Belly Fat and Fracture Risk
Take Home Points:
- Media headlines covering a 2012 conference study suggested that actual fracture risk was measured and also failed to properly clarify what was meant by "belly fat."
- The fat you can grab (subcutaneous) is different from the internal fat covering your inner organs (visceral fat).
- The conference data found increased visceral fat was associated with reduced bone mechanical properties (reduced bone stiffness and strength) in adult males, but did not examine fracture incidence in those men.
- Findings as presented in the final study publication confirmed the conference data, but did not see correlations with subcutaneous fat.
- Excess visceral fat may negatively affect bone strength/stiffness, but muscle mass may be an important protective factor for bone health in overweight individuals.
In 2012 a study examining body fat and fracture risk received widespread media attention. This was one of those cases where the media got a bit too carried away. At that time the data was not from a peer-reviewed journal. It was therefore not a completed study assessed for quality by other scientists. Instead, the data was from a presentation at the Annual Meeting of the Radiological Society of North America (RSNA). Presentations at conferences are a way for scientists to stay connected and get feedback before final publication. However, the data presented at these conferences is often incomplete and with low sample sizes (few data points). Proper statistical analysis of the data may also not be possible in these situations.
At the time, not having attended the meeting, I could not comment on the integrity of the data (though one of the co-authors was a past collaborator of mine, so I had no reason to suspect anything but high quality data). That being said, the media coverage of the study was problematic. For example, a call from my father informed me that TV news stations were saying the study said “men with excess belly fat are at risk of hip fractures.” Further, the headline on medicaldaily.com was “Belly Fat Increases Fracture Risk in Men.” Yet another report headlined “Men with Belly Fat at Risk for Osteoporosis.” Why were these headlines problematic? For one, the study didn't actually assess fracture risk. Second, because men were looking down, seeing fatty bellies and assuming the study applied to them. Did it? Maybe.
What is Belly Fat?
Ultimately, the term “belly fat” is ambiguous. The general public is inclined to think belly fat is the fat that can be grabbed on someone’s belly (also called subcutaneous fat). It could be this fat, but it could also be the fat you can’t feel, which is in your abdomen and surrounding your internal organs (also called visceral fat).
The above image taken from Makino et al., 2008 clarifies the difference in types of belly fat. Subcutaneous fat (the kind you can grab on yourself) is indicated in pink whereas visceral fat (the type overing your internal organs) is indicated in red.
Some of the media coverage cited above addressed this distinction in their text of their articles, but you have to read carefully for it. In addition, the picture choices were misleading to many.
The Conference Data
The reports of the conference data focused largely on visceral fat measurements. As mentioned one can have no obvious beer belly and still have excess visceral fat (and vice versa). So, you cannot simply look in the mirror and translate the results of this study to your health. Several other caveats are as follows:
- The mean (average) age of men in this study was 35. This is not an age where osteoporotic fractures are common in men and extrapolating a currently weaker bone structure, to later life bone fractures has limitations (this study did not track actual fracture incidence in the participants).
- The determination of bone strength and stiffness (bone mechanical properties) was done using finite element analysis (mathematical modeling) and not with tissue mechanical testing (actually loading the bones in a testing machine).
- Is visceral fat really the driving force behind the differences in strength/stiffness of the bone? It might not be – the authors report that the men with weaker bones had less muscle mass in addition to the increased visceral fat. Perhaps having less muscle is of equal or more importance than the visceral fat levels?
Despite the caveats, this is interesting data that suggest younger, obese men may be setting themselves up for bone issues later in life.
The Published Study
Some time has passed since the initial presentation of the data at the conference and we now have the results in a peer-review publication which you can access here. The paper confirms the conference data in that visceral fat was higher in those with lower bone mechanical properties (weaker/less stiff bones). What about subcutaneous fat - the kind you can grab? This was measured, but the results weren't as strong; subcutaneous fat tissue did not correlate significantly with either bone stiffness or strength. Only by looking at visceral fat tissue alone or total abdominal fat tissue (visceral + subcutaneous) did the authors see the correlation. But again, actual fracture risk was not calculated. Finally, the more thigh muscle individuals had, the better their mechanical properties were - again confirming the conference data. So, as mentioned above, muscle mass is likely a key player in determining bone strength and stiffness - even for people that are carrying a fair bit of excess fat.
The conference data released in 2021 did match the final published study quite well. However media coverage mislead many to think that the fat you could see and grab at your belly was associated with fracture risk. In reality the data found that more internal, visceral fat (and total fat) was associated with weaker and less stiff bones. That these bones are more likely to fracture is a possibility, but one that was not investigated in the study.