calories burned question. BRM etc. any nutritionists out there?



steevo

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Dec 14, 2003
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Ok this might seem kind of neurotic, but I will go ahead.

Yesterday I rode 7:33, for 136 miles. Some hard efforts, some easy efforts, hilly to rolling the whole time. I am a 5'9 137lb racer. I estimate my brm to be about 1700 calories per day.

I estimate 500 calories per hour burnt on a ride like this and consumed 3500 calories yesterday, which covers calories burnt. However, if you spend 80% of your awake time on the bicycle, does your BRM then go down? Should I have consumed closer to 5000 calories to only have a 500 calorie or so deficit, this is huge difference.

Also those of you with powertaps and whatnot, what is a good estimate of your caloric expediture for ride times/distances/races etc?
 

cuocciom

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Jan 13, 2006
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I'd like to add a bit to your question. I bought a PowerTap SL this winter. I can track work in kilojoules for each ride. I've read that kilojoules roughly equals kilocalories burned; if true, then I should look at my kilojoules and have a fair idea of what I've burned. On the other hand, I've read that there is a conversion of 4-5 calories per kilojoule. If true, then I'd be burning 4-6 thousand calories a day for 35-50 miles of riding. This seems ridiculous to me.
My problem, I believe, is that I don't know what kilocalories and kilojoules mean in non-scientific terms. Are there others out there who share my confusion? Can anyone explain this in terms that a non-physicist can understand?


steevo said:
Ok this might seem kind of neurotic, but I will go ahead.

Yesterday I rode 7:33, for 136 miles. Some hard efforts, some easy efforts, hilly to rolling the whole time. I am a 5'9 137lb racer. I estimate my brm to be about 1700 calories per day.

I estimate 500 calories per hour burnt on a ride like this and consumed 3500 calories yesterday, which covers calories burnt. However, if you spend 80% of your awake time on the bicycle, does your BRM then go down? Should I have consumed closer to 5000 calories to only have a 500 calorie or so deficit, this is huge difference.

Also those of you with powertaps and whatnot, what is a good estimate of your caloric expediture for ride times/distances/races etc?
 

ric_stern/RST

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Nov 11, 2002
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Hurstpierpoint
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cuocciom said:
I'd like to add a bit to your question. I bought a PowerTap SL this winter. I can track work in kilojoules for each ride. I've read that kilojoules roughly equals kilocalories burned; if true, then I should look at my kilojoules and have a fair idea of what I've burned. On the other hand, I've read that there is a conversion of 4-5 calories per kilojoule. If true, then I'd be burning 4-6 thousand calories a day for 35-50 miles of riding. This seems ridiculous to me.
My problem, I believe, is that I don't know what kilocalories and kilojoules mean in non-scientific terms. Are there others out there who share my confusion? Can anyone explain this in terms that a non-physicist can understand?

Lets have a quick go...

Each Kcal is worth ~4.18 Kj, i.e., to arrive at energy expended (from PT/SRM data) in the usual 'food' metric (Kcal) you divide your power meter reading by 4.18. Lets say you rode for several hours and expended 2000 Kj, your Kcal expenditure was therefore, 479 Kcal.

HOWEVER, humans are ~ 20 - 25% efficient when cycling. Therefore, to arrive at the actual expended energy in Kcal you need to multiply that 479 by a factor of 4 to 5, i.e., 1916 to 2395 Kcal. Somewhere between these two figures is the likely amount of energy that you expended in Kcal.

If you knew how efficient you were you could more accurately calculate it, BUT, efficiency alters for the absolute power you ride at, the cadence you use, and environmental and topographical conditions. Unless you ride round with a portable gas analyser (to measure the air you breathe out) you won't know the exact value of your efficiency.

We can therefore, ballpark our energy expenditure in Kcal simply by saying that whatever your power meter said in Kj is the Kcal expenditure, i.e., in this case you expended ~ 2000 Kcal.

Cheers
Ric
 

wilmar13

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Nov 30, 2003
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ric_stern/RST said:
We can therefore, ballpark our energy expenditure in Kcal simply by saying that whatever your power meter said in Kj is the Kcal expenditure, i.e., in this case you expended ~ 2000 Kcal.

OK, but what about calculating total calories burned that day... in my case for example I estimate my BMR to be about 2500 cal/day (6'4", 190lbs) so lets just say it avgs around 100 cal per hour. Now I go on a 5 hour ride and "burn" 4000 calories. Is the contribution of my BMR being counted there based on the work I did or if I count my total caloric requirements for the day as 6500calories (4000exercise + 2500BMR) did I over estimate it from double counting during that 5 hour period (not subtracting out 500 calories from the 100 cal/hr BMR)?
 

kmavm

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May 16, 2005
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wilmar13 said:
OK, but what about calculating total calories burned that day... in my case for example I estimate my BMR to be about 2500 cal/day (6'4", 190lbs) so lets just say it avgs around 100 cal per hour. Now I go on a 5 hour ride and "burn" 4000 calories. Is the contribution of my BMR being counted there based on the work I did or if I count my total caloric requirements for the day as 6500calories (4000exercise + 2500BMR) did I over estimate it from double counting during that 5 hour period (not subtracting out 500 calories from the 100 cal/hr BMR)?
"Basal" metabolic rate is just that: "basal." It's the amount required to keep you alive, breathing, your heart beating, neurons firing, immune system working, etc. These biological processes don't stop just because you happen to be on a bike, and your power meter cannot measure how much energy you're using to grow your toenails, move your diaphragm in and out, etc.; it can only measure the work you put into your bicycle's drivetrain. So, yes, your caloric expenditure on the bike should generally be added to your BMR.

Note that gas-exchange is a different kettle of fish; if you're measuring kcals via gas-exchange, then all those basal elements really ARE being counted. This is one of the reasons oxygen economy appears to be lower at lower intensities; a greater portion of the oxygen you're taking up is going to holding your body upright on the bike, sucking your gut in so you look cool, etc., compared to the energy being used to push the bike forward.