Many consider body mass index (BMI) to be an imperfect measure of obesity, for a number of reasons. It might misclassify someone who is short and muscular as obese. Or, it might miss people who have levels of excess body fat that put them at increased risk of obesity-related diseases, such as heart disease, stroke, cancer and type 2 diabetes. In addition, commonly used BMI cutoffs to diagnose obesity can miss people with excess body fat.1 However, since the current databases that are available measure overweight and obesity based on BMI, that is what is presented in this Module. More work needs to be done in this area.
1Vogel. L. Overweight or overfat? Many Canadians are both. CMAJ 2017 Sept 18; 189(37): E1202-E1203.
A number of factors can affect weight, including growth spurts. Some children and youth grow in weight before they grow in height and some grow in height first and then their weight catches up. It is normal for there to be a mismatch in height and weight during a growth spurt. As well, weight does not capture one’s body composition. Someone with strong bones and lots of muscle mass will weigh more even if they are the same physical size as another. BMI can also be inaccurate. It also does not tell us how much of the body composition is muscle and strong, dense bones. This can result in a mistake in how we categorize people.2 A number of athletes fall into the overweight and obese categories of BMI.3,4 It is also important to know that in Canada, people whose BMI is between 25-30 (misnamed the overweight category) live the longest and the differences between mortality rates among people who fall into the group below (20-25) or above (30-35) are only slightly higher. This means that for most Canadians, who have a BMI between 20-35, they live equally as long. It is only at the extreme ends of the BMI range that BMI starts to shorten life span. While there are some health risks associated with being in the obese BMI categories, this is not true for the overweight category.5
2Rothman KJ. BMI-related errors in the measurement of obesity. International Journal of Obesity 2008; 32: S56–S59.
3Overweight Olympians: Guess the BMI of top athletes. New Scientist. May 15, 2014. https://www.newscientist.com/gallery/obese-olympians/
4Chen B. If BMI Is The Test Of Health, Many Pro Athletes Would Flunk. Shots: Health News from NPR. February 4, 2016. https://www.npr.org/sections/health-shots/2016/02/04/465569465/if-bmi-is-the-test-of-health-many-pro-athletes-would-flunk
5Orpana, H. M., Berthelot, J. , Kaplan, M. S., Feeny, D. H., McFarland, B. and Ross, N. A. (2010), BMI and Mortality: Results From a National Longitudinal Study of Canadian Adults. Obesity, 18: 214-218. https://onlinelibrary.wiley.com/doi/full/10.1038/oby.2009.191