• Question: How do you calculate how many calories someone has burned after exercise? If you know, specific details would be nice as it is a crucial part of an experiment I'm going to run.

    Asked by watsonjc06 to Carol, Ellie, John, Philip, Rebecca on 3 Jul 2012.
    • Photo: Rebecca Lacey

      Rebecca Lacey answered on 3 Jul 2012:


      Hi watsonjc06 – loving your questions!!

      I’ve just discovered a great online calculator which will let you do this. It has loads of different types of activities to choose from (it even includes washing up!!). Have a go:
      http://www.healthyweightforum.org/eng/calorie-calculator.asp

      The amount of calories you burn depends on the intensity of the exercise, your body weight and the length of time you do the activity for.

      Some of the highest calorie burning activities are running and high impact aerobics

    • Photo: John Welford

      John Welford answered on 3 Jul 2012:


      Good question. I’ll try to give you plenty of detail to help with your experiment. First it’s worth talking about calories…

      Calories are a measure of energy, similar to joules, in fact 1 calorie = 4.184 joules. They tend to be used for food energy, but in quite a confusing way. The small calorie is the base unit, however the large Calorie (usually capitalised) is what food is often labelled in, this is equivalent to a kilocalorie (or 1000 small calories, or 4184 joules).

      Just like any other machine, the human body is used for the transfer of energy. It takes in chemical energy in the form of food and it either stores it as chemical energy (e.g. fat) or it converts it to mechanical energy as movement, or thermal energy as heat. Just like everything else it is subject to the law of “conservation of energy”, which means it doesn’t create or destroy any energy, it just converts it. This means that a simple equation can be written:

      energy in = energy out + energy stored

      If you remember that then you’ll never need to pay for an expensive diet plan!

      The “energy in” part is easy to measure as it is written on pretty much all the food packaging you buy. It is the “energy out” that you are interested in. There are a few different ways that this can be measured or estimated.

      One of the most simple is to look at the amount of energy put into the exercise you are doing. So for example if you are cycling then some basic calculations about the friction of the tyres and bearings will tell you the energy needed to move your mass a certain distance. This will be a massive underestimate as your body isn’t very efficient, so a lot of energy is given off as heat (that’s why you get hot when you exercise!). The type of calculators where you input your activity and some details about yourself are calculating the “energy out” using some approximations based on the average person. The more information you have to put in about yourself (gender, age, weight, etc) the better their estimates will be.

      These are not likely to be very accurate, but they might be good enough for what you need. The one Rebecca has linked to looks pretty good.

      The next step would be to take some measures from your body during exercise. Heart rate is a common one. This can provide a much better estimate of calories burned as it is actually taking a measurement from you! There will still need to be some other details about you put into the calculation in order to estimate the energy burned at different heart rates. I know that some gym machines can do this and combine it with measurements from the equipment to give a pretty accurate estimate of calories burned.

      The ultimate method would be to accurately measure your whole body temperature as you exercise, this is pretty difficult to do in practice though! Combining that with measurements of the energy put into the exercise (measured from a treadmill or exercise bike) would give you a super accurate result. Here are some details about a suit for doing exactly that:
      https://www.ncbi.nlm.nih.gov/pubmed/8162925

      Whilst this method is not very practical in humans, we do use it in the lab here to measure the output of motors. They are run inside a sealed box in which the temperature is measured, the output energy from the motor is also measured. This gives us a really accurate measure of the efficiency of the motor (the amount of energy wasted as heat).

      Phew that was a long one, hope it helps!?

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