Is Fat but Fit a Myth?

Researchers out of London used BMI values from parents and their adult children to evaluate a common adage, fat but fit, that being overweight can promote longevity.

The claim that being “fat but fit” is advantageous for increasing one’s lifespan has grown over the years. It is thought that an elevated body mass index (BMI) is a factor in living longer whereas remaining thinner longer can be damaging and lead to illness. Historically, this has never been proven to be true but the somewhat hyperbolic association of these factors persists. However, BMI is strongly heritable, according to the authors of a recent study, and this is an important metric they used in determining the relationship between parents’ health and their adult children’s BMI.

Where Did “Fat but Fit” Come From?

The belief of fat but fit results from observational studies indicating that mortality and BMI have a “U shape” relationship.  This means that mortality increases at a low BMI and a very high BMI and seems to be lowest at a slightly elevated BMI.

Researchers and health care professionals, however, have speculated that the elevated mortality at low BMIs is likely the result of pre-existing health conditions that lead to both low BMI and also mortality. To test this theory, they used parent-child data to analyze the mortality rates of parents with the BMI of their adult children. Knowing that BMI is highly heritable, this information was able to tell researchers what the parent’s BMI would likely be if it were not for their pre-existing health conditions.

The researchers utilized the data of 60,199 parent-child pairs as subjects and examined deaths between January 1984 and December 2009. The data collection considered both the children’s BMI and their parents’ cause of death and a statistical analysis shows the relationship between those parents with high BMI and their children who had higher blood pressure, lower physical activity, and a greater propensity for drinking alcohol and smoking tobacco.  Also, of the 18,365 parental deaths measured up to 2009, any children that mimicked parental BMI patterns were also to be at risk for increased mortality.

Myths Busted

These results, published in the International Journal of Epidemiology, and a subsequent discussion published in the British Medical Journal, highlight the need for more conventional research methods similar to the ones utilized in this study. The authors tout the strength of their study by its massive sampling size, but also acknowledge the assumptions they had to make in order to relay this data effectively. The largest assumption made was the use of a child’s BMI as an instrument for comparing datasets; the variability of such an instrument reduces the precision of estimates. In the case of parent-child BMI relationships, this instrument (data set) is caused by the exposure (parental procreation resulting in child/dataset).

Overall, these results put the discussion of BMI back into a category of personal awareness and accurate public knowledge. Further causal studies like this are needed to solidify a link between high BMI and elevated mortality from cardiovascular disease, stroke, and diabetes. However, conventional knowledge should now reflect that the myth between high BMI and lower mortality in life is on shaky ground. Indeed, the “fat but fit” myth has been busted.

Written by Cooper Powers, BSc

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.