Obesity and Mortality

How does carrying around excess poundage on one's person impact mortality?  That question has occupied the minds of many as the worldwide obesity epidemic continues, and the doomsayers everywhere predict a host of dire consequences.  Some of the data is unassailable: there's no question that osteoarthritis, type 2 diabetes, high blood pressure and other chronic conditions enjoy a linear relationship with increasing body mass, as Rick and I have described so often on PodMed.  Now comes a study in the Journal of the American Medical Association concluding that overweight and so-called grade 1 obesity do not negatively impact mortality; in point of fact overweight seems to confer a survival benefit.  Yikes!  Let's look more closely at this study.

The first thing to note about this study is it's a meta-analysis. A total of 97 studies reporting hazard ratios for all cause mortality using standard body mass index (BMI) categories were included.  Further study characteristics included the fact that all of the studies were prospective and enrolled adults.  In total the sample included almost 3 million people and over 270,000 deaths.

Rick points out in the podcast that one reason this analysis was valuable is because obesity has been subcategorized based on BMI.  Thus overweight is a BMI between 25 and 30, grade 1 obesity between 30 and 35, grade 2 as 35 to 40, and grade 3 as greater than 40. While it's sad that we have such prevalence of obesity and profound obesity that it must be stratified even further, the health impact of the condition is better defined using these criteria.

So what did this analysis tell us?  Surprisingly, it showed that folks who fell into the overweight category had significantly lower  all cause mortality than those in the normal weight group or any group of obese individuals.  People who fell into the class 1 obese category enjoyed the same mortality risk as those who were of normal weight, while those in categories 2 and 3 experienced higher all cause mortality.  Well.  Does this mean that people who are wandering around with an extra 10 or so pounds relative to their height are actually better off than those supposed normal weight individuals?

One hypothesis that has been advanced in explaining the  results suggests that people who are already on the cusp of illness, such as those with cancer, would have been included  in the normal weight category and therefore would skew the results toward a higher risk of death.  This and other potential causes of bias in the analysis are rejected by the authors, although they do identify potential errors as a result of self-report of height and weight data.

Rick and I conclude that for now, we're not prepared to say that being overweight is likely to be a more healthful state than being of normal weight, and we are convinced that greater levels of obesity are most definitely deleterious.  We also are both fans of modification of BMI tables to somehow adjust for greater amounts of muscle mass.  As Rick reveals in the podcast, according to current tables he's on the cusp of overweight, which he most certainly is not, as a result of extensive lower extremity musculature developed over years of cycling.

Other topics this week include transfusions in people with gastrointestinal bleeding in NEJM, fructose, glucose and brain activity in JAMA, and in the same journal, SSRI use during pregnancy and risk of stillbirth and neonatal death.  Until next week, y'all live well.

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December 14, 2013 at 9:32 pm

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KMJvet February 8, 2013 at 12:36 pm

Hi agree with Eric. You said you both have higher than you'd expect BMIs due to a high muscle mass because you're avid cyclists. Unless you're sprinters, I doubt it. Endurance athletes have lower muscle mass as an adaptation to that chronic stress and minimizing weight. If you think think about the muscle mass of top marathon runners, you'll see this. So, unless you've had an actual body composition done by dexa, I wouldn't assume you're "not that skinny" BMI is related to your muscle mass. Unless your cycling is in the form of high intensity interval training and not endurance riding, I'll bet you a quarter, it's not. I'm not saying that BMI is all that good a parameter to be used in epidemiological research--it's terribly flawed. They do it because it's easy. And then of course, there's the issue of people who are metabolically obese (such as leptin levels & insulin resistance) and are yet of normal BMI.

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Eric Levine January 13, 2013 at 1:47 pm

Hi, Very interesting information! Why is "overweight" a BMI between 25 and 30, while "normal" is less than 25 (I can't remember where the "underweight" range falls)? If so many people are overweight or obese, maybe "normal" should be called something else, since it apparently is not usual.

Also, regarding muscle mass, my understanding is that it takes a fairly significant effort to increase one's muscle mass enough to affect the BMI measurement, such as a rather intense weight-lifting regimen. Would aerobic exercises like bike riding, swimming, and running really increase muscle mass that much? Marathon runners and long-distance bike riders don't look all that muscular to me, so maybe it's more a matter of decreased body fat from all that aerobic exercise rather than a large increase in muscle mass.

I don't really respect the BMI measurement all that much. I think it needs to be taken in context with other health factors, such as blood pressure, heart rate, and other attributes. It seems that the BMI is blown out of proportion, especially given the information you presented here.

Thanks for a great program - Podmed is one of my favorites!

-eric

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Elizabeth Tracey January 18, 2013 at 10:28 am

Hi Eric, and many thanks for writing. I looked into this idea you present about muscle mass and its impact on BMI and wasn't able to find very good information. Here's an anecdote I know Rick won't mind if I share: he's quite tall and bears a striking resemblance to a grasshopper in that his legs are very muscular and big, while his upper body is really quite lean. If you saw him in street clothes rather than cycling gear you would probably describe him as a beanpole. Yet his BMI is just shy of 25! I'll keep researching this question and if you find anything else please let me know. Thanks again for writing and your kind comments.

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Madison February 3, 2013 at 12:58 am

Short and sweet article. My dooctr advised me pretty much the same thing. Eat smaller portions and eat atleast three meals a day in addition to healthy snacks; nuts, fressh and dried fruits. Avoid the usual junk food snaks, soda and don't touch the salt shaker. I guess it all boils down to healthy & balanced meals, instead of how much you eat.

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