The Body Mass Index (BMI) Says Nothing About Your Health

obesity ratesIf you go to your physician’s office and inquire about your weight status, he or she will measure your height and weight to derive your BMI (weight in kg divided by height in m squared). Then they will compare your BMI to that of established criteria to decide whether you are underweight (30 kg/m2) . Often times, this measure alone determines whether or not you receive lifestyle treatment.

But how useful is this measure anyways? What does it tell you about your health? And finally, how helpful is it to measure when assessing the effect of a lifestyle (diet/exercise) intervention?

Before I get into the various limitations of BMI, I must point out that the measure is quite useful across large populations, as it is well correlated with the degree of adiposity, and of course it is extremely simple to measure in clinical practice.

Nevertheless, here are some of the key issues with BMI, particularly when used on an individual basis.

1. BMI does not differentiate between the Michelin Man and The Terminator

Ok, we might as well just get this abundantly obvious problem out of the way. I have heard countless times how one buff celebrity or another (e.g. Tom Cruise, Arnold Schwarzenegger, The Rock etc.) would be classified as overweight or obese according to their BMI due to their excess amount of muscle. Yes, this is absolutely true. BMI is a measure of relative weight; fat mass and muscle mass are not distinguished. Here’s what is equally true: the large majority of the general population with a BMI in the overweight or obese range does not look like Jerry Maguire or the Terminator. Also, if you seek advice from your physician about your “excess weight”, in case you have body dysmorphia and cannot yourself decide, they will quickly be able to assess whether your excess weight is due to your bulging muscles or your rolls of adipose tissue. So while this is an obvious problem, I would argue not the main issue.

2. BMI does not differentiate between apples and pears

For over 60 years, we have known that independent of how heavy a person is, the distribution of their body weight, or more generally the shape of their body is a key predictor of health risk. It is now well established that individuals who deposit much of their body weight around their midsection, the so called apple-shaped, are at much greater risk of disease and early mortality in contrast to the so called pear-shaped, who carry their weight more peripherally, particularly in the lower body. Thus, two individuals with a BMI of 32 kg/m2 could have drastically different body shapes, and thus varying risk of disease and early mortality.

Fortunately, a very simple measure allows you or your physician to decide whether your elevated BMI is of the apple or pear variety: waist circumference. Current thresholds suggest that a waist circumference above 88 cm in women and 102cm in men denotes abdominal obesity. Interestingly, for the same BMI level, those individuals with an elevated waist circumference have a greater risk of diabetes, cardiovascular disease, mortality, and numerous other health outcomes. Thus, as studies from our laboratory have consistently suggested, waist circumference may be a more important measure of obesity and health risk than BMI. Currently, most researchers would agree that waist circumference should be measured along with BMI to adequately classify obesity-related health risk.

You can measure your own waist circumference by using a tape measure and wrapping it around your abdomen, at the level of the top of your hip bones. Make sure you measure at the end of exhalation, without sucking in your gut – you’re only fooling yourself!

3. BMI does not always budge in response to lifestyle change

Given the number of papers my supervisor, Dr. Ross, and I have published on the topic, I would argue this is the biggest drawback of using BMI: it doesn’t always change even though you may be getting healthier. This is particularly so if you adopt a physically active lifestyle, along with a balanced diet, but are not necessarily cutting a whole lot of calories. This lack of change in BMI or body weight is all too often interpreted as a failure, resulting in the disappointed individual resuming their inactive lifestyle and unhealthy eating patterns.

However, as we have argued most recently in a paper in the Canadian Journal of Cardiology, several lines of evidence suggest that weight loss or changes in BMI are not absolutely necessary to observe substantial health benefit from a healthy lifestyle. Thus, an apparent resistance to weight-loss should never be a reason for stopping your healthy behaviours.

First, it is well established that increasing physical activity and associated improvement in cardiorespiratory fitness are associated with profound reductions in coronary heart disease and related mortality independent of weight or BMI. Second, exercise (even a single session) is associated with substantial reduction in several cardiometabolic risk factors (such as blood pressure, glucose tolerance, blood lipids, etc.) despite minimal or no change in body weight. Third, waist circumference and abdominal fat (arguably, the most dangerous fat) can be substantively reduced (10-20%) in response to exercise with minimal or no weight loss. In fact, significant reductions in fat mass often occur concurrent with equal increases in muscle mass in response to physical activity – equal but opposite (and beneficial!) changes which are not detected by alterations in body weight on the bathroom scale, and thus BMI.

So in the end, while BMI surely has its strengths in ease of use and pretty good reliability in large populations, on an individual basis, the greater focus should be on healthy behaviors: physical activity and a healthy diet. And if you must measure something, check your waist circumference.

Enjoyed this post? Please sign up for future updates on similar topics to be delivered to your email inbox or RSS reader.

Peter

Reference:

Ross R, & Janiszewski PM (2008). Is weight loss the optimal target for obesity-related cardiovascular disease risk reduction? The Canadian journal of cardiology, 24 Suppl D PMID: 18787733

Category: News | 12 Comments

This picture captures why “no sugar added” is a meaningless concept

It seems that every time I go to the grocery store I see more products proudly announcing that they have “no sugar added”.  Typically these claims are seen on juice and other products that contain a high sugar content.

