Science Matters: What’s that about BMI?

As marches are happening in 360 cities around the world, I invite you to join me on this virtual march to stand up for the importance of science in your life and for your health.

At the heart of science is an essential balance between two seemingly contradictory attitudes — an openness to new ideas, no matter how bizarre or counterintuitive they may be, and the most ruthless skeptical scrutiny of all ideas, old and new. This is how deep truths are winnowed from deep nonsense.” – Carl Sagan

In today’s time, nutrition information can especially be challenging to decipher. You can find citations on all platforms including Buzzfeed, Facebook, the News, the radio, Women’s Health Magazine, the Cosmopolitan… type into the Google search bar, and you’ll get a million hits of varying nutrition claims and diets. Each quoting studies and conclusions contradicting the next article you read or hear. In a world where everyone is a “nutrition expert”, you never know what to make of the advice. What has come out of this whirlwind of vastly available information? Distrust in science, in all disciplines. Or perhaps disappointment leading to a loss of credibility. In the field of nutrition, especially. Possibly due to the fact that nutrition is a more new and emerging science, so even our national nutrition guidelines are changing with time. Initially we said to avoid feeding infants high allergen risk foods, whereas now it is suggested that exposure to peanuts early in childhood can prevent the development of peanut allergies (Dietitians of Canada, 2017).

What am I getting at here? I think what needs to happen is for everyone to step back and remember what the art of science is and what it means to be a scientist. What I am trying to say is that there is nothing wrong with challenging research, developing competing hypotheses and looking for flaws in studies. After all, that is how science works. It is not black and white, and not everything is like the laws of thermodynamics; there is no “absolute” zero for everything. It starts with the development of a hypothesis and then the creation of a way to measure and test the theory. It depends heavily on what question is asked – that, I think is something many of us have forgotten.

Take BMI (body mass index) for instance.

It is widely recommended and used as the “best measure of obesity in population and clinical studies”. It is what is used to label someone as “healthy weight”, “over-” or “under-” weight; and thereby informing if they are “at risk” for obesity-related conditions or premature mortality. It is a simple measure of height divided by height squared (kg/m2 ). It dates back to a Belgian statistician in the 1850s, Adolphe Quételet, who described this index as a way to characterize the level of adiposity in sedentary adults.

Let us think critically about the information we just gathered for a moment….

The equation is a measure of two parameters: 1) height, and 2) weight. We know that health and obesity-related comorbidities are associated with excess body fat (or adipose).

So why is it that we are using this index to define whether or not a surgery is safe to proceed, or to give nutrition advice for health? It is an inexpensive, quick, simple and non-invasive surrogate measure of of body fat. Key word here being “surrogate” – in other scientific words, it is an index of high precision, but low accuracy.


Sure, studies have shown that BMI levels correlate with body fat and with future health risks. But we need to remember that correlation is not equation.

BMI may be an appropriate measure for screening adiposity-related health risks, however, it does should not be used as a diagnosis given its clinical limitations. The relationship between BMI and body fat is altered by factors such as age, sex, ethnicity and muscle mass or body composition. BMI does not distinguish weight between excess fat vs. muscle or bone mass, and provides no indication of the distribution of each component amongst individuals.  In truth, health is complex and the human body is an intricate network of systems; “you simply cannot step on a scale and decide whether you’re healthy or not” (Dr.Arya Sharma, founder of the Canadian Obesity Network).

As healthcare professionals, it is important to see each patient or client as an individual and to assess obesity and health risks using a variety of indices and parameters. To withhold weight bias and determine the most appropriate intervention and medical advice for mental and physical health by gathering all data available, keep an open-mind and using our clinical judgement whilst remembering the limitations in the way data had been collected.

“…because evidence-based decision-making ensures implementation of the best interventions” – Jane Bellman, MEd, RD

Science is not everything. But it is critical to governing, decision-making and protecting human health and the environment. It challenges our current reality and our existence. It challenges our belief systems and sparks curiosity – a key ingredient in creation, innovation and propels us forward in our decision-making, and provides framework for our governing and our practice. By filtering through the evidence, it provides us parameters to develop healthy public policies by more than just opinions. Finally, in the words of a colleague: “It illuminates the wonder we can have our amazing planet… the wonder in the small and big! [And most of all,] It keeps us humble…” – Ruth Vo.

I challenge everyone to be scientists with your everyday encounters with nutrition knowledge (and with all other domains of information). Be critical, analytical, and most of all, do it with an open-mind and heart full of curiosity in wanting to know the “why” and “how”.

Here are a couple of red flags for identifying nutrition misinformation:

  • Recommendations that promise a quick fix.
  • Dire warnings of danger from a single product or regimen.
  • Claims that sound too good to be true.
  • Simplistic conclusions drawn from a complex study.
  • Recommendations based on a single study.
  • Dramatic statements that are refuted by reputable scientific organizations.
  • Lists of “good” and “bad” foods.
  • “Spinning” information from another product to match the producer’s claims.
  • Stating that research is “currently underway,” indicating that there is no current research.
  • Non-science based testimonials supporting the product, often from celebrities or highly satisfied customers.

Dietitians as health care professionals, are committed to using their specialized knowledge and skills in food and nutrition to improve the health of the population. Dietitians are one of a kind; they are uniquely trained to advise you on food, healthy eating and nutrition.

Dietitians must be part of a regulatory body, just like doctors, pharmacists and nurses. Only qualified health professionals can legally use the term “dietitian”. In many places, there are no laws to protect the title “nutritionist.” When in doubt, check your local regulatory body. A dietitian is your smart choice for credible advice on healthy eating.

(From Colorado State University’s Factsheet on Nutrition Misinformation)


Cited Sources 

Dietitians of Canada (2017). Introduction of Allergenic Foods to Infants, especially Peanuts: Interim Guidance for Canadian Dietitians.

Sharma, Arya (2007). Why I Don’t Like BMI.

Sharma, Arya (2008). Accuracy of BMI for Diagnosing Obesity.

Sharma, Arya (2008). Obesity Classification: Time to Move Beyond BMI?

Centers for Disease Control and Prevention. Body Mass Index: Considerations for Practitioners.

Colorado State University. Nutrition Misinformation: How to Identify Fraud and Misleading Claims.




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