How Many Calories Are You Burning?

How fast is your metabolism? Your body’s resting metabolic rate (RMR) is the cornerstone of your BODYSIM model. It dictates how much you can eat in a day, how quickly your body will lose fat or put on muscle, and very importantly for most of us–how likely that slice of pizza is to turn into fat. Most of our BODYSIM clients have some idea about what their RMR is, often based on rule of thumb calculators or a body fat measurement. However, even if that RMR is calculated from a DEXA scan body fat measurement, several more critical factors determine your true RMR.

Metabolism Calculators Are Too Limited

Body composition is the most important factor determining metabolism¹. A 200-lb male football player with 15% body fat will undoubtedly need more calories than a 120-lb female runner with 20% body fat. Calories are fuel and differently shaped bodies need different types of fuel! However, even in this study covering healthy college-age athletes, there is still variance. You may know that genetics and hormones can play an important role². Specifically, your RMR is determined beyond just your body composition (pounds of muscle and fat) by your levels of thyroid hormones, your percentage of fast-twitch versus slow-twitch muscle fibers, and how your history of exercise training has affected the number of mitochondria in your cells.

Interestingly, you can measure your RMR fairly accurately using a technique called “indirect calorimetry.” This is very different from an RMR _estimate_ you may see printed on your DEXA scan or that you can get from your fitness tracker or an online calculator. Those are all simple estimates because they don’t account for any of the factors we just mentioned above. Indirect calorimetry measures your RMR directly and indeed accounts for those factors by having you rest peacefully while wearing a mask that measures your total oxygen consumed and carbon dioxide produced over a representative time. From these numbers, a metabolic cart can directly quantify how much energy your body consumed on average during that time. We’ll see later, however, that this is only 1 of 5 necessary components of your Total Daily Energy Expenditure (TDEE), and that this number will change significantly on your fitness journey.

How does BODYSIM move beyond a calculator? The BODYSIM model works as a detective, using all the clues you provide to learn the solution to your metabolism: muscle and fat mass, calories and macros eaten, and exercise. It includes these other factors, and how your body changes over time, to determine your true RMR.

Fat Free Mass (FFM) Baseline

We start with the most trusted correct measurement for body fat: the DEXA scan, or DXA. While studies show that Biological Impedance Analysis (BIA) measurements are accurate to about +/- 3.1-7.5 percentage points [https://doi.org/10.1017/s0007114522003749 ] on average compared to DEXA, that is all dependent on factors such as the environment, ethnicity, phase of menstrual cycle, and underlying medical conditions, in addition to the BIA technology used. We have user’s whose BIA values are consistently >10% over or under their DEXA values. We ask that you get a DEXA scan after about a week of logging food in order to start with a baseline that we will then adjust based on fat free mass.

Daily Data is Key

However, we aren’t just interested in a single point in time, but in how your body changes and adapts to the changes you make (or don’t make–we’re still interested in your body’s adaptation to activities and consumption, even habitual!). Daily data is absolutely essential to proper training of the BODYSIM model.

We use your macro consumption combined with daily changes in muscle and fat mass to get insights into your true RMR. As part of this, we break down your total daily energy expenditure (TDEE) into more granular component parts:

• Basal Metabolic Rate (BMR)
• Thermic Effect of Feeding (TEF)
• Thermic Effect of Hypertrophy (TEH)
• Exercise Activity (ExA)
• Non-Exercise Activity Thermogenesis (NEAT)

Your BMR is the energy expended to keep you baseline alive, as if you were forever asleep. Your TEF is the energy expended to digest your food. Interestingly, protein takes much more energy to digest than other macronutrients. Your TEH is the energy required to synthesize new muscle in response to your workouts, above and beyond the energy of those workouts and the structural protein necessary to compose that new muscle. Your ExA is the extra active energy expended in your dedicated workouts beyond your passive BMR during that time. This difference is why your fitness tracker app may report both “Total” (including BMR) and “Active” calories (only ExA). NEAT is the energy you expend on all other (non-exercise, as per the name) activity beyond just sleeping: walking the dog, sweeping the floor, folding laundry.

The only way to get a truly accurate value for all of these is baseline with your DEXA, log all calories, weigh in daily with your Withings scale, and run that through a science-based model.

Fitness Trackers (Usually) Overestimate

What about calories burned from exercise? One of the biggest variables in day-to-day calorie expenditure is calories burned from exercise. While rules of thumb based on weight, speed, type of exercise (swimming vs running vs weight lifting…) can be directionally correct, this number can vary drastically depending on your efficiency and overall fitness³. The same principle, although less easily quantified, applies to resistance training. While it varies between muscle groups, the increase of intensity from 20% of your 1 rep max (1RM) to 80% of your 1RM intensity will roughly triple the energy demand⁴.

BODYSIM corrects your exercise expenditure estimate by taking reported calories burned from any or many exercise trackers and combining it with your daily weigh-ins, food logging, learned RMR, and a science-based model. For example, my Garmin watch consistently overreports calorie expenditure, especially for my easy runs. In the image below, you can see the correction of one 2 hour run at endurance pace.

While it may take some time and careful data collection to reflect your true values, training your digital twin to accurately measure your data collection is crucial.

Your Resting Metabolism Increases With Training

Even if your RMR and your TDEE were difficult to determine, once learned, you could make a nutrition plan to match, but it’s actually much more complicated because your body’s energy demands are as dynamic as your body itself. As you train, your body composition changes, your hormones change, and your increased mitochondrial biogenesis all work to alter your RMR and TDEE. You can see in the plot below that in the course of a single year, one of our team members’ non-exercise components of TDEE increased 450 calories / day. That’s a huge change that a static diet and exercise plan cannot absorb. A plan to achieve your goals requires continuous adjustments to your nutrition and exercise targets to account for these changes in your RMR and TDEE.

With your BODYSIM digital twin, you get a live feed of how your resting energy expenditure adapts to the changes you are making to your body, allowing you to adapt your strategy and make real change continue to happen.

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