As Peter and I discuss frequently here at Obesity Panacea, the relationship between body weight and health is not always as neat and tidy as you might expect (For all the details, check out Peter’s 5-part series on metabolically healthy obesity). A recent paper published in the International Journal of Obesity by Drs DK Childers and David Allison examines a number of these issues, and suggests ways that they may be at least partially resolved.
In the intro to this new paper, the authors point out 3 confusing issues surrounding the relationship between body weight and health:
- The relationship between body mass and mortality in epidemiological studies is “U-shaped” – high health risk for individuals with a body mass that is very high or very low, and low risk for individuals with intermediate body mass (e.g. 18-28 kg/m2 or so, depending on sex and ethnicity). However, the “ideal BMI” is far from clear (some studies have found that having a BMI well into the “overweight” range may be associated with minimal health risk), as is the causality of the relationship between low BMI and health risk (e.g. does being underweight cause disease, or does having a disease cause you to lose weight?). Given the wealth of research on obesity, it’s surprising that we really have only a very general idea of the relationship between body weight and health!
- Even more confusing, whatever the “ideal” BMI is, it appears to increase with age. The nadir of the U-shaped curve shifts slightly to the right in older populations. Why? No one really knows.
- Finally, there is the so-called “obesity paradox” – the idea that although obesity is typically associated with increased risk of death and disease in the general population, among individuals who have certain illnesses or injuries, obesity is actually associated with increased survival time. In other words, obesity is generally bad for your health, but in certain situations it may actually offer a protective advantage.
All of the above issues are quite controversial and not terribly well understood. However, in their new paper, Drs Childers and Allison suggest that the obesity paradox may actually help to explain the U-shaped nature of the relationship between body weight and disease, as well as the reason why the ideal body weight increases with age. From the paper:
Specifically, we suggest that the obesity paradox in which obesity hypothetically (a) causally monotonically increases MR among persons in the absence of major injuries or other diseases; yet (b) causally monotonically decreases MR among persons in the presence of certain major injuries or other diseases; can explain both the U-shaped curve and the increasing nadir with age. The existence of major injuries and diseases that satisfy (b) at the association level (not necessarily causal) is well founded. Let the collection of all such injuries or diseases be called afflictions of type A. We take no position here regarding the veracity of the causal aspects of point (b) above, but merely evaluate the consequences and potential explanatory power of it being true.
Now I should point out that this paper is entirely stats based, and what the authors did was simply calculate what the relationship between body weight and health would look like if the obesity paradox is true. Here’s basically what they assume:
In the absence of disease or injury, BMI has an exponential relationship with health risk – the higher the BMI the higher your estimated health risk. However, when you have those diseases for which body weight is protective, there is a general decrease in risk as your BMI goes up. So what happens when you make those assumptions? Well, you wind up with a relationship between BMI and health risk that very similar to the one we see in the general population – a U-shaped relationship between BMI and health risk at any given age, but with the “ideal” BMI increasing slightly as age goes up.
Now this paper is incredibly speculative, the models exclude a number of potentially important factors (e.g. body fat distribution, physical activity levels, diet, etc) and doesn’t necessarily mean that the obesity paradox is the driving force behind the observed relationship between BMI and health. But it does seem to suggest that the obesity paradox could be one explanation for this relationship, and one worthy of further study. Now that’s not terribly straight-forward in humans (for obvious reasons we can’t randomize people to different body weights and then subject them to injury or illness to see what happens!), but the authors suggest that animal models could be used to see whether the findings in the paper hold up under closer scrutiny. The authors also point out that for some individuals at high risk for specific illnesses, weight gain may actually be protective, although I think they realize that it’s not really a viable option, especially for the time being (talk about personalized medicine!). I should point out that for the record, there is still very little evidence that you should gain weight to live longer (and even then only in incredibly specific circumstances – e.g. right before you are going to suffer a debilitating injury), despite what the mainstream media may claim from time to time. As usual, lots of questions, not a whole lot of answers, but it at least sheds some light on the reasons behind the complicated relationship between body weight and health risk.
Thanks to Amby Burfoot for suggesting we look into this interesting topic!
Childers, D., & Allison, D. (2010). The ‘obesity paradox’: a parsimonious explanation for relations among obesity, mortality rate and aging? International Journal of Obesity, 34 (8), 1231-1238 DOI: 10.1038/ijo.2010.71