What’s the difference between BMI and BMR?

Performance nutritionist Stephan Smith explains the difference between BMI and BMR, and why BMR is more relevant for athletes

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The world of sports science and nutrition is awash with abbreviations – BMD, HR, HRV, BMI, ATP – the list goes on. But two we see quite often are BMI and BMR, and here we explain what each means in greater detail, and why BMR is a more relevant number for athletes.

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What is BMI?

BMI stands for Body Mass Index. Your BMI is determined from your body mass and height and is calculated by the following equation:

BMI = body mass in kg

            Height in metres2

BMI is used mostly by health professionals to determine a person’s risk factor for diseases, such as cardiovascular conditions, certain cancers or type 2 diabetes. While a BMI between 18.5 kg/m2 and 25 kg/m2 is considered normal, the different classifications are:

<18.5 Underweight
18.5-24.9 Normal weight
25.0-29.9 Overweight
30.0-34.9 Obesity class I
35.0-39.9 Obesity class II
>40.0 Obesity class III (extreme obesity)

Now, you may be reading this as a fit and healthy individual, and after working out your BMI realise you fall into an overweight or obese class. This is one of the problems with BMI – it isn’t the best measure for everyone, particularly fit and active people with a high amount of muscle mass (which triathletes fall into!).

For example, let’s say we have two men who are the same height and both weigh 100 kg, but one of these men is a bodybuilder with a low body fat percentage while the other is overweight and doesn’t exercise. By the BMI system, the bodybuilder would be in the same class as the overweight man.

What is BMR?         

Another abbreviation we often see in training magazines is BMR, or Basal Metabolic Rate. Now, just to make things more confusing, BMR is often interchanged with RMR (Resting Metabolic Rate), which is also referred to as REE (Resting Energy Expenditure)!

Despite the different abbreviations, BMR, RMR and REE are all simply techniques used by scientists to measure the amount of energy a person requires to perform normal bodily functions. What a lot of people don’t realise is that even if you lie on the couch all day your body would still burn a certain number of calories to keep you alive, to provide energy for functions such as breathing, blood circulation and thermoregulation.

The difference between the tests is down to the pre-test conditions. For a true BMR measurement, the test should be performed on someone in a darkened room, after eight hours sleep and a 12hr fast, and requires the person to sleep in the testing facility.

RMR or REE tests are less restrictive, and normally only require someone to arrive after an overnight fast, without drinking any caffeine. Both tests involve a person lying or sitting still for about 30mins while they have their breath analysed for oxygen and carbon dioxide, similar to what you see when someone’s completing a VO2 max test. Your RMR is correlated to the amount of muscle mass you have, with more muscle leading to a higher RMR. This makes sense, as muscle tissue requires a large amount of energy to maintain.

To summarise, BMI stands for Body Mass Index and is related to your height and weight. Whereas BMR, or RMR, represents the amount of calories your body requires to perform essential functions.

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Stephen Smith is a SENr registered sports nutrition consultant, and is the owner and founder of Race Faster. He is currently researching gut health and the effects of exercise on the gut for his PhD. You can follow him on Twitter @stephensmithPN