Cycling UK Forum

The only way to be sure of your energy expenditure is to actually measure it while you are doing it; for example ride strapped to a machine measuring your O2 consumption.

I'm curious to know why this obsession with calories?

Your calculation, even if you do make it more complicated, will only supply you with a generalisation. And then, once you have this figure, what are you going to do with it?

The calorific value figures for food can be up to 20% out because food is variable. On top of that, the calorific value of food is measured in a bomb calorimeter, in other words, it is burnt and the heat produced measured. This is rather a long way from what a digestive system does. (EDIT oops, not any more, see below). It is also accepted that the actual calorific value of food depends on how it is prepared before you eat it, and possibly what you eat it with.

For example, your body will extract fewer calories from a raw carrot, compared to a cooked carrot, and fewer from a whole cooked carrot compared to a liquidised cooked carrot. The amount you chew it may make a difference. And nobody really knows quite how many calories you take in from soluble fibre which is broken down by bacteria in your gut. Which are different in different people; and the digestion of women is different to the digestion of men; and the efficiency and length of our guts varies, both by individual and age. And so on.

And once you've digested your food, all those differences in efficiency Mick was talking about come into play.

So, unless you make intimate friends with a seriously tooled up sports physiologist, the best you can hope for is a fairly vague rule of thumb, even if you do take into account wind resistance, rolling resistance, the molecular composition of tarmac, horoscope etc etc.

*Edit - I've just double checked on the Bomb calorimetry thing and I'm waaay out of date on that one. These days, the calories are found by calculation from chemical analysis of digestible components with reference to standardised tables. This is because fibre has a calorific value in a calorimeter that is not the same in a body. The standard agreed guestimate is 2 calories per gram of soluble fibre but, as I mention above, is not known for sure. In short, the calorie value obtained is an estimate, not a measurement.

Well said.

Incidentally, Audax UK once published a value of 8 kCals/min as a guideline for Randonneurs.

This was taken out of the Handbook. Prob because it was misleading.


For the really serious, measure you urine Specific Gravity, weigh yourself and do a % BodyFat caliper test every morning.

I reckon that there are calories and there are calories. I understand completely what you were saying, Hypocacculus, and that is why I mentioned efficiency earlier on. No one had mentioned efficiency. How a car is tuned and what engine is fitted can make a whole heap of difference to the fuel consumption. Compare that idea to the human machine riding a bike.

A good cyclist will have a well-tuned body and his engine will be large and efficient, and also be generally un-stressed. Compare that to an un-fit non-cyclist. He will have a small engine working very stressed at the top end of his output and really in-efficient too.

Any calorie calculation method can only be used comparatively for one individual person. ie compare how you improve as you get fitter. I have a Garmin Edge 705 and it assumes many things in its calculation. What it doesn't use is heart rate - funny when it is supplied with a HR monitor!

I upload my rides into Ascent, and Ascent re-calculates just about everything from the raw data that comes from the 705. Most of the figures tend not to be the same as the 705's - especially calories.

I take my calorie consumption with a pinch of salt - if you'll pardon the poor analogy! - but what I do do, is to read the info as an interesting fact and no more than that. I can fairly reliably contrast and compare the info because it is me on my bike on a chosen ride. Even the same rides can be different, probably due to wind, how fast I'm going, how I'm feeling, and how fast my heart beats. The fitter and well-er I feel, the fewer the calories consumed.

My figures are of no practical use to me or anyone else. They are just interesting.

I have a Garmin 605. It calculates 'Elevation' using 3D GPS positioning, so the kCals it shows are not worth a jot. They are well over due to fluctuations in calculated elevation.

As far as I can gather from Garmin, the unit has a 'cookbook' bicycle roadload curve of J/s vs speed. To the base curve, it adds on Joules based on the kg figures its told and the rate of vertical movement.
If I stand still and wave the thing up and down, it reckons I'm using 50 kCals a minute raising my arm up and down.

The 'cookbook' roadload curve ( to my calculations ) is based on a racing bike with the rider wearing shorts and SS jersey in 20 Deg C calm air.
With minimal elevation fluctuation riding along a flat road, it gives approx 7 kCals / min at 15 mph, no matter how heavy the 'Bike lbs'.


