In a cohort study comparing a poor diet (exposed) to a healthy diet (unexposed), what does relative risk measure?

Understanding Relative Risk in Cohort Studies

In a cohort study comparing poor diet (exposed) to healthy diet (unexposed), relative risk measures the ratio of the probability of developing heart disease in the exposed group compared to the unexposed group—essentially comparing the risk between the two groups rather than measuring risk in isolation.

What Relative Risk Actually Measures

Relative risk (RR) is fundamentally a comparative measure that quantifies how much more (or less) likely the exposed group is to develop the outcome compared to the unexposed group 1.

Key Characteristics:

• RR is a ratio of two probabilities: It divides the risk (probability) of disease in the exposed group by the risk in the unexposed group 1

• Formula: RR = [Risk in exposed group] / [Risk in unexposed group] 1

• Interpretation: An RR of 1.25 means the exposed group has 25% higher risk than the unexposed group; an RR of 0.76 means 24% lower risk 1, 2

Why the Answer is "Comparison"

The question presents options about what RR measures. Let me clarify why each option is correct or incorrect:

• Option A (Odds of heart disease in exposed group): Incorrect—this describes odds, not relative risk, and focuses only on one group 3

• Option B (Risk of heart disease in exposed group): Incorrect—this is just the numerator of RR, not the complete measure 3

• Option C (Probability based on diet): Partially correct but incomplete—RR specifically compares probabilities between groups, not just a single probability 1

• Option D (Comparing something): Most accurate—RR is inherently a comparative measure between exposed and unexposed groups 1, 3

Practical Application in Diet-CHD Studies

In the dietary pattern literature, RR is consistently used to compare disease risk between dietary exposures 1, 2, 4:

• Meta-analyses report RR comparing highest versus lowest dietary pattern adherence 1
• For example, prudent diet patterns show RR = 0.76-0.80 for CHD, meaning 20-24% lower risk compared to those with poor dietary patterns 1, 2, 4
• Western dietary patterns show RR = 1.05-1.46, indicating increased risk compared to healthier patterns 1, 4

Important Caveats

RR has significant limitations that researchers must understand 5:

• RR shifts toward the null value as outcome prevalence increases, regardless of true effect strength 5
• RR is not purely a measure of effect magnitude—it's influenced by baseline prevalence 5
• The comparative nature means RR cannot be interpreted for a single group in isolation 3