A Little More Each Day

One working mama learning to run & to maintain my 100+ pound weight loss!

Weight Loss Wednesday: TMI on BMI

on November 6, 2013

Sorry for the title 🙂

Like I mentioned in my November goals post, I’m currently struggling with deciding exactly what my goal weight is going to be. There really isn’t a great scientific answer about what a healthy weight really is but BMI (Body Mass Index) is one of the more commonly cited measures used to define healthy weights.

Body Mass Index (BMI) is a formula (kg/m2) used to approximate your body fat based on height and weight. The formula has actually been around since the 1800s, but became an area of increased focus in the US in the 1970s, when we began seeing an increase in obesity and obesity-related diseases. For adults over age 20, the same formula applies (BMI is adjusted for age and gender if you’re less than 20 years old). Early research showed that this measurement correlated well to direct measures of body fat like underwater weighing and dual energy x-ray absorptiometry. It is useful for population-wide studies because it is very cheap and easy to obtain, requiring quick and simple measurements that are frequently obtained during normal medical care.

According to current BMI classifications (these  have changed over the years):

  • Underweight = BMI less than 18.5
  • Normal weight = BMI 18.5 – 24.9
  • Overweight = BMI 25 – 29.9
  • Obesity = BMI over 30 (Class I = 30 – 34.9; Class II = 35 – 39.9;  Extreme/Class III/Morbid Obesity = BMI over 40)

For reference, my BMI when I joined Weight Watchers in July 2012 was 41.8 and is currently 25.5. (274.6 lbs to 167.7 lbs at 5 feet, 8 inches tall)

By using BMI to look at large populations of people, relationships have been identified between elevated BMI and heart disease, high blood pressure, diabetes and other things that we now think of as obesity-related disease. It has been very helpful as a research tool on a population-wide basis. However, that doesn’t necessarily mean it is useful on an individual basis and resources such as the CDC make it clear that this is a screening tool, not something diagnostic. An individual’s specific risk encompasses a lot more than this specific number – it includes their lifestyle choices (diet, exercise, tobacco), past medical history and family history. Unfortunately, some physicians, some insurance companies and other entities (like Weight Watchers in my case) look just at this number.

The CDC (Centers for Disease Control) and NIH (National Institutes of Health) are both clear about the limitations of BMI as an individual diagnostic test, including it’s overestimate of “obesity” in people with high muscle mass or large body habitus (like athletes) and underestimate of “obesity” in people with low muscle mass (like the elderly or the “skinny fat”). It also doesn’t take into account other things about body habitus that we now suspect are independent risk factors for obesity-related diseases, like waist circumference. Having increased fat stores around your waist (defined as a waist circumference >35 inches in women and >40 inches in men) is associated with heart disease and diabetes. BMI alone doesn’t take that into account, and so a pear shaped woman (lower risk) could have the same BMI as an apple-shaped woman. There are guidelines for incorporating both of these into assessing risk, but I can honestly say I’ve never had a physician ask or measure my waist circumference (and I really do have a good primary care physician).

The BMI group that has caused the most controversy recently is the range into which I fall: the “overweight” group (BMI 25-29.9). There have been several recent studies (large population surveys looking at BMI and various outcomes, not a single person) that have shown that people in the “overweight” group have no increase in risk compared to those of “normal” weight and may even have less risk of health problems (see the articles cited below). This may be because this group is one in which the correlation between body fat percentage and BMI is particularly poor or may be because patients who are slightly overweight are more likely to be engaged in medical care than those at normal weights and receive early screening for potentially life-threatening disease. That has not been determined at this point, although there are lots of theories. Over and over, studies have confirmed that those with BMI less than 18.5 and greater than 30 are at significant health risk, so it is a tool with some value, but like all tools, it must be used responsibly.

Ultimately, a healthy weight for you is something you need to talk about with your physician and something that is defined by far more than just BMI. I guess this means I should take my own advice and do that – is a BMI of 25.5 good enough for me, with my really great diet (if I do say so myself) and activity levels balanced by my history of high blood pressure, gestational diabetes (a risk factor for type II diabetes) and strong family history of diabetes? I think so, but maybe an official conversation about that is warranted and a letter to change my goal weight at Weight Watchers (if that is what I decide to do) is something I can get at that time. (This is really why I blog – talking to myself about things to help decide what direction to take, to continue moving forward in the healthiest possible direction. This conversation about BMI has been helpful for me even if isn’t for anyone else!) I know I could still lose another 10-15 pounds. It isn’t like I don’t have the body fat to spare. But honestly, for the first time in my adult life I am perfectly happy with the number on the scale (if BMI was out of the picture) and with my body.

You are always, always more than a number.

Sources for further reading:

National Institutes of Health

Centers for Disease Control

Romero-Corral A, et al. “Accuracy of Body Mass Index to Diagnose Obesity in the US Adult Population.” Int J Obes June 2008

Prentice AM, et al. “Beyond Body Mass Index.” Obesity Reviews August 2001

Romero-Corral A, et al. “Association of Body Weight with Total Mortality and with Cardiovascular Events in Coronary Artery Disease: A Systematic Review of Cohort Studies.” Lancet August 2006

Flegal KM, et al. “Excess Deaths Associated with Underweight, Overweight and Obesity.” JAMA April 2005

Schneider HJ, et al. “The Predictive Value of Different Measures of Obesity for Incident Cardiovascular Events and Mortality.” J Clin Endocrinology and Metabolism April 2010

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8 responses to “Weight Loss Wednesday: TMI on BMI

  1. Daphne says:

    It was strange to see the topic of your post today because I spent the morning reading up on BMI and waist/hip ratio. 🙂 I weighed in under 200 pounds for the first time in 17 years (and on my 35th birthday no less!) so I was researching BMI to determine what weight I need to be at to move out of the obese category in order to set my next goal weight. After reading several articles and your blog post I think I’ll go ahead and work towards a BMI under 29.9 just to have a focal point to move towards but not worry about it *too* much. Thanks for all the great info!

    P.S.
    I found your blog through Runs for Cookies a while back ago and have enjoyed visiting here every day since. I always learn something! Thanks for that!

    • Glad my rambling helps somebody! I definitely celebrated both the less than 200 lb milestone (good for you!! I know how huge that is!) and being only “overweight” – that definitely seems like a good point to stop and reassess.

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