Let us begin with a revision of some of these terms:
Intervention: Any activity that is deliberately introduced to change/ modify a condition. In health, such activities could include medicines, surgery, health education, counselling, etc. The condition could be a disease state, or a behaviour.
Primary prevention: Action taken before the onset of disease, to prevent its occurrence. Typically, primary prevention involves health promotion (increasing physical activity, eating healthy foods, etc.) and specific protection (immunization, chemoprophylaxis).
Coronary heart disease: The most common type of heart disease, usually caused by narrowing of blood vessels (arteries) that supply blood (oxygen and nutrients) to the heart. It may result in a ‘heart attack’.
What was the (research) question the reviewers were trying to answer?: Coronary heart disease (CHD) is largely the result of ‘lifestyle’ problems. Risk factors include high cholesterol, excess salt intake, high blood pressure, smoking, excess weight, diabetes, sedentary lifestyle, high fat diet. Experts have long promoted ‘lifestyle modification’ to prevent the occurrence of this problem. Do these interventions actually prevent coronary heart disease?
What did the reviewers do?: They searched for randomized controlled trials that assessed intervention(s) aimed at reducing more than one risk factor for coronary heart disease (multiple risk factor intervention). Then they sorted the trials by quality and some other criteria. Finally, they pooled the results of all selected trials to see if such interventions really work.
What did they find?: Trials attempting to reduce more than one risk factor (at the same time) produced small changes in blood pressure, cholesterol and smoking. However, they had little or no impact on the number of people suffering from, or dying of CHD.
How does one explain these findings?: We need to understand a few points in this context-
1. The trials that were included in this review attempted to reduce multiple risk factors. This does not imply that such interventions are useless- all one can say is that attempting to reduce multiple risk factors simultaneously does not seem to work.
2. Trials require funding. While it is desirable to follow up subjects for many years, it increases the cost of the trial tremendously. At least 50% of the trials (out of 55 trials) in the review lasted 12 months. However, the longest trial lasted for 12 years. It may take decades for CHD to develop. Subjects may have had changes in the short term, but these may not have persisted long enough to impact the risk of developing or dying from CHD.
3. The small changes reported could have resulted from bias. That is, there might have been an error.
4. The review does not extend to single intervention trials.
5. Other study types (designs) have not been considered in this review. The evidence from those studies may be different from that obtained in this review.
Bottomline: We need to have more evidence (trials) to settle the matter. As of now, multiple intervention trials do not seem to be effective in preventing Coronary Heart Disease.