They might tell you, you burnt 2000kcal today or 300kcal during this sport training. But how do they get these numbers?
And afaik studies showed that they are incorrectly measured. How do the experts check that the data is correct? I mean you need the real numbers but for that you used the smart watch (or similar device).
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They don’t. They make guesses based on the data available to them and the algorithms that were programmed into them.
Its never an actual true number, just a calculation. Based on the activity your weight and other factors it would calculate energy spent. Taking it to very simple terms its like calculating the energy or calories to move a weight from point x to point y at a certain speed. Depending on the activity there might be other factors, which consider experimental measurements to make the calculation as accurate as possible. But again, its only a calculation. Use it for trends, as a guide, but not as an absolute true number.
It’s an algorithm that runs primarily on on body weight, distance travelled, heart rate, and activity input. If I know that someone weights 150 lbs and I can tell by GPS that they travelled 3 miles on a run, I have a good idea of how many calories they burned. Similarly, if that person indicated that they were doing “gym cardio” (e.g. a Zumba class) and their average heart rate for 45 minutes was 142 bpm, I can make an educated guess about what they burned.
I think the second part of your question is “how do experts accurately measure calorie expenditures” and the answer is that they hook up something that captures your breathing in and out and calculate how much CO2 you’re producing. A good answer is the tl;dr answer in this thread
[https://www.reddit.com/r/askscience/comments/rbo8b/how_do_scientist_measure_how_many_calories_are/](https://www.reddit.com/r/askscience/comments/rbo8b/how_do_scientist_measure_how_many_calories_are/)
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