As Yoni Freedhoff has pointed out in the past (emphasis mine):

[These claims are] there to make you feel that the product inside the box is a healthy one.

A quick peek at the back of the box is probably in order.

Take Mott’s Fruitsations Unsweetened Strawberry Fruit Rockets for instance. Reading the ingredients you’ll find that they include both, “Concentrated Strawberry Puree“, and, “Concentrated Fruit Juices“.

And what are concentrated purees and juices?

Sugar.  Plain old sugar.

I know that some people will say that sugar in juice is different from table sugar or high fructose corn syrup because it is “natural”.   I disagree.  But let’s say that we accept that the sugars found in juice are somehow “better” than added sugars, and that “no sugar added” is a term that has value.  What then to make of the below picture, courtesy of freelance science writer David Despain:
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Category: Miscellaneous, News, nutrition, Obesity Research | Tagged | 6 Comments

Juice is not natural (!!!)

orange juice

The health-impact of juice is a contentious issue. Yes, it’s full of vitamins and minerals. But it’s also full of sugar.  How much sugar?  The below graph compares the calories and sugar content in a 12-ounce can of Coca-Cola, Pepsi, and Tropicana Orange Juice.  I’ve hidden the names.  Which one is the “healthy” orange juice?

Calories and Sugar Content in 12 ounces of Tropicana, Pepsi, and Coca-Cola


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Category: News | 22 Comments

Exercise protects you from stress

womenactiveThere are few of us who can honestly say they are not stressed out at least some of the time. Too much to do, not enough time, looming deadlines, financial concerns, health problems, etc. can all cause us to feel on edge.

Your heart rate and blood pressure soar, you start perspiring, sleeping becomes a challenge, you’re irritable, and so on.

As you might have imagined, chronic psychological stress negatively impacts on your physical health, increasing the chances of countless chronic diseases. Additionally, stress can also reduce your lifespan.

Findings on a more cellular level suggest that psychological stress expedites the aging process of your body’s cells. Specifically, stress has been correlated with telomere shortening of a cell’s chromosomes. Every time a cell divides, and it replicates and shares the genetic information wound up in its chromosomes with its new copy, the new cell retains a slightly smaller end section of the chromosome, termed the telomere. When the telomere gets to a critical length, the cell reaches a point it can no longer divide properly.

When this begins to occur on a systemic level, you are in trouble.

And this is essentially what happens with aging, leading to cellular senescence.

So what effect, if any, does exercise have on this negative impact of psychological stress on cellular aging?

A recent study investigated this very question in a sample of 63 healthy post-menopausal women who were assessed for stress via questionnaire, physical activity levels over a 3 day period, and telomere length via a process I barely understand, so I won’t try to explain (it is Friday, after all).

For purposes of comparison, the women were divided into sedentary (< 33 minutes during the 3 days) or active (>33 minutes during the 3 days) – not a very high bar for activity.

Not surprisingly, participants with higher levels of stress were less likely to exercise, have higher BMI, less years of education, and shorter telomere length.

In terms of exercise protecting you from the negative cellular effects of stress, the authors found the following:

Among sedentary individuals, a 1 unit increase in perceived stress was associated with a 15-fold increased risk of having short telomeres (in the lower tertile of telomere length in the entire sample).

Among active individuals, a 1 unit increase in perceived stress had NO RELATIONSHIP with telomere length.

In other words, those who are active (and just barely so, based on the categorization in this study: 11 mins per day) seem to be protected against the cellular damage caused by cognitive stress

Keep in mind these analyses accounted for differences in BMI, education, age, and anti-oxidant use.

Bottom line:

Just in case you needed another reason to be physically active, regular activity may protect your cells from the damage caused by daily stresses of modern life. Unfortunately, those people who could benefit the most from physical activity – stressed individuals – are least likely to be active.

Peter

Puterman, E., Lin, J., Blackburn, E., O’Donovan, A., Adler, N., & Epel, E. (2010). The Power of Exercise: Buffering the Effect of Chronic Stress on Telomere Length PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010837

Category: News | 2 Comments

On the road again

First day of school, 2014

First day of school/work, 2014

The past 12 months have been quite busy for my wife and I.  In September I defended my PhD at the University of Ottawa/Children’s Hospital of Eastern Ontario.  In December we moved across the country to Halifax so that I could begin a post doctoral fellowship at Dalhousie University.

Coincidentally, the day that we drove out of Ottawa in a 14 foot Uhaul, we also found out that my wife is pregnant.  That day was also a massive snowstorm (if anything makes driving a massive truck in a snowstorm even more stressful, it’s knowing that you’re about to be a first-time parent!).  After a short time in Halifax, we have now moved again – this time to Prince Edward Island, where I have begun work as an Assistant Prof in Applied Human Science*.  That makes for 2 separate moves within 7 months (and a single pregnancy!).

