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That's one big femur!

Well new research based upon analysis from the femurs of individuals suggests that yes, they are.

Researchers found that the heavier an individual was, the wider the shaft of that person’s femur. The researchers hypothesize that the femur of an overweight person is more robust because it bears more weight, but also because overweight individuals move and walk differently to compensate for their greater mass.

This finding is sure to give validation to those who have forever claimed that their weight is because of their larger bones. That of course is true, as the skeleton can be quite heavy, and determines much of your weight. However, we must realize that the increase in bone (femur) size is in response to the weight of the individual, and not a factor that simply made them larger.

In other words, being overweight would have to proceed the change in femur size, as the body would increase the femur only in response to the increase in weight, otherwise there is no biological impetus.

An argument against this position would be that people can be born with big bones and then become overweight, and that is true, but because widespread obesity (at least in the US) is relatively new in our evolutionary history, it is more likely that our skeletons are individually adapting (not evolving) to the increased weight, than humans evolving larger bones to compensate for obesity (as it requires less time adapt than evolve (lifetimes vs. thousands of generations)).

The mechanism for this increase in femur size is linked to how obesity affects the body and the skeleton.

An exploration into biomechanical research in obesity shows numerous significant differences in walking strategy between overweight and normal weight individuals. These compensatory acts may alter force movement pathways and magnitudes through the femoral diaphysis, triggering ML elongation through BFA.

Cutting through the jargon, overweight people makes changes in their gait and locomotion in order to accommodate their increased mass. This leads to unnatural changes in the stress applied to the skeleton, and causes a biological response to strengthen the bones against the new stresses.

Let’s also keep in mind that this is preliminary research:

While this research is still intended to be a first step, these results do show promise in future efforts to identify obesity using skeletal remains and further highlight the multifaceted nature of long-bone cross-sectional properties. Given these results, future research on a more contemporary or forensically appropriate sample is warranted.

However, the initial findings of this study show that having big bones does not make you fat, being fat gives you big bones.

Journal Reference:

  1. Gina M. Agostini, Ann H. Ross. The Effect of Weight on the Femur: A Cross-Sectional Analysis*. Journal of Forensic Sciences, 2011; 56 (2): 339 DOI: 10.1111/j.1556-4029.2010.01648.x