Saturday, May 17, 2014

A better measure of health? Waist Circumference over BMI

People do say the strangest things to me and the other day was no exception.  I was standing poolside, about to enter a lane, when a man already in the pool, in an adjacent lane, looked up at me - speedo clad, in cap and goggles - and said, "I'd like your body mass index please."  Seriously, those were his exact words.  I wasn't sure what to say or how to interpret what he said.  It reminded me of how police say, "I'd like your ID please."  In other words, I didn't know if he wanted me to tell him what my body mass index number was or if he wanted to have the same number as I had.  In response, as I laughed and jumped in the pool, I said, "No you don't, it's too low."  And so the discussion began.. as I pointed out to my new "friend" that BMI is helpful for understanding weight changes and obesity in the population, but is usually less helpful for me and you.  "OH?," he said. I explained, "waist circumference or waist to hip ratio might be better indicators of health."  To which he replied, "Ok, so what is it you do for a living?"  Well....

The next day, I ran across a research study on waist circumference and health. The study is published in the journal Mayo Clinic Proceedings.  I have previously discussed the different measures of weight status, e.g., BMI, Waist Circumference, and Waist to Hip Ratio.  A less discussed measure is the Waist to Height Ratio, which I mention again at the end of this post.

The man in the pool, like many of us, was not aware of the BMI debate.  Researchers and obesity specialists offer reasons why the BMI may not accurately measure a single persons' health status (related to weight) and also why the BMI may not accurately measure the number of persons who have health related levels of adiposity (fatness).  BMI is determined by a formula that uses a persons weight and height to create a ratio (i.e., weight(kg)/height(m)^2).  A person is said to be overweight if their BMI is over 25 and obese if it is over 30, but it is possible to be lean and fit and have a high BMI (in other words, a person might be 'heavy' and not have a lot of body fat, or they may have body fat in the legs which is not as harmful to health as body fat in the waist area, and the BMI doesn't catch those distinctions (see Snijder et al, 2006 ).  A popular example of when a BMI may be inaccurate is seen in athletes.  A fit football player may be heavy because he is solid muscle and his BMI could be 30, such that on paper he is qualified as obese.  Still, BMI can be a good measure of weight related health risk for most of us.  

In regards to trying to capture the rate of obesity for whole populations, BMI is tricky. Every year or so we are told the % of people in a state or country who are overweight or obese.  That number is not a true average of the population but one estimated from a sample of people who were probably called on the telephone and asked their height and weight.  (Some surveys, like the NHANES in the US do collect actual measurements in lab settings).  The usual way of telling researchers in writing, in person, on line or on the phone, ones height and weight, is called self report.  Self reported BMI is said to be a limited measure of overweight and obesity because people can make mistakes and people can, well.... many of us want to be thinner and taller, and might fudge our numbers on purpose.  So even if we aren't athletes, and our BMI accurately reflects our health status, we might not provide researches with the right numbers.  This probably means that the estimated overweight and obesity rates of the population are HIGHER than the reports indicate.

As BMI does not accurately reflect weight related health status for individuals with less common body types and can be incorrectly calculated from 'bad' data, some researchers suggest that waist measurements (waist circumference, waist to hip ratio, and the waist to height ratio) are better indicators of health than the BMI. (See for example, Czernichow et al , 2011; Bener et al, 2013; Vazquez et al, 207).  These studies do indicate that waist circumference (WC) is a better predictor of heart disease, diabetes and mortality than BMI.

To be fair,  some of the studies involve self report of waist measurement and could also have errors, but the WC does capture belly fat and that is what is currently seen as the instigator of disease.  I would expect that and some evidence supports this, people are less likely to purposefully misstate their waist circumference than their height or weight.

The most recent study* (the one I saw in the journal Mayo Clinic Proceedings) on waist circumference found a positive relationship with waist circumference and all causes of death, heart disease death, respiratory death (i.e., COPD) and cancer.  In other words, as the inches (or centimeters) of a persons waist increased beyond a baseline reference number (85 cm for men, 65 cm for women) so did the risk of dying from any disease during the study period.  The researchers compared the likelihood of dying from any cause at 5 cm intervals, or a couple of inches.  For women (all white people in this study), the increased risk of death was pronounced even at the first 5 cm mark (i.e., 70 cm), but for men, the increased risk of death did not become evident until 100 cm.  The effect size - amount of increased risk - was bigger for heart and respiratory disease than for cancer.

In the same study, the researchers compared mortality rates between high and low waist circumference groups.  The low group for women included those with waist circumferences less than 70 centimeters and the low group for men included those with waist circumference less than 90 centimeters.  To be clear, the researchers created 6 WC categories for men and 7 for women.  The differences in likelihood of death during the study period were seen between the 'referent' low category and the high category (50% higher likelihood of death), but there was no difference between the categories of normal or slightly elevated waist circumference and the low category.  That is why it is important to look at the continuous scale  - the 5 cm increments discussed in the above paragraph.  Those data show that a persons risk increases with every couple of inches they add to their waist.  

IMPORTANT NOTE: Remember the comments at the start of this post?  BMI does not accurately reflect health risk when the weight part of the equation is misleading.  An athlete may have a high BMI because of lean tissue, but a low weight person could have excess stomach fat.  This is especially true for older persons, the number on a scale may not reflect the extra inches on a person's waist. That is why the researchers in the Cerhan study, even after confirming that BMI told pretty much the same story as the WC, suggested that at all levels of BMI and especially in the old old adult (i.e., over age 75), WC also be measured by clinicians.  In some people the WC is going to be a better indicator of disease or death risk than BMI and it looks like WC is just as good as BMI in the other cases.. 

You can see the summary for the study here.  The data came from 11 different studies and in all but one of the studies, the people measured their own waists and reported the results to research staff.

*Cerhan, James R. et al.  A Pooled Analysis of Waist Circumference and Mortality in 650,000 Adults
Mayo Clinic Proceedings , Volume 89 , Issue 3, 335 - 345

 (There is some discrepancy among health agencies on what normal or low risk is for the waist circumference; the CDC, AHA, and NHLBI in the US say 35 inches or less for women and 40 inches or less for men, this would be ~ 88 cm for women and 102 cm for men, much higher than the low group used for comparison in the Cerhan study, but remember what I said about the categorical analysis and the incremental analysis.  For women in the study, the risk increased at the first 5cm point (70cm) where as for men, the risk first increased at the second increment.  The International Diabetes Federation suggests a lower target waist circumference; 80 cm women and 90 cm men). 

There is also the waist to height ratio which has gotten a lot of praise for being able to accurately predict disease.  You can calculate yours by using the tools on this website. [be aware of the ads on the waist to height calculator page, or better yet beware them - they seem spamy]

2 comments: said...

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Deirdre Dingman said...

Thank you Maggie,
I will check out the infograph and in the meantime I hope my followers, readers, will see your comments and visit your link.