As Hypocacculus says, the only way of determining true kJ consumption is to analyse O2 and CO2 of inhaled and exhaled breath.

Cyclenut wrote:

Ayesha wrote:Motive energy is generally regarded as 25% of total energy used, but this % falls when the weather is brassic and you're not wearing thick enough clothes.

Of course that must cause quite a lot of variation, probably more than this supposed gender difference. Regarding that, I think it's a bit fishy that nobody has so far ventured a number, or even so much as an opinion as to whether it's men or women who convert food into work most efficiently. Maybe it's a myth. Or if not a complete myth, I daresay any gender difference may be no more significant than the difference in food conversion efficiency between, say, fat people and thin people, tall and short people, those with high versus low blood pressure, etc, etc.

What we need is more numbers and fewer opinions.

After scratching through some old papers ( Royal college of Nursing ) I finally came across the page which states women's rate of kCals/hr/m^2 body surface area is lower than a man.

The calcs give a figure of kCals/hr/m^2 for a skin area of 1 m^2. Women is less than men.

In other words, women conserve energy. If they eat too much, they put on fat quicker. ??

[something to do with the relative position of the reproductive organs. Inside rather than outside ]

So how does that equate to METS? This is why decent exercise machines ask for the subject's sex.

For men and women climbing hills on bikes, they both have to expend the appropriate number of Joules to get to the top, but the woman gives out less energy as heat??
The difference, I'm afraid, is given out as noise through talking.

Ayesha wrote:The calcs give a figure of kCals/hr/m^2 for a skin area of 1 m^2. Women is less than men.

Interesting. What are the figures?

snibgo wrote:

Ayesha wrote:The calcs give a figure of kCals/hr/m^2 for a skin area of 1 m^2. Women is less than men.

Interesting. What are the figures?

For nutritional requirements while in a hospital bed, the patient is measured for 'frame size'.... Wrist circumference and height.

The pateint's 'ideal' BMI is calculated ( to disclude excess fat %, as fat doesn't need feeding ).

Body surface area is calculated based on the patient's 'lean mass'.

For a cyclist, can we assume he/she is classed as an 'Athlete' and does not have any 'excess fat % ??

kCals/hr/m^2 is then assessed upon the patient's age.

For a 25 Yo, Males give out 39.5 kCals/hr/m^2 and girls give out 37.

For a 55 Yo, males 37, and ladies 35.

Its not linear. For a 15 Yo, boys 46 and girls 43.

Ayesha wrote:

snibgo wrote:

Ayesha wrote:The calcs give a figure of kCals/hr/m^2 for a skin area of 1 m^2. Women is less than men.

Interesting. What are the figures?

For nutritional requirements while in a hospital bed ... Body surface area is calculated based on the patient's 'lean mass' ... kCals/hr/m^2 is then assessed upon the patient's age.

For a 25 Yo, Males give out 39.5 kCals/hr/m^2 and girls give out 37.
For a 55 Yo, males 37, and ladies 35.
For a 15 Yo, boys 46 and girls 43.

Interesting numbers - but - that's figures for how many calories to merely keep warm, not do any work. I'm not a biologist, but I believe that a body's mechanisms for maintaining it's ideal operating temperature of 37º are not the muscles - except in emergencies, when we get cold enough to shiver. I also believe that women have a slightly thicker layer or subcutaneous fat than men. So a woman of the same lean mass will nevertheless be better insulated (and no inconvenently external sexual heat sinks!) so will not need as many calories to keep warm.

So this still doesn't really get us any closer to knowing if there's any difference in how efficiently people convert food into work.

I'm guessing some more here, but it seems likely that there will be a major difference between aeobic and anaerobic work, with aerobic being most efficient, since that's what we're evolved to do mostly, with anaerobic only for emergencies as it were, and since the consistent driving force of evolution is to do more with less. (Our current desire to find ways of consuming more food than is good for us being a very recent and doubtless temporary preoccupation of a fortunate few!) Anyway: assuming anaerobic is less efficient, perhaps the best way to burn calories on a bike is to go somewhere very hilly and go eyeballs-out up all the climbs and freewheel down the other sides. Maybe that way you'll burn even more than doing the same total work in the same time, but at a constant rate on a flat road.

