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Tyler Norman, Ph.D.
Manager of Corporate Outreach
ACSM Exercise Physiologist
NCFI Corporate Fitness Specialist
West Hills Athletic Club, WMU

The Goldfish Analogy…Basal Metabolic Rate vs. Body Mass Index

I often explain the difference between Basal Metabolic Rate (BMR) and Body Mass Index (BMI). They're easy to mix up because the anacronyms are so close and they are both generally used in the context of weight management. Although completely different, they do parallel each other in some ways. BMR is the number of calories that your body needs to sustain itself at a state of rest. If you laid in bed all day, it's the number of calories your body will use over the course of the day. To determine an individual's Total Energy Expenditure (TEE), an activity factor must be included in the calculation. If you are very active or have a job that is physical in nature, you would have a higher activity factor and subsequently burn more calories over the course of the day than someone who is less active or had a sedentary job. In the application of weight management or energy balance, BMR is an important piece of the puzzle. We all know that if we want to lose weight, we must create a caloric deficit. But how do I know what a deficit is? Without the BMR calculation we are simply guessing. Conversely, if I want to gain weight, I must create a caloric surplus. How do I know what a surplus is? It's the guessing that usually leads to little or no progress in our weight management efforts. Additionally, it's important to know our BMR so we don't create too much of a caloric deficit, putting us at risk of going into what's known as "caloric debt," which is when our metabolism uses our precious muscle for fuel. BMR can be calculated by a fitness professional or dietician after conducting a body composition analysis, which tells us the percentage of fat weight vs. lean weight. Once you know your BMR and subsequently your TEE, you can now accurately determine a caloric surplus or deficit. It is generally recommended not to exceed a 500 calorie per day surplus or deficit. I sometimes use the fishbowl analogy to explain BMR. The goldfish grows to the size of the bowl, but if you put it in a bigger bowl, it gets bigger. Your BMR determines the size of your fishbowl. If your averaging 2000 calories a day to maintain your current bodyweight, and you want a smaller fishbowl, consume 1500 calories per day and you get a smaller fishbowl. BMI Is a generic calculation using your height and weight to determine if you are underweight, normal weight, overweight, obese, or morbidly obese. Its primary application is to determine if we are at risk for a variety of coronary risk factors like hypertension and diabetes because of our weight. While it is generally used by the medical and insurance communities, fitness professionals like myself do not place much value on BMI as it does not account for lean vs. fat weight. Subsequently, anyone with above average muscle mass will be labeled overweight or obese. For example, anyone with significantly above average muscle mass like Serena Williams, Michael Phelps, and Michael Jordan are categorized as morbidly obese.

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