# What drives calorie burn - stress on body (e.g. heart rate) or output/watts?



## BantamSLK (Sep 19, 2009)

Hey folks - this is likely a stupid question, but I didn't see it elsewhere on the forum so I figured it didn't hurt to ask...

I understand that very roughly, you can equate 1 kJ or work done to 1 kcal burned. However, I always had the (probably wrong) assumption that the calories you burned was more a function of how hard your body was working, rather than the output. By that I mean, as an example, a rookie cyclist working at 160 heart rate and putting out 160 watts would burn about the same as a advanced cyclist working at the same 160 heart rate but putting out 300 watts. 

However, if the kJ to kcal math works (even roughly), then the advanced cyclist at 160 watts (and lets say only 110 hr) would put out the same as that rookie cyclist at 160 watts and 160 heart rate. Is that right? Basically, as you get fitter, and can put out more watts at the same heart rate, do you actually burn more calories per hour as well?

Again, sorry for the long-winded and probably silly question - but I never really understood this well (and the implications of getting fitter and being able to burn more calories at a lower heart rate are really interesting).


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## looigi (Nov 24, 2010)

Heart rates are highly variable between individuals and even variable within an individual and only indirectly related to power output. The thing that makes 1kJ=1kcal approximation work is that the average person is only 25% efficient, but again that's the average person and it will be different between individuals and is somewhat trainable in an individual. HR is best used to determine how relatively hard an individual is working compared to the max he can work, not as an absolute measure.


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## Alex_Simmons/RST (Jan 12, 2008)

BantamSLK said:


> Hey folks - this is likely a stupid question, but I didn't see it elsewhere on the forum so I figured it didn't hurt to ask...
> 
> I understand that very roughly, you can equate 1 kJ or work done to 1 kcal burned. However, I always had the (probably wrong) assumption that the calories you burned was more a function of how hard your body was working, rather than the output. By that I mean, as an example, a rookie cyclist working at 160 heart rate and putting out 160 watts would burn about the same as a advanced cyclist working at the same 160 heart rate but putting out 300 watts.
> 
> ...


The rough correlation between work done and energy metabolised is that the kJ of work done refers to the amount of mechanical work done at the cranks of the bicycle, whereas the energy "burned" refers to the Calories metabolised by the body.

Humans riding bicycles are not perfectly efficient, only so much of the energy we metabolise reaches the cranks, typically around 19-24%. So that's roughly 1:5 to 1:4 ratio. It just so happens that there is 4.2kJ per Cal, hence those two ratios approximately cancel each other out to provide the 1:1 ratio.

HR isn't relevant. If you put out more power, you will metabolise more energy.


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## BantamSLK (Sep 19, 2009)

Great - thanks for the information folks

I had always thought that the fitter you got (as defined by say, watts produced at a given HR) the fewer calories you burned as your body no longer had to put out as much effort, but I guess that's not actually right.


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## looigi (Nov 24, 2010)

Actually, it is right to some extent. Being fitter and having better neurological adaptation to the activity improves efficiency, meaning you do burn fewer calories for the same output. That's what I meant when I said it was "somewhat trainable" in an individual. 

I though of an analogy for HR vs power that might be of use. I have experience with engines that have oil temperature gauges. Oil temperature roughly tracks power output, going up with power output and going down as power goes down. Its response is slow so it averages and lags actual power output. Also, it will be affected by other factors like engine installation and variables like air temperature. So two engines putting out the same power might have different oil temperatures, or engines running at the same oil temperature may have different power outputs. HR is like oil temperature. It gives you a time averaged and lagged indication of relative effort for a given individual that can be influenced by other factors.


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## Kerry Irons (Feb 25, 2002)

BantamSLK said:


> Great - thanks for the information folks
> 
> I had always thought that the fitter you got (as defined by say, watts produced at a given HR) the fewer calories you burned as your body no longer had to put out as much effort, but I guess that's not actually right.


Note the comment from Alex Simmons that your metabolic efficiency can range from 19-24%. That's a BIG range - one person might 25% more power from a calorie burned than another person. How much of this is truly trainable has not been thoroughly researched. Metabolic efficiency is relatively hard to measure so most people just guess that a fit athlete is about 24%. But whether someone comes off the couch and 19% and then can train themselves up to 24% is not completely clear.


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## Alex_Simmons/RST (Jan 12, 2008)

Kerry Irons said:


> Note the comment from Alex Simmons that your metabolic efficiency can range from 19-24%. That's a BIG range - one person might 25% more power from a calorie burned than another person. How much of this is truly trainable has not been thoroughly researched. Metabolic efficiency is relatively hard to measure so most people just guess that a fit athlete is about 24%. But whether someone comes off the couch and 19% and then can train themselves up to 24% is not completely clear.


Yes, although I'd say that assuming one is at least modestly fit, their fundamental baseline efficiency level won't change a huge amount (e.g. the gross efficiency of pros isn't that much different on average to the average amateur club rider), so if it does change it'll do so relatively slowly over many years.

As you say, there's not a lot of information on chronic changes to efficiency.

Of course there are acute impacts to efficiency, e.g. heat, cadence, inertial loads, amount of high power efforts using anaerobic energy pathways, relative fuel substrate utilisation (which in turn is affected by lots of things including diet) etc etc, so that the ultimate efficiency ratio on any given day and during any part of a ride can vary.

