Understanding Confounding Variables in Observational Research
The correct answer is A: stress level and diet are important factors—specifically, they represent confounding variables that may explain part or all of the observed association between exercise and heart disease reduction.
What This Study Actually Demonstrates
This study illustrates a fundamental challenge in observational research: the presence of confounding variables that create uncertainty about causal relationships 1. When researchers found that people who exercise have 30% less heart disease, they also discovered that these same individuals tend to have lower stress levels and better dietary habits—two factors independently associated with reduced cardiovascular disease risk 1.
Why Stress and Diet Matter as Confounders
The Biological Plausibility
- Stress independently affects cardiovascular outcomes through multiple mechanisms including elevated blood pressure, increased inflammatory markers, and altered metabolic function 2, 3
- Dietary quality directly influences heart disease risk through effects on lipid profiles, insulin sensitivity, inflammation, and endothelial function 1, 4
- Both stress and diet can influence cardiovascular disease independent of physical activity levels 1, 4
The Statistical Problem
The researchers disclosed these factors because they recognize that the observed 30% reduction in heart disease among exercisers may not be entirely—or even primarily—due to exercise itself 1. Instead, some or all of this benefit could result from:
- Lower stress levels in people who exercise 1, 2
- Better dietary choices among physically active individuals 1, 5
- The combined effects of all three factors working together 4
Why the Other Options Are Incorrect
Option B: "Exercise directly decreases heart disease"
This statement cannot be definitively concluded from this study design 1. While exercise does have direct beneficial effects on cardiovascular health through multiple mechanisms (improved endothelial function, reduced inflammation, favorable effects on lipid profiles), this particular study's design does not allow researchers to separate the direct effects of exercise from the confounding effects of stress and diet 1.
- The American Heart Association acknowledges that while physical activity is beneficial, observational studies showing associations must account for confounding variables to establish causality 1
- To claim direct causation, researchers would need either randomized controlled trials or sophisticated statistical methods to control for confounders 4
Option C: "This study only represents healthy people"
This statement makes an unfounded assumption not supported by the information provided 1. The study description gives no indication that participants were limited to healthy individuals—in fact, studies examining heart disease outcomes typically include people across the health spectrum 1.
The Broader Context: What We Know About Exercise and Heart Disease
Despite the limitations of this particular observational study, extensive evidence from multiple study designs does support that exercise independently reduces cardiovascular disease risk 1:
- Randomized controlled trials demonstrate that exercise training reduces cardiovascular mortality by approximately 27-31% even when controlling for other factors 1
- Meta-analyses show exercise-based cardiac rehabilitation reduces all-cause mortality and cardiac events 1
- The mechanisms are well-established: exercise improves endothelial function, reduces inflammation, favorably modifies lipid profiles, and decreases blood pressure 1
However, the most powerful cardiovascular risk reduction occurs when exercise is combined with stress management and dietary modification—exactly the pattern this observational study revealed 4. The Lifestyle Heart Trial demonstrated that comprehensive lifestyle changes (including exercise, stress management, and diet) produced regression of coronary atherosclerosis, whereas control patients showed progression 4.
Clinical Implications
When counseling patients about cardiovascular disease prevention, address all three factors simultaneously rather than focusing on exercise alone 1, 4: