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We’re sure you’ve heard of Body Mass Index (BMI) – it’s that number (or so we’ve been told) tells us if we are fit or not. But what’s Basal Metabolic Rate (BMR)? And should we be focusing on BMR instead?
That’s what this article aims to decipher. Let’s look at what BMI and BMR are, how they are calculated, their key differences, and what role they can play in our health and fitness.
Understanding BMI and BMR
What is BMI?
Devised by Adolphe Quetelet in the 1800s, Body Mass Index (BMI) has become the de facto method for determining fitness levels. Here’s the formula:
Based on this calculation, adults are categorised into the following ranges:
- Under 18.5: Underweight
- 18.5 to 24.9: Normal weight
- 25 to 29.9: Overweight
- 30 and above: Obese
What is BMR?
Let’s say you’re lying in bed all day and doing absolutely nothing! Now, you might think your body isn’t working at all, but that’s not true. Even at rest, your body keeps on performing basic life-sustaining functions like breathing, circulation, cell production, and nutrient processing. Basal Metabolic Rate (BMR) is nothing but a measure of the number of calories your body requires to perform all these activities.
Once you understand your BMR, you can manage your caloric intake and design an effective diet plan.
Calculating BMR:
The Harris-Benedict equation is a widely used formula to estimate BMR:
Key Differences Between BMI and BMR
Before you decide to use BMI and BMR for mapping your health journey, it’s important to understand some key differences:
Purpose
- BMI (Body Mass Index) is primarily used to categorise individuals based on their body weight relative to their height and assess associated health risks. It provides a quick, simple method to gauge whether someone is underweight, normal weight, overweight, or obese, and by extension, if they are at increased risk for health problems like heart disease, diabetes, and hypertension.
- BMR (Basal Metabolic Rate), on the other hand, quantifies the minimum amount of energy (in calories) that your body requires to complete its normal functions at rest, including breathing, circulation, cellular production, and nutrient processing. BMR is critical for calculating caloric needs, which can guide dietary interventions and weight management plans.
Components
- BMI calculations are straightforward and only require two variables: weight and height. The formula is widely accessible and easy to compute, which makes BMI a universal tool for preliminary health screenings.
- BMR incorporates a broader range of factors, including age, biological sex, height, and weight, reflecting its more complex nature. The most commonly used formulas to estimate BMR, the Harris-Benedict equations, account for these variables to provide a more personalised measure of caloric needs.
Application
- BMI is widely used in public health surveys and clinical settings as a standard tool for identifying potential weight-related health issues across populations. It helps in epidemiological studies, enabling health authorities to monitor obesity trends and allocate resources for public health interventions.
- BMR offers more personalised information, which makes it invaluable for individuals seeking to manage their weight through tailored dietary strategies. Knowing your BMR can help you understand how many calories you should consume to lose weight, maintain weight, or even gain weight healthily.
Limitations
- BMI’s major limitation is its inability to differentiate between fat mass and muscle mass. This can lead to misclassification, particularly for athletes and bodybuilders who may have a high BMI due to increased muscle mass, not excess fat.
- BMR’s calculation limitations stem from its dependency on factors like muscle mass, which the basic formulas do not directly measure. Additionally, BMR can be influenced by factors such as genetics, environmental temperature, and hormonal status, which are not accounted for in the standard equations. This requires individuals to adjust their caloric intake based on lifestyle and activity levels, which can complicate dietary planning.
So, What’s Better For Weight Loss?
If your goal is to lose weight (and by that, we mean fat), then you need to create a caloric deficit. In other words,
Calories In should be less than Calories Out
If your goal is to put on weight, then you need to create a caloric surplus. In other words,
Calories In should be more than Calories Out
Now, how do you decide how many calories to consume? That’s where BMR comes in along with another key number – Total Daily Energy Expenditure or TDEE.
BMR, as we’ve seen, is the measure of how many calories your body requires to keep the lights on, so to speak. TDEE is the sum total of calories that your body requires to carry out all your daily activities – walking, running, buying groceries, going to the gym, your post dinner walk, and much more.
In short, TDEE = BMR + All Physical Activities
This free tool from FITTR can give you your BMR and TDEE.
Once you know these two numbers, you can then decide how many calories to consume in a day as per your fitness goal.
