A more accurate way to track calorie expenditure

Don’t be disheartened, just don’t expect exact precision when it comes to estimating CI/CO. Having estimates gives you a target and if you get to a point where you’ve stalled out you have some data to look at to see where the estimation problem in. It could be as simple as one of the foods you eat has inaccurate calorie information, or the outdoor HR-based estimate is off, but you at least have things to go off of.

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I am on a diet now and curious if i can still build the muscles with the calorie deficit?

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Who is telling you have expended 300 kCal? If MyFitnessPal is using heart rate to compute calories, then they are most definitely wrong. If you have a power meter, TrainerRoad can measure energy expenditure at the pedals, and from that infer total energy expenditure. The energy expenditure at the pedals is the kJ number (kilo Joule is another unit for energy just like calories, 4.1 J = 1 Cal). The second number TR gives is in kCal, and that’s total energy expenditure. Because on average the efficiency is about 20–22 %, that tracks closely to the conversion between Joule and calories. By accident the conversion factor between Joule and calories almost cancels this efficiency percentage, and the number for total energy expenditure in calories is close to the total energy expenditure at the pedals in Joule. While there is some variability in there, it is typically quite small, i. e. you won’t see people with a 30 % efficiency. So TR’s number (and that of any other platform using power data) is going to be much, much, much closer to the true energy expenditure than any software using heart rate only.

Edit: I need to correct my numbers: TR uses 24 % efficiency, so my numbers were too low. Sorry about that.

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That is really hard. The last 10KGs I lost I set a goal to at least not lose any muscle/fitness. One of the key features for this is to make sure you get around 2grams of protein per KG of bodyweight. With this I was able to keep my ftp around 290 while keeping the calories at a 500 deficit.

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Yes. How much muscle depends on your genetics, and how much muscle you’ve had in the past. It’s harder to grow new muscle than regain muscle you’ve lost previously. Keep your riding efforts moderated if you’re very serious about wanting to maximize muscle gain in a kcal deficit.

You’d be well-advised to lift weights, of course. Here’s a great thread here on the TR forum.

Also eat plenty of protein.

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If you’re using a Garmin cycling computer it uses the Firstbeat algorithm to estimate calories, which is WAY more accurate than just about any other HR based estimate.

To be fair to them, I had expected results going back to when I had a 920xt too, and for none power meter stuff with my 245. The 245 gives me pretty similar to when use a power meter on my commute.

Watts x 3.6 x hours.

Just a little clarification. I’m not sure if I’ve seen that multiplier before, but I like it. seems to match my calculation however I got it I can’t remember.

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Energy (joules) = Power (watts) x Time (seconds)

Work J = Watts x 3600 x hours
Work kJ = Watts x 3.6 x hours

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Watts is energy per unit time , measured as Joule per second (J/s)

So 1 W over 1 hour, amounts to 3600 J of work done.

However, when we talk about calories we normally refer to the energy in the food we need to eat to compensate for the energy expenditure from the work done.

Calorie is also just a unit of energy and going from Joule to calorie gives a factor of about 0.24. But also we need to eat roughly 4x as many calories to compensate for a certain amount of work done, because it also requires energy to stay alive and keep all of the internal machinery running. This factor of 4x is roughly a wash against the factor of 0.24 going from Joule to calorie. So we can say that 3600 J of work done equals 3600 calories you need to eat.

Now the final step is that when we talk about food calories, the unit that we are all used to is actually kilocalories (1000 calories). so 3600 calories gives 3.6 kcal.

That is how the factor of 3.6 is derived, so doing 1W over an hour can be offset by eating 3.6 kcal of food.

The same logic also gets us to the total amount of work done in kJ, being equal to the amount of kcal that need to be eaten.


Very good, except ~75% of energy produced dissipates in the form of heat!

that is exactly the 4x factor


Oh, I thought you attributed it to requiring energy to stay alive and keeping internal machinery running.

Ah I see, your issue is more with the labeling.

No issue, just a clarification. Always seems crazy to me to think that if I’m pedaling at 250 watts, I’m also running a 750w internal heater!

Well, we are better than combustion engines, with a typical 20% efficiency.

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This was probably the clearest and level-headed discussion on nutrition I’ve heard in a long time. Thanks for the tip.