# How do we know how many calories are burnt?

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For example one burns about 200 calories while taking a walk.
Who figured that out and how?

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>For example one burns about 200 calories while taking a walk. Who figured that out and how?

First off, if you have a thing on your phone or wrist telling you how many calories you burned, it’s mostly just taking a wild ass guess.

Having said that, the way to figure out how many calories you burn during a given activity is to put on a breathing mask while performing that activity, and measuring exactly what gasses you’re in- and exhaling; then with that knowledge you can use algebra to figure out what you used as an energy source and how much of it, which will tell you how much energy you used if you know how much energy those energy sources contain.

Or to give the simplified version, you compare how much oxygen you inhale to how much CO2 you exhale to figure out of you’re burning fat or carbs, then multiply how much of each you’ve burned by how much energy they contain.

Fitness trackers like fitbits basically take the basal metabilic rate which is the optimal calory expendature to keep everything running smoothly, and estimates it together with heart rate and activity level.. so most of it runs off the data you put into it so thats for instance why your fitness watch just assumes you will burn like 1300-1800 calories a day. As that’s kinda whats to be expected as a baseline of burning just to keep your body running.. if your watch can track blood oxygen. I.e vo2 max the values tend to be more accurate too

Its not perfect for instance some trackers may be up to 30%-50% off… for instance they overestimate burning of calories when walking. Some underestimate it Sounds like a lot but others are much worse. At higher intensity workouts most fitness trackers formulas tend to be better for instance both garmin and fitbit have been able to boast an accuracy within like 4% of the most probable true value. So your fitness tracker may sometimes overestimate it because it expects better metabolic health.

We measure the heat evolved. A calorie is the energy needed to raise the temp of a kg of water by one degree. It is a misnomer, when applied to nutrition. Your body does indeed produce heat as a bi-product, but it does not correlate to the food you have eaten, or the fat you have stored. The true answer is that thinking of nutrition and metabolism in these terms is inconclusive at best, fraudulent at worst. The entire concept of calorie counting or calculation as it pertains to nutrition is deeply flawed for several reasons. It’s scam science.

We can do indirect calorimetry. One liter of oxygen consumed equals 5 kcal (how that was determined, I don’t know). So we can do experiments to measure how many calories are used, such as having someone walk on a treadmill using a mask connected to a metabolic cart that measures how much oxygen they are using, then convert that to calories.

This process is used often in measuring efficiency and economy (for, say, a new type of athletic shoe or bike handlebar system).

A calorie is simply a measurement of energy and you can convert it to Watthour, Joule, etc..

If you walk, your body supplies energy to move you around.

Knowing how much you weigh, how far you walked and the change in geographic height between start and destination point will let you calculate how much energy (calories) you would need for that. This is purely theoretical.

In practice you always need more energy as you also need to keep your body at temperature (sweating or shivering), you might swing your arms a lot during walking, you might have very heavy shoes, etc…

For fitness trackers, it is a calculated approximation, based on the average human.