Lifestyle Factors and Chronic Disease Mechanisms

Risk is not certainty

A non-communicable disease cannot be passed from person to person. Its incidence is the number of new cases in a population over a stated time. A risk factor is linked to an increased rate of disease: it changes the likelihood of becoming ill, but does not determine what will happen to any one person. Risk factors include lifestyle choices, such as diet, smoking and alcohol consumption, as well as substances in the body or environment.

If people exposed to a factor develop a disease more often, the two are correlated. That alone does not prove that the factor caused the disease. A causal mechanism explains how it could do so, and evidence for such a mechanism is stronger for some risk factors than for others. Diseases often involve several interacting factors rather than a single cause.

Diet, smoking and exercise: the heart and blood vessels

Cardiovascular disease affects the heart or blood vessels. The coronary arteries supply the heart muscle with blood and oxygen. A diet high in saturated fat can raise blood cholesterol; fatty material may then build up in artery walls. In coronary heart disease, this narrows the coronary arteries, reducing blood flow and the oxygen reaching heart muscle. Smoking can damage artery linings, adding to cardiovascular risk. Regular exercise can lower blood pressure, reducing strain on the heart. Diet, smoking and exercise therefore matter together: changing one factor may reduce risk, but it does not erase the effects of the others.

Obesity and Type 2 diabetes

Obesity is a risk factor for Type 2 diabetes. In this disease, body cells become less responsive to insulin. Insulin normally helps cells take glucose from the blood, so reduced responsiveness makes blood glucose harder to control. Excess body fat can affect this process. Obesity raises the likelihood of Type 2 diabetes; it does not mean that everyone with obesity will develop it.

Alcohol, the liver and the brain

The liver processes alcohol. Heavy alcohol consumption can damage liver cells and cause a build-up of fat in the liver; continuing damage may lead to liver disease and eventually liver failure. Alcohol also affects nerve cells in the brain and can impair brain function. These are different outcomes of the same lifestyle risk factor, not one single disease.

Smoking, lung disease and cancer

Cigarette smoke damages cells in the airways and the alveoli, where oxygen passes into the blood. This increases the risk of lung disease. Smoke also contains carcinogenic chemicals that can damage cells and increase the risk of lung cancer. More generally, carcinogens are cancer risk factors: ionising radiation is another example. Damage to DNA can lead to uncontrolled cell division. Smoking and cancer are linked, but a smoker is not certain to develop cancer, and smoking is not the only cause.

Across a local community, a nation or the world, differences in exposure to diet, alcohol and smoking can affect how often these diseases occur. Interpreting those patterns requires the same distinction throughout: a higher disease incidence alongside a risk factor is an important clue, while a supported causal mechanism helps explain the link.