance of developing a disease, but a statistical link is not proof of cause. A causal mechanism is a biological explanation of how the factor produces the disease. For smoking, the mechanism is well established: tar and other chemicals in tobacco smoke are carcinogens that mutate DNA in lung cells, so smoking causes lung cancer. For diet, saturated fat raises LDL cholesterol, which is deposited in coronary artery walls, narrowing them and causing cardiovascular disease. However, for some factors the mechanism is unproven. A correlation may exist because a third variable, such as age or poverty, influences both the factor and the disease.

What earns the marks

• Define risk factor as anything that increases the probability of developing a disease.
• Distinguish correlation from causation: a link does not prove that the factor causes the disease.
• Describe a proven mechanism, such as carcinogens in tobacco smoke mutating DNA and causing lung cancer.
• Explain that for some factors the mechanism is not proven, so a third variable may explain the correlation.
• Use specific examples such as saturated fat raising LDL cholesterol and narrowing coronary arteries.

Common mistakes and how to put them right

• Saying any correlation proves causation.

  Put it right by stating that a causal mechanism must be identified.

• Confusing risk factor with cause.

  Put it right by explaining that a risk factor raises probability but may not be the direct cause.

• Assuming all lifestyle factors have proven mechanisms.

  Put it right by giving an example where the mechanism is uncertain.

Exam tips

• Use the phrase causal mechanism and explain what it means in your answer.
• Link each risk factor to a named disease and a biological process.
• When discussing an unproven factor, mention a possible third variable such as age or poverty.