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  • Calories burnt from exercise

    Published: 03-11-2021

    How does one calculate burnt/expended calories from excercise?

    This is what I've been asking myself over the last couple of days as I've been on a cut.

    I've learnt that one can use:

    • Excercise to calorie burnt reference tables (based on a concept known as Metabolic Equivalent of Task)
    • Body-worn fitness trackers

    Metabolic Equivalent of Task

    Metabolic Equivalent of Task (MET) is a number used for the estimate of exersion for a particular activity, and therefore by proxy can be used to calculate calories burnt.

    More specifically it is the ratio of undertaking a particular task contrasted with simply existing at rest. For example sitting at rest is 1 and playing soccer is 6: "Playing soccer is x6 more calorific than sitting at rest".

    Different activities have different MET units associated with them. A low unit like 1 or 2 is typically for low exersion activities like sitting and a jog might be 6+.

    One MET corresponds to an energy expenditure of 1 kcal/kg/hour.1

    When using MET to calculate calory burn one must factor in:

    • The length of the activity
    • The participant's weight
    The length of the activity correlates with the amount of calories burnt. And a heavier person burns more calories doing the activity than a ligher person.

    MET scores are categorised by exersion levels:2

    • Score of 3 and under:
      • Light intensity: No significant body changes
    • Score of 4 to 6:
      • Moderate intensity: An increased heart rate and breathing
    • Score of 6 and above:
      • Vigerous intensity: Like moderate intensity, but with increased sweating & inability to complete sentences between breaths
    Example activities METs
    At rest 1
    Walking slow 2
    Walking (2.5mph/4kmh) 3
    Golf 4.5
    Cycling on flat at 10–12 mph (16–19 km/h) with light effort 6.0
    Soccer 10.0

    So to calculate the calories we must take into account the met score and a person's bodyweight in kilograms. The constants 3.5 and 200 are also used which are more specifically explained here.

    The formula for calories per minute is:3

    METs x 3.5 x BW (kg) / 200 = Kcal/min.


    Q: So if Jane (weighs 70kg) plays soccer (MET of 10) for 30 minutes what is her estimated calories burnt?

    A: 367.5 Kcal (Hover to find out)

    Why:
    10 x 3.5 x 70 = 2450
    2450/200 = 12.25 Kcal/min
    12.25 x 30minutes = 367.5 Kcal

    Criticisms of MET

    The baseline resting MET of 1 (1 kcal/kg/min) is derived from the oxygen consumption of one individual. A 70kg, 40 year old man. Using the MET system on different people presents the obvious inaccuracies as everyone's physiology is different.

    A study was undertaken consisting of around 750 people of varying physiology.4 It found that each individual's oxygen consumption (how 1 MET is derived) was different. This meant that everyone obviously has a different baseline of energy consumption and the original 1 MET cannot fit all.

    The study concluded for a large sample of mostly similar people the base line MET level of 1 overestimates by an average of 20%.

    Fitness Trackers

    Fitness trackers like a fitbit use a combination of metrics to measure calories burnt:

    • A step counter
    • A heart rate monitor
    • It's knowledge of your weight and other physiology (height, BMI, age, sex, etc)
    • An accelerometer which cross-references your movement with a library of movements uses that to pick an activity. The activity has an associated MET score or some likely derivitive or equivalent.

    As described above I already understand how MET can be used to estimate calories. Combined with a step counter and a heart rate monitor I can assume these gadget average out calories burnt across these other metrics. Furthermore these are usually linked to apps which you insert your age, sex, height and weight.

    The problem is my fitness watch thought I did 2000 steps when in fact I was stationary in the car driving. It seems far more accurate that you take not of your own activity levels without the reliance of faulty accelerometer which congratulate you on your 2 mile breast stroke along the motorway.

    What I've learnt

    I think MET scores are only good for rough estimations and fitness trackers which try and guess what activities you use have to be constantly babysat to make sure they aren't feeding you false information. If you are too trusting with this stuff it can really set your diet plans back with not enough or too much food eaten. I have no idea what people are getting for spending more than £30 on a fitness watch?

    My advise for anyone trying to track calories burnt from activity is the following:

    • Listen to your body. Your body is far more sophisticated at telling you what it wants than Apple or Fitbit. If you are hungry and tired then you are probably going to drop weight.
    • Use patience and adopt the scientific method. Log stuff down. No idea how much your 5x5 Squats consume? You probably will never know without data. Eat 400kcal surplus and see if you get heavier? Didn't work? Write that down. Eat 400kcal and log down the results. Now you have a coincidence. Try for the third and you might have a pattern. Weight training is a long process and worth while process!

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