Fitness trackers like Garmin and Wahoo are fantastic tools that give athletes data they need to meet performance goals. For endurance athletes—who have insane energy demands—the calorie estimate information can be particularly useful for optimizing nutrition plans.
But there’s a problem: Calorie estimates are not always accurate.
Here, we will dive into how fitness trackers calculate energy expenditure, why these numbers aren’t reliable, and what you should use instead.
How Do These Devices Calculate Calories?
There are a lot of methods that fitness trackers use to calculate calories, such as estimating based on data like activity type, age, and physique. However, the fitness trackers with heart rate monitors tend to be the most accurate.
These work on the idea that calories burnt is proportional to heart rate. To convert heart rate into calories, the device must factor in efficiency.
What Is Exercise Efficiency?
In exercise physiology, the term efficiency refers to how much of your body’s energy goes into mechanical work (physical movement). The rest of the energy goes to heat.
Most fitness trackers assume an efficiency of around 24%, meaning that 24% of energy goes into movement and 76% goes into heat (yes, our bodies lose much more energy to heat than movement!). However, in reality, efficiency varies between individuals and activities.
How Does Efficiency Affect Calorie Counts?
Depending on efficiency, the calorie estimate on your fitness tracker could be way off. Cycling, for example, typically has an efficiency somewhere between 20% and 25%.
Let’s say you generate 3,500kJ of mechanical work during a long cycling session. At a 24% efficiency, this translates to roughly 3,500 kcal burned.
3,500 kJ ÷ 0.24 = 14,583 kJ (or 3,485 kcal)
But let’s say you actually have a 20% efficiency, meaning your body is losing a lot of energy to heat production. Your total energy expenditure would be roughly 4,200 kcal.
3,500 kJ ÷ 0.20 = 17,500 kJ (or 4,182 kcal)
That’s nearly a 700-calorie difference from the fitness tracker’s estimate!
Let’s be clear: Most people aren’t doing 3,500 kJ of mechanical work per session. For them, variations in efficiency won’t affect calorie estimates too much. However, for endurance athletes who track calories over long, intense workouts, even a 1% difference in efficiency can make a big difference!
Can You Figure Out Your Individual Efficiency?
There are various tests which can be used to determine individual efficiency. These tests typically use information from power meters, HR monitors, and metabolic carts (for VO₂ measurement). However, these test results are not reliable indicators because efficiency is variable.
For example, efficiency can vary due to...
- Climate
- Recovery status
- Hydration status
Even fueling status can affect efficiency because low glycogen stores could force the body to rely on less-efficient energy systems! So, unless you are exercising in completely controlled lab conditions, you can't be sure of your efficiency at a given moment.
Pro Fact: Each gram of sweat that evaporates from your body dissipates approximately 2.43 kJ (0.58 kcal) of heat, so efficiency decreases in hot conditions.
Calorie Counts Are Also Accurate
Even if we assume that Garmin’s calorie estimates are perfectly accurate (which they aren’t), there’s still another major problem—we don’t actually know how many calories we’re eating.
Food manufacturers don’t actually test each product for calorie content. Instead, they rely on a formula, plugging in standard macronutrient numbers to estimate the numbers you see on nutrition labels.
Where do those calorie numbers come from?
A method from the 19th century called bomb calorimetry. Scientists would burn food in a sealed furnace to measure how much heat was released to determine calorie amounts.
But our bodies don't "burn" food.
Take nuts as an example. They are high-calorie according to bomb calorimetry. But the body doesn't actually absorb all of these calories (if you've ever looked at your poo after eating a lot of nuts, this is obvious!).
As biologist Rob Dunn notes, there are a dizzying array of factors which affect how many calories a food really has. Everything from processing method, digestibility, and even individual gut bacteria affect how many calories you really get from food.
On top of issues with bomb calorimetry, the nutrition label may also be inaccurate. Studies such as this one found that serving sizes were often off and carbohydrate content exceeded label statements 7.7%!
The bottom line? You can meticulously count calories in all the foods you eat, but those numbers are just estimates.
Pro Fact: When it comes to calorie count accuracy, sports nutrition products like energy gels are most accurate because they only contain a few simple, well-measured ingredients. But, as Dr. Tim Podlogar notes, variability skyrockets with whole foods because fat, protein, and fiber affect uptake.
So, What Does Work?
So, if calorie estimates and calorie counting are inaccurate, how are we supposed to track energy expenditure? Dr. Tim Podlogar suggests this practical approach
- Use fitness tracker data as a guideline: The 24% efficiency is a good starting point for most athletes. However, Dr. Podlogar says that elite athletes may want to use a 21% or 22% efficiency for calorie estimates instead, because those are the numbers he is seeing in the lab.
- Adjust based on energy balance: Over time, you will figure out if there is an error in calorie estimates because you will either gain or lose weight. It’s a lot of trial and error but over time you’ll get intake correct.
- Listen to your body: The body is highly adaptable. If you are consuming too many calories, your body will respond by being more active (such as fidgeting more). On the flip side, if you aren’t getting enough energy, you will find it difficult to train.
Did you learn something? This question actually came from one of our listeners during the Nduranz Q&A episode. Listen to the full episode for more answers!