Power (watts) and calories

I've been using power output to measure my cycling workouts. It would be useful to know the incremental caloric consumption for a given average wattage over time. There should be a relationship. I'm making a guess that an average of 190 watts for an hour, is roughly equivalent to 1000 kcal. The formula to estimate kcals would be kcals= time in hours x average watts x 5.26. My guess is that this approximation would have to be adjusted for changes in physiological efficiency at different intensity levels. Is there any research available on this topic - or does someone know a better formula?

 

To work out energy expenditure in joules, you multiply average power (/1000) by time in seconds. Thus, the work done in 1-hr at 190 W avg is 684 kj (0.19 x 3600). To convert this to kcal you divide by 4.18, i.e., 164 kcal.

However, the human body when cycling is only around 25% efficient (normal range ~ 20 - 26%), thus, this needs to be calculated, to work out the human energy consumption, i.e., 164 / 0.25 = 656 kcal.

To work out the exact amount of energy expended you'd need to go to a sports science lab and have your expired air analysed. As a *good* ballpark figure, it's best to estimate energy expenditure simply by quoting the work done in kj as kcal, i.e., in the above example just say you 684 kcal.

Efficiency changes with both cadence, and absolute power output, and also with fatigue. At higher cadences, efficiency *decreases*, while at higher powers it *increases*. As you fatigue efficiency decreases.

Ric

 

Ric - thanks for the input. Looks like you've done, or read, some research on the topic. I was hoping for a somewhat higher number, since 190 watts/ hr average on my Computrainer (no wind or hills) results in a 20 mph/ average speed. If this burns only 684 kcal, then I'm only doing about 34 kcal per mile. Then again, maybe that's the reason I'm not losing weight as quickly as I wanted.
Bob

 

Part of this (efficiency) falls into my PhD!

If you're looking to loose weight, then upping your training (volume and/or intensity) and moderately decreasing your energy intake (~ 250 - 500 kcal per day) will result in about a 1lb per week weight loss, which is about the best most people can aim for.

Ric

 

Ric - Your advice is spot on. The hard part for me is not increasing my training volume. But I find making modest changes in my diet very difficult. It seems to be much easier to make a big change in diet.
Bob

 

Bobkny,

Seems like any reduction in food is a big change for me!

Two easy ways to reduce food consumption are to (1) eat smaller amounts at each sitting or (2) cut out snacking. 250 - 500 kcals per day only amounts to aprox 1 chocolate bar or 2-3 slices of bread or 2-3 apples or... This isn't a massive amount to loose from your diet each day!

Unfortunatly, restricting energy intake more than Rik has recomended is a bad idea. Although it can lead to dramatic weight loss in the short term, there are longer term undesirable consequences.

 

Hmm,

What kind of undesirable consequences would those be?

Last year i managed to loose 42kg in 5 months (averaging around a 2kg/week weight loss)..

it's been 6 months after those 5 months now, and i'm still thin (lost maybe another 5kg in those past 6 months..

Soo, i'm wondering what consequences could i expect from this fast weight loss?

 

Sorry I was being lazy... should have written 'there may be longer term consequences'.

Well done on your weightloss DevilMan, this must have had a significant effect on your performance that you must be pleased about.

Unfortunatly people who lose weight quickly can be more likely to put weight back on than those that lost it slowly. Undesirable consequences of rapid weight loss range from diabetes, kidney problems, increased cancer risk, skin problems, etc. For some there are also psychological problems. These risks are magnified when the weightloss comes from protein diets, no carbs diets, diet pills, etc. However it seems that you lost it with less eating and more exercise which is the safest and most effective way to lose weight.

You lost 47 kg in 11 months, as I weigh 60 kg this level of weight loss for one month would obviously be unhealthy for me. To lose this amount of weight you must have been carrying a little excess in the first place and there are likely to be health benefits (particularly reduced risk of heart disease).

All in all wieghtloss of 1 to 2 lbs a week is preferable for continued weightloss and maintained health.

 

It sure had a significant effect, because i didn't ride a bike for like 15 years (and did never any exercising).. When i started my training rounds my avg speed was around 18-20km/u ..now i managed to do the same rounds in 32-34km/u average

And my weight dropped from 112kg to around 64kg

Now i'm even too fast for my riding buddys ;-)

And i indeed managed to loose the weight just by reducing the calories and a lot excercising (12.000km in one year ).

To me this also seemed the way to go, all that diet stuff will get the pounds of, but when you start eating normally the pounds will get back on..

My thoughts on weight loss was that it needs more of a complete lifestyle change than just eating less.. If you can't commit to changing your lifestyle it will probably be very hard to stay slim.

