Fit over thin: new research reveals why fitness trumps weight loss in cutting mortality risks

Cardiorespiratory fitness, body mass index and mortality: a systematic review and meta-analysis. Image Credit: DenisProduction.com / Shutterstock

A recent study published in the British Journal of Sports Medicine investigated the relationship between cardiorespiratory fitness (CRF), body mass index (BMI), and mortality risks from cardiovascular diseases and all causes.

Through reviews and analyses of data from multiple cohort studies, the researchers evaluated how CRF and BMI jointly influence mortality, highlighting the significance of fitness as a predictor of health outcomes independent of weight.

Background

Obesity is a widespread health issue linked to higher mortality risks from all causes, especially cardiovascular diseases. Despite global public health efforts, obesity rates continue to rise, affecting nearly two-fifths of adults worldwide. While health strategies have traditionally emphasized weight loss, these approaches often fail to maintain long-term benefits.

Furthermore, observational studies and clinical trials have indicated that intentional weight loss alone does not consistently reduce the mortality risk.

CRF, which reflects the body’s efficiency in oxygen use during physical activity, has shown a strong inverse relationship with mortality rates.

While increasing evidence suggests CRF may mitigate the health risks of obesity, it is not widely included in clinical guidelines for risk management. The study highlights the need to consider fitness as an independent and essential determinant of health, rather than relying solely on weight-loss strategies.

About the study

In this systematic review and meta-analysis, researchers evaluated the joint effects of CRF and BMI on all-cause and cardiovascular mortality.

They conducted a literature search from 1980 to 2023 and identified studies from a wide range of research databases, such as PubMed, Web of Science, and SportDiscus.

Eligible studies included prospective cohorts with CRF assessed via maximal or peak oxygen uptake (VO2peak) exercise tests, direct measurements of BMI, and analyses of combined CRF and BMI impacts on mortality. The inclusion criteria also specified that the included studies comprised a reference group of normal weight-fit individuals.

Populations with certain chronic diseases, such as diabetes or renal conditions, were included, while those with conditions such as cancer, liver failure, or neurological disorders were excluded. Extracted data included study design, participant demographics, CRF and BMI classifications, and hazard ratios with 95% confidence intervals.

CRF was categorized into fit and unfit groups based on study-defined thresholds, while BMI was grouped as normal weight (less than 25 kg/m²), overweight (between 25 and 29.9 kg/m²), and obese (equal to or greater than 30 kg/m²).

Additionally, moderator analyses were performed to examine the effects of variables such as age, sex, chronic disease status, and follow-up duration.

The researchers included 20 studies with 398,716 observations in the final analysis to ensure diversity in sex and geographic representation. This approach also addressed the limitations in earlier reviews by enabling more generalizable findings on the role of CRF and BMI in predicting mortality risks. The study employed a robust three-level meta-analysis model to account for potential variances in data from related cohorts, ensuring more conservative and reliable estimates.

Key findings

The study found that CRF significantly reduced mortality risks, irrespective of BMI. Fit individuals, regardless of being normal weight, overweight, or obese, exhibited no statistically significant increase in the risks of all-cause mortality or cardiovascular disease compared to normal weight-fit individuals. Conversely, unfit individuals in any BMI category showed markedly higher risks.

Normal weight-unfit participants showed 1.92-fold higher all-cause mortality risk, while overweight-unfit and obese-unfit groups exhibited 1.82- and 2.04-fold increased risks of all-cause mortality, respectively.

The risk of CVD-related mortality was more significant for overweight-unfit and obese-unfit individuals (2.58-fold and 3.35-fold greater, respectively), while normal weight-unfit individuals had a 2.04-fold higher risk of CVD-related mortality than the reference group.

Additionally, the moderator analyses revealed no significant differences in CRF and BMI effects based on age, sex, or follow-up duration for all-cause mortality. However, follow-up duration was found to influence CVD-related mortality, with shorter studies showing higher CVD mortality risks for unfit individuals.

Notably, achieving CRF levels above the 20th percentile of age-adjusted fitness standards was sufficient to yield significant risk reductions, emphasizing the accessibility of fitness as a health goal.

Cumulatively, these findings emphasized the role of CRF in mitigating health risks associated with excess weight. However, the study cautioned that while CRF attenuates the risks of elevated BMI, it does not eliminate them entirely, especially for CVD-related outcomes.

Conclusions

To conclude, the study underscored the importance of CRF in reducing mortality risks, regardless of BMI. Fit individuals across all weight categories had significantly lower risks compared to their unfit counterparts.

These findings supported prioritizing CRF improvement through physical activity over weight-focused interventions alone. While weight-loss efforts should not be discouraged, the findings highlight that CRF-focused approaches may be more sustainable and achievable for long-term health outcomes.

Incorporating fitness assessments into clinical practices and public health strategies could also improve long-term health outcomes and reduce the burden of obesity-related mortality risks. Future research should explore more precise measures of adiposity and investigate diverse populations to refine these findings further.