Using Basal Metabolic Rate to understand risk of weight gain

Our bodies require regular energy intake - generally from food and drink - in order to survive and function. After digesting carbohydrates, fats, proteins, etc. in our food, our body then needs to use this energy for things like blood circulation, maintaining body temperature, breathing, and brain function, even before talking about daily physical activities, such as walking or exercise. Roughly speaking, this is the definition of Basal Metabolic Rate (BMR) - "the energy required to sustain the body while you're physically and mentally not doing anything".

How high or low a person's BMR is, depends on various factors, including age, body composition, and gender.

BMR tends to be highest during childhood and puberty, which explains why it seems many teenagers can seem to eat an unlimited amount of food without getting fat! It tends to remain relatively constant in adults, and gradually declines as you start to age.

A large factor affecting the ease of weight gain is besides diet, genetics, medication, or hormones, is BMR - simply put, it affects how much energy your body burns on a daily basis.

Besides the aforementioned factors, what has the biggest effect on BMR? Muscle mass! Muscle burns a lot more calories than fat! An athletic person with a higher muscle percentage will need more energy to sustain their body, than someone else of the same weight, but with more fat and less muscle. If the amount of energy/calories consumed each day is more than the energy spent, then weight gain is likely.

Let's look at the body composition of two different people:

Julia

Kevin

Julia is significantly lighter than Kevin in weight. However, what's important to note is that Julia also has significantly lower Skeletal Muscle Mass and Basal Metabolic Rate, compared to Kevin.

We can see that Kevin's body requires more energy to maintain their muscle, and thus it's generally harder for him to inadvertently gain weight, since their bodies uses up energy more quickly. In general, all else being equal (same age, gender) - if two people have the same body weight, the person with more muscle will have a higher BMR.

Now, to be clear, overall BMR doesn't necessarily make that much of a difference, by itself. If you want to lose weight, your diet and level of activity generally has a more noticeable impact on the number of calories consumed and spent, than BMR. However, all else being equal, conducting strength training to increase your muscle mass and thus BMR will make it easier to keep weight off long term, compared to just a temporary reduction in calories and exercising more each day.

To use BMR to estimate Total Energy Expenditure (TEE), which is the amount of energy you actually use each day (since, if you're reading this, that means your body is not in a vegetative state that only uses a minimum of energy), you need to multiply your BMR with a Physical Activity Level (PAL). This is actually pretty straightforward!

Now, there's no one perfect answer for how to calculate TEE. For example, the World Health Organization's data suggests:

But we can break it down further, to closer approximate your activity level and calculate how many calories you might burn in a day:

Let's imagine that Julia and Kevin both work at desk jobs with little exercise. In this case, their Total Energy Expenditure for a day would be:

Julia: BMR 1086 x PAL 1.2 = 1303

Kevin: BMR 1740 x PAL 1.2 = 2088

We can see that even if two people do the exact same thing in a day, the amount of energy burned will not be the same, due to difference in BMR. This difference becomes even more pronounced with increasing levels of activity!

So now let's imagine that Julia and Kevin both play sports 4 days a week, for a PAL of 1.5. Their TEE for a day would be:

Julia: BMR 1086 x PAL 1.5 = 1629

Kevin: BMR 1740 x PAL 1.5 = 2610

An almost 1000 difference in calorie expenditure! That could be more than 2 cheeseburgers (depending on the size of the burger, of course), and it adds up. No wonder it's easier for Kevin to avoid weight gain if he wants - it can be easier to keep caloric intake under 2610 than 1629.

So how much should you eat a day? Obviously, this depends on your goals: are you trying to increase weight and muscle? Or are you trying to burn fat and get lighter? Using body composition analysis, you can say good-bye to simplified "2000 calories per day" recommendations, and determine the appropriate amount of activity and energy intake, based on your specific needs.