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Megan Best
Megan Best

RDN, CPT

Why BMI is bullshit

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Yes, you heard me right. And yes, I am a Registered Dietitian who fully stands by this statement. So let’s talk about it…

What is BMI?

Body mass index (BMI) is a measurement that only takes height and weight into account. To calculate your BMI, divide your weight in kilograms by your height in meters squared.

BMI was introduced nearly 200 years ago by a Belgian named Lambert Adolphe Jacques Quetelet. He was a mathematician (not a physician or other healthcare professional) who came up with the calculation to give a quick and easy way to measure the degree of obesity of the general population in order to assist the government in allocating resources. Besides not taking into account proportions of bone, muscle and fat in the body, BMI is also purported to have racist undertones based on some of Quetelet’s affiliations. PLUS many obesity experts have come out and said they don’t believe we should still use this metric as an indicator of health. Yet, if you step into your doctors’ office they most likely have a large BMI chart glaring at you from the wall.

Various health professionals use BMI to calculate whether a person is underweight, normal weight, overweight, or obese. However in many cases, especially with athletes, it has no relation to the amount of body fat a person carries. Some people’s weight and height measurements put them in the overweight or obese category while their body composition may be lean and muscular. On the other hand, some people’s BMI indicates that they are healthy, when they are carrying too much fatty tissue.

Better Indicators of Health

Body Fat

Whether or not your BMI indicates that you are overweight, it can be more productive to find out where your body fat lies.

When most people refer to body fat or wanting to lose fat, they are referring to subcutaneous fat– the kind that lies in a layer just beneath your skin. This fat, while many consider it less than ideal, actually produces some beneficial molecules (like Adiponectin, a hormone that helps protect against diabetes and inflammation). Visceral fat, on the other hand, is a key player in many health problems.

Visceral fat lies beneath the abdominal wall in the spaces surrounding the liver, intestines, and other organs. Researchers have linked visceral fat to a number of chronic conditions, including:

  • Cardiovascular Disease
  • Some cancers
  • Asthma
  • Dementia

What is generally considered “healthy” body fat ranges from 21-35% in women and 8-24%. Females get a little bit more leeway as we are simply built to carry more, and as each gender ages the “acceptable” numbers creep up a bit.

It is, however, important to keep in mind that body fat is going to vary in different populations, and just because some have a higher range does NOT make them unhealthy (particularly of subcutaneous fat- please refer to aforementioned visceral fat slide). Some of the factors that may affect body fat that are not within a person’s control include:

  • Age
  • Gender
  • Race
  • Ethnicity
  • Genetics

So how do we determine our body fat? There is not a simple calculation that produces accurate results, however there are a variety of ways to measure body fat (which also vary in accuracy):

  • Skin Calipers (the least reliable)
  • Bioelectrical Impedance Analysis (like an Inbody scale)
  • Air Displacement Plethysmography/Bod Pod
  • Dual X-Ray Absorptiometry (DEXA) Scan
  • Underwater Weighing (the gold standard)
Now, let’s talk about other ways to measure health.

Metrics that have nothing to do with body size:

  • Weightifting Personal Bests
  • Ability to increase workout volume
  • Faster mile times
  • Longer distances run/rowed/biked
  • Improved health markers

Other data

  • The way clothes fit
  • Energy levels throughout the day
  • Mood
  • Relationship with food

The Bottom Line

Whether we’re discussing BMI, body fat, or other measures of health, these guidelines are general and apply on a more broad, public health level.

Certainly if you or your family member falls outside of healthy ranges, it’s worth meeting with a healthcare professional or participating in health promoting behaviors like exercise and good nutrition (if not already). However, just because someone falls outside of these parameters does not automatically make them unhealthy.

Don’t judge based on appearance alone. Dig deeper. Especially if you are a healthcare professional.

Sick of being judged by your BMI and want to work on other measures of health? We focus on all things non scale victories in my 1:1 coaching program.

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