Cardiorespiratory fitness, polygenic risk, and breast cancer in postmenopausal women: a prospective cohort study

In a prospective cohort study of postmenopausal women, higher cardiorespiratory fitness was associated with a reduced risk of breast cancer and was found to attenuate the excess risk linked to high polygenic genetic susceptibility, particularly among older women and those with a higher body mass index.

The Gist

The Big Picture: Genetics vs. Lifestyle

Imagine your body is a car.

• Genetics (The Polygenic Risk Score) are like the factory settings of the car. Some cars come from the factory with a slightly higher risk of rusting or breaking down (a higher genetic risk for breast cancer).
• Cardiorespiratory Fitness (CRF) is like how well you drive and maintain the car. Do you drive gently? Do you change the oil? Do you keep the engine tuned up? This study asks: If you have a "risky" car (high genetic risk), does taking excellent care of it (high fitness) stop it from breaking down?

What They Did

The researchers looked at data from over 13,900 postmenopausal women (women who have gone through menopause) in the UK.

1. They measured their Genetic Risk (the factory settings).
2. They measured their Fitness by having them ride a stationary bike and seeing how efficiently their hearts and lungs worked.
3. They followed them for about 11 years to see who developed breast cancer.

The Main Findings

1. Fitness is a Shield

Just like a good maintenance schedule keeps a car running longer, higher fitness was linked to a lower risk of breast cancer for everyone. The fitter the women were, the less likely they were to get the disease.

2. The "Genetic Risk" Factor

Some women had a "high-risk" genetic blueprint. Naturally, these women had a higher chance of developing breast cancer compared to women with "low-risk" genes.

3. The Magic Interaction: Can Fitness Fix a "Bad" Blueprint?

This is the most exciting part of the study. They wanted to know: Does being fit cancel out the bad luck of having high-risk genes?

• The Result: Yes, but with a catch.
• The Catch: The "cancellation" effect mostly happened in older women (60+) and women who were overweight.
• The Analogy: Think of the high-risk genes as a leaky roof.
  • For younger women or those with a very light frame, the roof might be leaking, but the house is so sturdy that the leak doesn't cause a flood yet.
  • For older women or those with more body fat, the "house" is under more stress (due to aging or inflammation). The leaky roof (genes) is more likely to cause a flood (cancer).
  • High Fitness acts like a super-strong tarp or a repair crew. When the house is under stress (older age or higher weight), putting up that tarp (getting fit) stops the flood completely. It neutralizes the danger of the leaky roof.

Who Benefited Most?

• Women aged 60 and older with high genetic risk: If they were fit, their risk dropped so much that it was almost the same as someone with low genetic risk. The fitness "tarp" worked perfectly.
• Women with overweight or obesity: Similar to the older group, fitness helped neutralize the genetic risk here.
• Women aged 40–59 or those with a healthy weight: The study didn't see a strong "cancellation" effect here. This doesn't mean fitness wasn't good for them (it still lowered risk generally), but it didn't seem to completely override the genetic risk in the same dramatic way it did for the older/overweight groups.

Why Does This Happen? (The "Why" Behind the Magic)

As we age, our bodies get a bit "rusty" (inflammation, hormonal changes). If you have a genetic tendency for cancer, this "rust" makes the problem worse.

• Fitness is like a rust remover and a shield. It reduces inflammation and helps the body's immune system spot and destroy bad cells before they become tumors.
• For women who are already under more biological stress (due to age or weight), this "rust removal" is the difference between getting sick and staying healthy.

The Bottom Line

If you are a postmenopausal woman, especially if you are over 60 or carry extra weight, getting fit is a powerful tool.

Even if your family history or DNA suggests you are at high risk for breast cancer, you are not doomed. Think of your genes as the weather forecast (it might look stormy), but your fitness is the umbrella. If you have a strong umbrella (high fitness), you can stay dry even when the storm is coming.

Key Takeaway: You can't change your genes, but you can change your engine. Keeping your heart and lungs strong is one of the best ways to protect yourself, especially if you are at higher genetic risk.

Technical Summary

1. Problem Statement

While it is well-established that higher cardiorespiratory fitness (CRF) reduces the risk of chronic diseases and that polygenic risk scores (PRS) can stratify genetic susceptibility to breast cancer, the joint association between these two factors remains unclear. Specifically, it is unknown whether modifiable physiological factors like CRF can attenuate the excess breast cancer risk associated with high genetic susceptibility in postmenopausal women. Understanding this interaction is critical for developing personalized prevention strategies that combine genetic risk assessment with lifestyle interventions.

