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How Accurate Is BMI?

Body Mass Index (BMI, kg/m2 ) has been around for quite a while now. According to Diabetes.co.uk, BMI is a useful measurement for most people over 18, and one of the best methods of assessing if people are overweight or obese. [2]. It is also a non-invasive and relatively cheap method of assessment, relying on valid weighing scales and height measurement and therefore can be carried out in most settings.


The latest NICE guidelines updated in 2014 [3] classify BMI as follows:

  • Classification BMI (kg/m 2 )
  • Healthy weight 18.5–24.9
  • Overweight 25–29.9
  • Obesity I 30–34.9
  • Obesity II 35–39.9
  • Obesity III 40 or more

  • They advise to use BMI as a practical estimate of adiposity in adults. Now the word ‘estimate’ is very important here. Why? This is because BMI is not a direct measure of adiposity. And thus the debate begins.

    Numerous studies have shown that BMI does not accurately measure body fat [4,5]. Some people who present with a normal weight for their height carry far too much body fat, classified as 30 percent or more for women and 20 percent or more for men, putting them in the category of normal weight obesity. [6]

    These people may appear slim, but they are vulnerable to some of the same medical risks that present in people who are clearly overweight, including type 2 diabetes and heart disease. A person whose BMI is high, but who carries a fair amount of subcutaneous fat in the hips and rear, may have a lower health risk compared to a person who has the same BMI but stores most of the fat in the abdominal area. Another study that used the BMI categories as the main indicator of health, showed that thousands of adults may be misclassified as cardio-metabolically unhealthy or cardio-metabolically healthy, for the reasons as described above. [4].

    There is unanimous agreement that the health risks of obesity and metabolic syndrome are associated more with central obesity than with total obesity. [7] Therefore, it is important to use other measures of assessment, in addition to BMI calculation to establish a person’s overall risk of cardiovascular disease. NICE advises to use waist circumference measurement alongside BMI certainly in people whose BMI is less than 35kg/m2. [3]

    Dr. Margaret Ashwell, the highly respected obesity specialist most noted for her Shape Charts, recommends using waist to height ratio as it is a more sensitive method of assessing early warning health risk compared to BMI. [7]

    We know that the use of body weight by itself and/or BMI has been criticized, particularly in athletic populations. [8]. BMI does not distinguish between body fat and muscle mass, which is why muscular athletes may have a high BMI because of increased muscle mass rather than additional body fat.[9]. In athletes, it is more important to assess the distribution of fat over the body [9]. Therefore, the use of total body fat % and subcutaneous fat patterns (using skin fold callipers) may be more effective than BMI in assessing fatness and obesity in physically active individuals and young adults. [9].


    So as Nutrition and Weight Management advisors, where do we go from here?

    Firstly clients need to be reminded that increased exercise is associated with a reduction in mortality and morbidity and this may lead to an increase in muscle mass, therefore no change to a person’s weight or BMI. Exercise can help reduce abdominal fat by 10 to 20 percent without a notable change in weight, so a person may be getting healthier but the BMI doesn't show it. [10]. In a non-clinical setting, BMI, alongside other valid measurements such as waist and hip circumference and total body fat %, are reliable and acceptable predictions of a person’s overall cardiovascular ill health risk. In addition to body measurements and objective data, assessing clients in a holistic fashion, including monitoring their dietary, lifestyle and activity patterns using Department of Health guidelines will provide a reliable set of health facts on which to base safe and reliable recommendations for clients and provide them with a positive understanding of their current and future relative ill health risk.


    By Susan Barry, dietitian and Future Fit Training tutor


    References

    1. Garrow JS, Webster JD (1985) Quetelet’s index (W/H2) as a measure of fatness. Int J Obes 9: 147–153.
    2. https://www.diabetes.co.uk/bmi/how-accurate-is-bmi...
    3. https://www.nice.org.uk/guidance/cg189/chapter/1-r...
    4. Tomiyama AJ, Humger AM et al, (2016) Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. Int J Obes 40: 883-886
    5. Romerro- Corral A, Somers VK et al, (20018) Accuracy of body mass index in diagnosing obesity in the adult general population. Int J Obes 32: 959-966
    6. https://www.livestrong.com/article/93472-problems-...
    7. Ashewell M, Hsieh SD, (2005) Effective global indicator for health risks of obesity and how its use could simplify the international public health message on obesity. Int J Food Sciences and Nut: 56(5) 303-307
    8. Moon JR, Eckerson JM, Tobkin SE, Smith AE, Lockwood CM, et al. (2009) Estimating body fat in NCAA Division I female athletes: a five-compartment model validation of laboratory methods. Eur J Appl Physiol 105: 119–130.
    9. Ogden CL, Carroll MD, Curtin LR, McDowell MA, Tabak CJ, et al. (2006) Prevalence of overweight and obesity in the United States, 1999–2004. JAMA 295: 1549–1555
    10. Saunders TJ, Palombella A et al, (2012) Acute Exercise Increases Adiponectin Levels in Abdominally Obese Men. J Nut & Met (10) 1-6.