Anyway enough (more or less) off topic babble, back to the topic of watts and calories (which i don't know much about ;-) )

 

As the originator of this thread, I want to thank the other contributers for their input and ideas. Although I introduced this topic to get a better understanding of caloric consumption in exercise- particularly as it relates to cycling power output, my intent was to see if I could use this measurement to better manage my body weight. My situation is somewhat different than Devil Man's, in that at about 5' 11" height, I have little difficulty in maintaining a body weight of around 160 lbs. In competition however, especially in climbing, I perform much better at around 150 lbs. It's taking off those ten lbs of winter weight gain, that I always find so difficult. What I've learned so far from this thread is:
1. Cycling is not as calorie intense as I thought. Probably because it is a non weight bearing activity. I thought that I was burning 800-1000 calories per hour, on my bike-- but 600-800 is more likely.
2. Conventional wisdom is to slightly reduce calories and increase training volume to produce a 1-2 lb per week loss.
3. Some people - myself and at least one other contributer - find it psychologically difficult to make small changes in their diet. And, for these types, it may be more effective to make a radical change for a short period of time.
Bob

 

DevilMan;

42Kg in 5 months is a LOT of weight to lose. You want to becareful about weight swings like that, especially as you get older. Your body reacts in unusual ways, especially if you have a few of those dramatic weight swings (loss/gain).

-CG

 

I know weight swings like this would be bad, but...

I don't have weight swings, i set a goal a year ago, that was to become a fit and slim person, and i don't plan on becoming fat ever again.

I'ts been a whole year now since i've started and i'm still as dedicated as i was in the beginning, although i've not lost any more weight and the last few months (as i already weigh 65kg's now ;-)

 

okay I'm dragging up a really old thread now!

I posted this question to Racermate but wanted to get an authoritative response as well.

I've been using the CT in 'fixed resistance' mode where the load-gen resistance is fixed (within normal limits etc etc) and one simply pedals in whatever gear and cadence. It's basically mag-trainer mode.

In this mode, I can call up a screen that displays instantaneous power and another that displays totalized CAL. A timer can be started and stopped to control the timed duration and the totalizers.

Average power at the e/o a timed interval is not reported but I figured the reported total CAL should be pretty close to the product of duration x average power. But the calcs turn out significantly lower than my perception of average power held times duration. I don't stare at the display by any means but check it every few minutes to make sure I'm around my power target.

I asked Racermate about the calculation of CAL and they told me it was based upon an assumed rider cycling efficiency of 26%. I realize CAL is actually kcal but I'll just stick to that nomenclature.

So from info in the thread I get:

CAL (out) = kJ (out)/4.184 where out denotes delivered power to the rear wheel

And if efficiency 26% = CAL (out)/CAL (in) or CAL (out) = CAL (in) x 0.26

then, CAL (in) x 0.26 = kJ (out)/4.184

or kJ (out) = CAL (in) x 0.26 x 4.184

or simply kJ = reported CAL x 1.088 or close to 9% higher than CAL as reported by the CT.

Sound correct? I have to say this ties in quite well with my personal estimate of average power over the interval.

 

Rick, it's funny because I was just looking at this yesterday.

Your math looks right. I calculate that at 26% efficiency, 1 kJ at the pedals = 0.919 kcal produced by the body.

I had a lab test last year and the report gives me my oxygen consumption and power at LT and at VO2max. From the numbers in the FAQ that I linked to above and a quick calculation, I was 24% efficient at VO2max and about 22% efficient at LT. Since my annual average IF is around 0.77 or so, I figure that I average out to ~21.5% efficiency. Twenty-six % efficiency is a bit high based upon what I've read but I guess that you can't change it.

 

thanks Steve. I'd forgetten about the old FAQ.

This was starting to do my head in a little as while doing L4 intervals at a manually noted level (checked every few minutes) of around 370-380W, the CT was telling me, on a 1:1 CAL to kJ basis, I was averaging only 345 or so. I though "geez" is my PE THAT far off? !!!

anyhow, it doesn't matter to me what CT uses as I'm only interested in back-calculating AP for working intervals (err.. as long as I know what it uses, that is!). 1.088 of CAL makes things work out just fine and in fact I noted a couple of efforts were about 5W higher than I'd estimated.

thanks again,

 

So you're not riding with a PT or SRM while on the Computrainer?

 

http://www.ncbi.nlm.nih.gov/pubmed/16160031

 

Are you sure you didn't mean this:

http://www.ncbi.nlm.nih.gov/pubmed/...ez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum

In any case, Armstrong might not be a very good example, since his efficiency, while above average during his peak years, wasn't exceptionally high.

 

nope - not this year. After adjusting my CT to match my trusty ole PT Pro in 2004 and checking it regularly over time up 'til last September, the typical variation in interval AP was <1%. Amazingly often 1W ...

In Sept, I checked it with a ramp test against the PT and my road SRM and haven't felt the need to put either (good) bike on there since then. My very old trainer bike's PT harness is shot so I need to replace that by spring when I'd get a little more angsty over things.

As my intervals and indeed training in general this time of year are just longish, steady-state affairs, I've no trouble mentally noting the key 2-3-4 stats for the workout. The next morning I pop the basic power x durations into one of the old AP/NP/TSS calc. spreadsheets and use that for logging purposes. I still use the beta PMS spreadsheet so popping the summary TSS data in there isn't much trouble either.

Basically my 'intervals' are quasi steady-state so I don't bother with the slight intra-interval NP/AP differences. A percent or two potentially low TSS isn't going to make much difference to me.