I’m really excited to be starting what is essentially my dream job.  I’ll be in a relatively small school doing both teaching and research, which was always my goal.  And it’s in the part of the world/Canada that my wife and I have wanted to be in all along.  I think that both the department and the community will be an excellent fit, and it’s been a very positive experience for us so far.

As I transition from post doc to prof, I wanted to take a moment to publicly thank my post doctoral supervisor Chris Blanchard, as well as his team at Dalhousie.  Although I was only in his lab for 7 months, he was extremely generous to me both in time and in resources.  We’ve begun some very interesting projects that will start to trickle out soon (one is currently under review, two others will be submitted shortly), and have begun prepping for some very cool future projects.  Chris and his team were excellent to my wife and I, and we both thoroughly enjoyed our time in Halifax.  I would also like to thank the Heart and Stroke Foundation for their funding of my work at Dal.  And it goes without saying that I am forever thankful of all the good folks at the HALO research group in Ottawa.

I should also mention that when I was putting together my application for this position I leaned very heavily on the tremendous resources that have been collected by Dr Becca in her Tenure Track Aggregator (in addition to the help of many colleagues).  In particular, I found the Prodigal Academic‘s  posts to be very helpful, despite being in a very different field and dealing with very different types of academic institutions.  Similarly, this handy list of questions to ask/prepare for from SERC was unbelievably helpful. If you are applying to an academic job, I cannot recommend that you click on those above links highly enough.  I also had a lot of help from the Centre for University Teaching at the University of Ottawa, and the Centre for Learning and Teaching at Dal.

However, while I found a ton of advice online about how to prep my application for my current job, I’m cognizant of the fact that grad school is not a perfect preparation for life as an academic.  And so I’d like to ask for input from anyone who has once been in my shoes as a new faculty member: 

  • What did you do that worked well?
  • What would you do differently?
  • How did you balance the responsibilities of teaching and research?
  • How did you transition from being a student/trainee to being a prof/mentor?

And from anyone who was ever a student:

  • What did your profs/supervisors do that you liked best?
  • What did they do that you liked least?
  • What would have improved your experience as a student?

I’ll be doing both teaching and research, so I’m interested in your thoughts on either or both.  If you’re uncomfortable commenting under your real name, feel free to comment anonymously (the email line needs to be completed, but as far as I know, it need not be a valid email address).  And if you’re completely uncomfortable posting online but would like to share, feel free to email me at saunders dot travis at gmail. I’d rather have comments available so that others can benefit/reply, but I’ll take advice in any form that I can get it.  

As for what this will mean for the Obesity Panacea, probably not much.  I will continue to post when I have time (aiming for 1-2 posts/week between Peter and me), focusing mostly on topics that I am currently working on (I will try to work in some posts related to my course content as well, so that it’s not 24/7 sedentary behaviour).  Peter and I began this blog almost 6 years ago, a month before I defended my MSc.  So for the handful of people who have been reading since then (I’m looking at you, Angie & Yoni!), you’ve actually seen a frightening amount of my professional development!

As always, thanks for reading, and I look forward to any advice you’d like to share!

Travis

*As always, a reminder that the opinions and info expressed on the blog belong to Peter or I (or the guest post author, as appropriate), and do not reflect those of any other institution or funding agency.  I hope that’s pretty obvious, but this is a good opportunity for a reminder.  It is also why I don’t specifically list my active academic affiliation(s) here on the blog, although they are by no means a secret.

Category: Miscellaneous, News | Tagged | 7 Comments

Walking meetings: a step in the right direction?

Image by Sangy23.

Image by Sangy23

Today’s post comes from Allana Leblanc.  You can find more on Allana at the bottom of this post.

Right next to coconut water, and standing desks, “walking meetings” are the newest buzzwords for the ever trendy workplace.  A quick Google search will show pages of magazine articles and news stories touting the benefits of taking your work to the streets.  Travis has mentioned them a couple times here, and a summer student with the Healthy Active Living and Obesity Research Group gave a compelling overview of why they are beneficial to health.

Not to beat a dead horse, but I want to share with you the results of a recent article on the benefits of walking meetings. But rather than the health benefits of walking meetings, I want to talk about the benefits for creativity.  Because in a series of 4 experiments, Oppezzo and Schwartz show that walk is your first step to a creative day (full text available here).

To test creativity, they used a couple different tests, but basically, during the sitting, or walking conditions, participants were audio recorded while they completed different word association tasks.  After the test was over, they were scored according to novelty and appropriateness of their responses

Experiment 1

Two conditions:

  1. sitting + test
  2. walking on a treadmill + test

Results: Walking on a treadmill produced an average creative output of 60% higher than sitting.

Experiment 2

Three conditions:

  1. sitting + test, followed by treadmill walking + test
  2. treadmill walking + test,  followed by sitting + test
  3. sitting + test,  followed by more sitting + test

Results: The walking condition, was associated with the highest test scores.  Order of the conditions also mattered so it wasn’t just that people were getting better at the tests.  Walking had a carryover effect such that the walking + sitting condition produced higher scores than the sitting + walking condition. The sitting + sitting condition produced the lowest scores for creativity.


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Category: Guest Post, Peer Reviewed Research, Sedentary Behaviour | Tagged | 4 Comments