2. Methodology

• Study Design: Prospective cohort study.
• Data Source: UK Biobank.
• Study Population:
  • Inclusion: 13,907 postmenopausal women of European ancestry.
  • Exclusion: Men, premenopausal women, history of cancer at baseline, non-European ancestry, missing CRF or PRS data, and extreme VO₂max values.
  • Follow-up: Median of 10.7 years (up to December 31, 2020).
• Exposures:
  • Cardiorespiratory Fitness (CRF): Estimated maximal oxygen uptake (VO₂max in mL·kg⁻¹·min⁻¹) derived from a validated submaximal cycle ergometer test. Participants were categorized into age-specific quartiles.
  • Polygenic Risk Score (PRS): A standardized genome-wide sum of per-variant posterior effect sizes for breast cancer (mean=0, SD=1), categorized into quartiles.
• Outcome: Incident breast cancer (first primary diagnosis, ICD-10 code C50) and breast cancer-specific mortality.
• Statistical Analysis:
  • Models: Cox proportional hazards models with age as the underlying time scale.
  • Adjustments:
    • Model 1: Adjusted for established risk factors (family history, Townsend deprivation index, alcohol, smoking, hormone replacement therapy, age at menarche, parity, age at first birth).
    • Model 2: Further adjusted for body fat percentage to test independence from adiposity.
  • Interaction Analysis: Evaluated both multiplicative (cross-product terms) and additive interactions. Additive interaction was quantified using the Relative Excess Risk due to Interaction (RERI).
  • Stratification: Analyses were stratified by age (40–59 vs. ≥60 years) and Body Mass Index (BMI <25 vs. ≥25 kg/m²).
  • Sensitivity: Excluded cases diagnosed within the first year to mitigate reverse causation; used multiple imputation for missing covariates.

3. Key Contributions

• Objective Measurement: Unlike previous studies relying on self-reported physical activity, this study used an objective physiological measure (submaximal cycle ergometer test) for CRF.
• Joint Risk Assessment: It is one of the first prospective studies to examine the additive interaction between objectively measured CRF and PRS regarding breast cancer incidence.
• Stratified Insights: The study provides granular data on how these interactions vary by age and BMI, identifying specific subgroups where fitness interventions are most effective.

4. Key Results

• Overall Associations:
  • CRF: Higher CRF was associated with a lower risk of breast cancer in a dose–response manner (p for trend = 0.001). This association remained significant after adjusting for body fat percentage.
  • PRS: Higher PRS was positively and progressively associated with incident breast cancer (p for trend < 0.001).
• Interaction Analysis:
  • Multiplicative Interaction: Not statistically significant (HR 0.72, 95% CI 0.46–1.15).
  • Additive Interaction: Significant negative interaction was found (RERI −0.84, 95% CI −1.56 to −0.12), indicating that high CRF attenuates the excess risk conferred by high PRS on an additive scale.
• Stratified Findings:
  • Age: The protective attenuation effect of high CRF against high PRS was significant in women aged ≥60 years (RERI −1.10) but not in those aged 40–59 years.
  • BMI: The attenuation was significant in women with BMI ≥25 kg/m² (overweight/obese) (RERI −1.23) but not in those with BMI <25 kg/m².
  • Risk Grouping: Women with high PRS and low-to-moderate CRF had the highest risk (HR 2.58). However, women with high PRS and high CRF did not show a statistically significant increase in risk compared to the reference group (low PRS/low CRF), particularly in the ≥60 and overweight subgroups.
• Secondary Outcomes: No statistically significant association was found between CRF and breast cancer-specific mortality.

5. Significance and Implications

• Clinical Translation: The findings suggest that improving CRF is a viable strategy to mitigate genetic risk, particularly for postmenopausal women who are older (≥60) or have overweight/obesity. This supports the integration of fitness assessments into genetic risk stratification models.
• Biological Plausibility: The authors hypothesize that in older women and those with higher adiposity, physiological stressors (immune senescence, chronic inflammation, increased peripheral estrogen) amplify genetic risk. High CRF may counteract these via improved metabolic regulation, reduced inflammation, and enhanced immune surveillance.
• Public Health Policy: The study advocates for personalized prevention strategies. While PRS identifies high-risk individuals, CRF assessment can help tailor lifestyle interventions, suggesting that fitness-enhancing behaviors are particularly crucial for those with elevated baseline genetic and metabolic risks.
• Limitations: The study is limited by the single baseline assessment of CRF (no longitudinal tracking of fitness changes), restriction to European ancestry (limiting generalizability), and the observational nature which precludes definitive causal inference.

Conclusion: Higher cardiorespiratory fitness is inversely associated with breast cancer risk and significantly attenuates the excess risk associated with high polygenic risk, specifically in postmenopausal women aged 60 and older and those with overweight or obesity. This highlights the potential of CRF as a modifiable factor in genetically informed breast cancer prevention.