Science summary: Exercise and energy compensation
“How accurate are the calories I burned on my fitness tracker after a workout?” is a question clients often ask me. It can be surprising, but the answer is not as simple as accurate or not. There is a lot of nuance to determining caloric burn and how to use that data to properly refuel post workout, more than can be captured in a limited fitness tracker.
There was a study published on calculating caloric burn earlier this year with a surprising result that gained a lot of traction in the scientific community. The scientists looked at energy compensation post-workout, basically how your body adjusts its energy burn after you exercise. They used at a large database of metabolic data to make this compensation estimate. The study found that over the course of the day, additional calories expected to have been burned during exercise was 28% lower (meaning only 72% of the extra calories burned from exercise translated into extra calories burned over the course of that day). Let’s break this down in an example: Let’s say on an average day, you burn 2000 calories at baseline, just being alive. If you went on a run and burned roughly 400 calories, your body would adjust that baseline rate to conserve 28% of that 400 calories, or 112 calories less. Thus, your baseline burn for the day is reduced by 112 calories to 1888, and your total burn is 1888 + 400 = 2288 not the expected 2400.
Now, why does this compensation occur? Your body is constantly burning calories to just keep you alive even when you are standing still, and it appears after a tough workout, it will reduce the energy spent on those processes. You may also be less likely to go on a walk later if you’ve tired yourself out on a run, or less likely to execute small movements over the course of the day such as fidgeting, dancing in the kitchen while cooking, or taking the stairs. The study also found that the degree of this energy compensation varies by body composition: a higher body fat percentage was associated with a higher energy compensation (less calories burned), meaning this varies from person to person.
In contrast, as you build muscle through exercise, your body needs more calories at baseline to fuel itself. So each workout as you additively build muscle, your baseline caloric needs increase. That being said, although caloric compensation may occur in the short term, long term your body increases its caloric need baseline with progressive muscle building workouts.
This study is a great example of why I find it more important to focus on refueling with what your body needs rather than what a number on your watch may dictate. If, for example, you’re trying to refuel post-workout, it’s my opinion that the best approach is not to just eat the exact number of calories your Apple Watch or FitBit says you burned. Instead, think about fueling your workouts over the course of the day, generally including a carb-heavy pre-workout snack (like a piece of fruit) and a post-workout protein and carb snack (like yogurt with fruit, or eggs with rice). You can even further optimize your fuel when you consider the type of workout you did and how you ate for the rest of the day (and even the day before!) and this is where your dietitian can really help you excel.
As always, remember, this is just ONE study, so just like the data on your fitness tracker, take it with a grain of salt. New data is always interesting and fun to use, but shouldn’t lead you to make major changes without the advice of people like your dietitian and medical doctor. I hope you learned something about the complicated machine that is your body, and look forward to discussing great pre and post-workout fueling options with you.
