Calling BS on the BMI!

Written by Suzanne Dietrich, RD,  Gut Instincts Nutrition Counselling

 With contributions from Carly Werner, RD & Katie McCrindle, MSW

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Please note that in this blog post we will be discussing weight, which we acknowledge may be triggering and sensitive reading material for some readers.

Genetics, for the most part, predisposes us to a certain set point weight, regardless of what the Body Mass Index chart tells us. Some people refer to the BMI as a tool to determine how much they should weigh and/or if they are healthy. This blog will explain why this can be problematic and introduces the Set Point Theory of Weight.  

What is the BMI & Where did it come from?

The BMI is a ratio of weight (kg) to height (m2). Period.

The BMI was developed by a man named Adolphe Quintlet in the mid-1800s who did not work in health at all - he was a mathematician, astronomer and statistician. He was curious about the relationship between a person’s height and weight and wanted to see what the “average” person would look like (here).

 How is BMI being used? 

 Researchers study the relationship between BMI and many factors including mortality and morbidity. The BMI was developed as a population measure rather than an individual assessment tool (here) and should not be used as such.

Some people calculate their BMI to see if their weight is “where it should be.” It is used inaccurately by some in the fitness or health field as a tool to convey the message of what a “normal weight” is for a person. However, BMI cannot determine what a person’s weight should be. In fact, I don’t know a tool that can. We don’t know what a “normal weight” is for a person based on calculations. 

The naming and use of the BMI categories are weight stigmatizing. 

BMI numbers have been categorized into sections in ways that stigmatize people based on their weight. The use of terms such as “underweight,” “overweight,” “normal weight,” and “obese” are problematic. As my esteemed colleague Fiona Willer, PhD, RD shared in her podcast (here) – “it would have been a lot less stigmatizing if they were labeled category A, B, C & D.” The weight categories are completely arbitrary. 

In addition, these categories suddenly changed in 1998 in the US which meant that many people fitting in the “normal weight” category went to bed one night and then work up, in the same body... to now be called “overweight!" The cut-offs were lowered by 2.3-2.5, which meant more people were not considered "normal weight." We have to remember that weight loss is big business not only in the diet industry but also in the pharmaceutical industry. These new categories helped normalize weight loss medications and bariatric surgery, which can have severe health consequences for some but profitable outcomes for these industries. 

Oddly, the BMI categorizations were not determined based on the distribution of BMI’s in human, well-nourished population, but in fact, they were determined based on stats of dead men who were measured in the 1940s (here and here). In our “thin-centric society” and “thin=healthy culture” all of these terms induce judgment about one’s weight and the categories imply that many individuals are the wrong size and their body is a problem that needs to be fixed. 

Because weight has been targeted as a body attribute that is personally modifiable or based on lifestyle choice there is much judgment placed on one’s weight. We don’t hear the message that someone’s feet are the wrong size! 

Doesn’t BMI Determine Health Status?...NO!!

The Centers for Disease Control shares that BMI is not diagnostic of the health of an individual (here). 

People of all sizes get sick.  If we look at what is really making Canadians healthy and unhealthy as stated by the Government of Canada (here) it is these key factors:

  • income and social status

  • social support networks

  • education employment/working conditions

  • social environments

  • personal health practices and coping skills

  • healthy child development

  • biology and genetic endowment

  • health services

  • gender

  • culture

Weight is not mentioned here. We cannot assume that a person’s size is equal to a person’s risk for disease. 

Doesn't a higher BMI indicate and increased death rate?...NO

In a systematic review of 97 journal articles, it was found that the individuals in the “overweight” category were associated with a significantly lower mortality rate, even compared to the “normal weight” category (here). 

What is Set Point Weight Theory?

So you can see why we are calling "BS" on the BMI! Despite a widespread belief (*sarcasm* thanks a lot, diet culture!) about being able to control our body shape and size with some food and activity changes, the set point theory maintains that we actually have very little control over our weight. 

Our set point weight is the weight range that our body wants to maintain - usually within about a 10-20lb range. There are mechanisms in place to keep our weight in that pre-programmed range. Think about how our bodies want to maintain other vital functions - temperature, blood glucose levels, heart rate. It makes sense that our weight would be another thing on the list.

Dieting can mess with our body’s natural weight range. Dieting (any sort of caloric restriction) feels like famine to our bodies and they adapt accordingly. Metabolic rate (the speed at which our body uses food fuel for energy) slows to be able to survive on less food. And hunger signals seriously increase. If you’ve ever been on a diet, you might notice that your sense of taste and smell are heightened. This feature pushes us to seek out food when our body believes it is scarce. 

Once the famine (ie: diet) is over, the body seeks and stores as much food energy as possible in order to have a better shot at surviving the next “famine.”This is the reason that most people who follow diets end up gaining back even more weight than where they started and this higher weight becomes your body’s new set point. 

How do you figure out your set point? 

It has nothing to do with consulting a BMI (Body Mass Index) chart. It’s a long term process of learning to listen to your body’s hunger and fullness cues. Your set point weight will be the range that your body naturally falls into when you are feeding and moving your body with mindfulness and when many of those key factors that influence health are in a good spot. 

If you would like more help determining your set point, visit our list of dieticians here.