Nutritional Pearls: Body Mass Index Vs Waist-to-Hip Ratio

Jonathan is a 32-year-old man with a BMI of 20 but with an elevated waist-to-hip ratio of .90. Despite his waist-to-hip ratio, at his latest appointment he asks you if it is necessary for him to continue weight loss efforts now that his BMI is in the normal range.

How would you advise your patient?
(Answer and discussion on next page)

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Answer: Men with BMI in the normal range but high waist-to-hip ratio have significantly higher mortality risk than those with both measurements in the normal range.

For years physicians and researchers have used body mass index (BMI) as a simple and cheap measure to help estimate an individual's risk of such chronic diseases as diabetes, high blood pressure, and heart disease. It's not perfect, however: those who are extremely fit and have very low body fat can still be classified as overweight or obese when you simply look at the interaction of weight and height.
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This is why waist-to-hip ratio (WHR) is also important, as it serves as an indicator of the location of your body fat: those with a higher WHR are much more likely to be carrying fat around their middle, which often means around their internal organs. As we've seen, that visceral adiposity, as it is called, carries a much greater risk of disease than just carrying fat around one's waist. That said, WHR is not perfect either: the best way to determine the location of one's body fat is through expensive and time-consuming body scans. An internist is hardly going to order such an expensive scan just to find out where a patient’s fat is, which is why smart physicians will use BMI, WHR, and their personal knowledge of the individual and their overall lifestyle to advise on their health.

The Research

A study just published in the Annals of Internal Medicine suggests that WHR is even more important than we may have thought.

The authors made use of data gathered through NHANES III (Third National Health and Nutrition Examination Survey), the most recent set of NHANES data (initially gathered 1988-1994 and concluding in 2006) that includes both waist and hip circumference measures. Over 15,000 men and women with at least an 18.5 BMI (the minimum of what is considered to be normal weight) and no history of cancer other than skin cancer were included in their analysis.

The BMI and WHRs of those participants who passed away before the conclusion of the study, along with their cause of death, were compared to the BMI and WHRs of those who did not.

The Results

Somewhat unsurprisingly, those men whose BMI was in the normal range but had a WHR of .90 or greater (considered centrally obese by the World Healthy Organization) were 87% more likely to die of any cause than those men whose BMI and WHR were both in the normal range. In fact, they were more than twice as likely to die from any cause than a man whose BMI was in the clinically overweight range (25 to 30) but did not carry his weight around his middle.

The results were similar for women: those with a normal BMI but a WHR above .85 were 48% more likely to die of any cause than those women whose BMI and WHR were normal. They were as much as 40% more likely to die of any cause than a woman with a BMI over 25 but with a normal WHR.

The risk of death from cardiovascular disease, specifically, was similarly surprising for both men and women: a man of normal BMI but high WHR was 78% more likely to die of heart disease than a man with BMI and WHR in the normal range. A woman of normal BMI and high WHR was over twice as likely to die of heart disease than a woman of normal BMI and WHR.

What’s the “Take-Home”?

Don't take this to mean that BMI is worthless. There is still a wealth of data linking greater BMI with increased risk of heart disease, diabetes, and cancer, among other diseases. What this does mean is that both BMI and WHR should be considered together in light of the greater risk to those with a higher WHR but normal BMI. If you feel you need to work on your waist, and not necessarily your weight, a Mediterranean style diet has been shown to help reduce that centrally-deposited fat.

Reference:

Sahakyan KR, Somers VK, Hodge DO, et al. Normal-weight central obesity: implications for total and cardiovascular mortality. Annals of Internal Medicine. 2015;163(11):827-835.