Reevaluating the “Fat but Fit” Myth

Research Overview

Researchers from London have conducted a study utilizing body mass index (BMI) values from parents and their adult children to assess the veracity of the adage that being “fat but fit” can enhance longevity. The notion that being overweight may contribute to a longer lifespan has gained traction over the years. It has been suggested that a higher BMI might correlate with increased longevity, while maintaining a lower weight could lead to health issues. Historically, this claim has lacked empirical support, yet the exaggerated perception of its validity continues to circulate.

Heritability of BMI

The authors of the recent study emphasize that BMI is highly heritable. This characteristic was fundamental in their analysis of the connection between parental health and the BMI of their adult children.

The Origins of “Fat but Fit”

Understanding the U-Shaped Relationship

The concept of being fat but fit stems from observational studies that suggest a “U-shaped” relationship between mortality and BMI. This relationship indicates that mortality rates are higher at both low and very high BMI levels, with the lowest rates occurring at moderately elevated BMIs.

Evaluating Mortality Factors

Researchers and healthcare professionals have proposed that the increased mortality associated with low BMIs might derive from underlying health conditions that contribute to both low BMI and higher mortality rates. To investigate this theory, the study analyzed parent-child data to examine mortality rates in relation to parental BMI. By understanding the heritability of BMI, researchers could estimate what a parent’s BMI might be, excluding the influence of pre-existing health conditions.

Study Parameters and Findings

Methodology

The research involved 60,199 parent-child pairs and analyzed mortality data between January 1984 and December 2009. It took into account both the BMI of the children and the causes of death among their parents. Statistical analyses revealed a correlation between parents with high BMIs and children exhibiting higher blood pressure, reduced physical activity, and increased tendencies toward alcohol consumption and tobacco use. Among the 18,365 parental deaths recorded up to 2009, children mirroring their parents’ BMI patterns were similarly at risk for increased mortality.

Research Implications

The findings, published in the International Journal of Epidemiology, along with a discussion in the British Medical Journal, underscore the necessity for more rigorous research methodologies akin to those employed in this study. The researchers highlight the robustness of their work, thanks to its extensive sample size, while acknowledging the assumptions necessary for presenting the data effectively. A key assumption involved using a child’s BMI as a comparative instrument, which could potentially diminish the accuracy of estimates. This relationship is inherently influenced by parental procreation, creating a complex dataset.

Conclusion: Debunking the Myth

The results of this study reinstate the discussion around BMI into a framework of personal awareness and informed public understanding. Further causal research is essential to establish a definitive link between high BMI and increased mortality risks associated with cardiovascular disease, stroke, and diabetes. Current knowledge indicates that the myth connecting high BMI with lower mortality is increasingly tenuous. Indeed, the notion of being “fat but fit” has been effectively challenged.

References

(1) Carslake, D., Davey-Smith, G., Gunnell, D., Davies, N., Nilsen, T.I.L., Romundstad, P. (2017). Confounding by ill health in the observed association between BMI and mortality: evidence from the HUNT study using offspring BMI as an instrument. International Journal of Epidemiology, 1;11. doi: 10.1093/ije/dyx246

(2) Hawkes, Nigel. “Being “fat but fit” does not increase longevity.” (2017): bmj5602.