Measuring efficiency requires quality lab equipment (metabolic cart to measure gas exchange and power output) and quality test protocols with people who know what they are doing.


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## looigi (Nov 24, 2010)

Alex_Simmons/RST said:


> ...Measuring efficiency requires quality lab equipment (metabolic cart to measure gas exchange and power output) and quality test protocols with people who know what they are doing.


Going back to the OPs question, HR can't be used for this as it's too variable in and among individuals and only loosely related to power and efficiency.


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## sdeeer (Aug 12, 2008)

looigi said:


> Going back to the OPs question, HR can't be used for this as it's too variable in and among individuals and only loosely related to power and efficiency.


A little something for everyone below.....

HR is crap for calculating energy expenditure, but most people know that. The amount of energy required to do work (AKA peddle a bike) depends on the amount of work you are doing. Higher intensities require more work and thus more calories, which requires more substrate oxidation and thus more oxygen. 

HR required for a given work rate (wattage for example) is dependent on the amount of oxygen delivered per beat. 

So....The Fick equation states that VO2 = Q * (a - v diff). 

Q is cardiac output and is composed of HR and Stroke volume (SV). Athletes have better preload which increases SV for a given work rate (watts) so their HR is lower. 

Bottom line is the more fit you are (ish) the more calories you burn when you heart rate is higher (indicating a greater oxygen delivery/cost), making HR a crap measure of energy expenditure. 

Wattage per minute and using KJ is a much better estimate of total caloric expenditure during work (riding). As Alex stated, typically efficiency on a bike is 19 - 24%. So the conversion of KJ to calories is not direct without know efficiency, which is rather hard to determine without extensive testing. 









and 1 kj = 1000 J = 0.23892 (.023) kcal So if efficiency is on the high end, then it all comes out "even." But knowing EXACT energy expenditure is a moot point (ish) because the system is very dynamic. What I mean is even if the value is incorrect, if your power meter is calibrated, your value will be consistently wrong by the same amount.....and that is close enough. Many people use calorie trackers for the energy in side (which again are typically off by 10 -20% in the best of circumstances).

Even in the metabolic wards / studies there is an error of 2 - 5%.......

and some fun advanced stuff for some to ponder. When you are running on a lower glycogen store and burning a greater amount of fat for ATP production, the oxygen cost of exercise increases, but energy expenditure goes down just a bit..









And.....As some motor units "wear out" late in exercise, you recruit higher order motor units which are often less efficient....

And.......there is the Crazy COOL role of thyroid hormone, slow component, oxygen kinetics, and substrate oxidation......

F-Yes......!!

Rabbit holes! [Shiz...diet and hormone and training interactions at the cellular and molecular level.......]









References that might not be too deep....

VO2 and the Low-But-Normal Thyroid | Runner's World

How Does Endurance Training Affect Your Thyroid (and Vice-Versa)? | Runner's World


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## duriel (Oct 10, 2013)

So... If I eat a candy bar, will I go faster?


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## aclinjury (Sep 12, 2011)

Here are two studies that deal with the topics of "cycling efficiency" and "optimal cadence" that come up on RBR every month.

They are great read, especially for those who are looking to understand more about the science. They are difficult reading for sure, but you don't have to be a physiologist to extract some useful info out of them.

The articles also necessarily discuss terms that get tossed around here a lot, terms like:

- gross efficiency
- mechanical efficiency
- economy
- work efficiency
- delta efficiency

http://d3epuodzu3wuis.cloudfront.net/R060.pdf
http://danpat.net/docs/pedrate.pdf

(Alex, I believe you gave me the link to the first article).


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## looigi (Nov 24, 2010)

What's the executive summary for those articles?


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## stevesbike (Jun 3, 2002)

basically that training down-regulates your thyroid and that training + caloric deficit down-regulates it even more, slowing down metabolism. Lots of endurance runners with hypothyroidism. Thyroid medication could be a PED. High intensity interval training also down regulates it even more than steady-state endurance training. 

Would be interesting to hear if sdeer knows of any studies examining whether certain diets have any mitigating effect on these results and how this might impact the anti-oxidant debate (do they have a mitigating effect on thyroid function since they appear to blunt the stress response signaling that's beneficial for other stress response mechanisms). 



looigi said:


> What's the executive summary for those articles?


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## berserk87 (Jul 24, 2014)

sdeeer said:


> A little something for everyone below.....
> 
> HR is crap for calculating energy expenditure, but most people know that. The amount of energy required to do work (AKA peddle a bike) depends on the amount of work you are doing. Higher intensities require more work and thus more calories, which requires more substrate oxidation and thus more oxygen.
> 
> ...


I have to channel my inner Homer Simpson here: "Mmmmm...math..."


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## phoehn9111 (May 11, 2005)

Not to derail the thread, but hypothyroidism as a consequence of excess TSS
appears to be a pretty serious malady. What's the word on symptom 
identification and, if possible mitigation or remediation.


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## aussiebullet (Sep 26, 2005)

Stop digging a hole would be the first thing that comes to mind.


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## looigi (Nov 24, 2010)

stevesbike said:


> basically that training down-regulates your thyroid and that training + caloric deficit down-regulates it even more, slowing down metabolism. Lots of endurance runners with hypothyroidism....


Interesting. My wife suffers from hypothyroidism (Hashimoto's) and likes to be quite active and has trouble keeping her weight down. Sounds like it might be a vicious cycle of some sort.


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