However: will all that anaerobic work so deplete one's glycogen stores as to cause one to stuff one's face upon return? Will, on the other hand, the purely aerobic work of the steady-state rider be more readily fuelled meanwhile by the metabolism of bodyfat and without stimulating appetite to the same extent? Unfortunately I can only speculate. However I do know that the lightest my wife and I have ever been (and both as fit as a fiddle) was when we got back from a long camping tour with a small saucepan!

Fascinating, Ayesha. Thanks.

I note for bed rest there is very little difference between the sexes (about 2 parts in 40), compared to the generally quoted GDA kcal of 2500 for males and 2000 for females. So if men and women do the same daily work, the GDA differences indicate that women use food energy more efficiently than men. (As you indicate they also do when at rest, ie when doing no useful work, but with less difference between the sexes.)

Just trying out the numbers: I'm a bloke, the thick end of 55 years old, roughly 2 sq metres surface area, so my bed rest requirements are 2 * 24 * 37 = 1776 kcal. Yeah, that figures.

When considering food conversion efficiency, where does the other energy go? If I am 25% efficient and eat 1000 kcal so I can expend 250 kcal cycling, does that simply create an extra 750 kcal of heat?

snibgo wrote:When considering food conversion efficiency, where does the other energy go? If I am 25% efficient and eat 1000 kcal so I can expend 250 kcal cycling, does that simply create an extra 750 kcal of heat?

Good question.

I've just returned from an 800mile ride (over 12 cycling days) and used (according to Ascent) 48,000cals or 4,000 per day average. I was 12st 9lb when I left, and 13st 2lb when I got back. ie put on 7lbs!

However, as usual for me after a long ride, I start to loose weight after I get back despite continuing to have a huge appetite. I am now 12st 10lbs five days after finishing. ie I've got back to my original weight. I expect to continue to loose weight for a week or two.

Why this delay? Why was I heavier at the end of the ride?

If I need 1,750cals just to "tick over" and another 4,000 to ride a bike for a day, that makes 5,750cals. Can I eat that much? How does that look on a dinner plate?

snibgo wrote:When considering food conversion efficiency, where does the other energy go? If I am 25% efficient and eat 1000 kcal so I can expend 250 kcal cycling, does that simply create an extra 750 kcal of heat?

Got it in one. That's why hard work makes us red in the face and sweaty. Both are to dispose of waste heat: by directing blood flow through the skin and providing moisture for evaporative cooling.

One of the neatest aspects of cycling is that the harder you work the better draught of cooling air that produces over the body, so you can cycle harder in warm weather than it's possible to do many other sorts of work or exercise without overheating.

snibgo wrote:Fascinating, Ayesha. Thanks.

I note for bed rest there is very little difference between the sexes (about 2 parts in 40), compared to the generally quoted GDA kcal of 2500 for males and 2000 for females. So if men and women do the same daily work, the GDA differences indicate that women use food energy more efficiently than men. (As you indicate they also do when at rest, ie when doing no useful work, but with less difference between the sexes.)

Ah yes, but the resting figures are per sq metre, derived from kg lean mass, so there you are comparing males with females of the same weight. Whereas the GDA figures compare Mr Average with Mrs Average - and Mr Average is bigger and heavier, with more skin, so he needs more calories just to keep warm at night and more to move his mass around once he's up and active.

According to Adultdata, in the UK in 1998 the mean weights for a man and a woman were 79.75kg and 66.7kg. So in terms of kcal per kg that advice suggests a consumption of 31 for men and 30 for women. Remarkably similar. That's an even smaller difference than the difference in resting figures. So it seems that to a first approximation there can't be any difference worth bothering about between the locomotive efficiency of men and women.

CJ wrote:... Mr Average is bigger and heavier ...

Ah, yes, of course. So it sounds as if the GDA kcals are based on a kcal/kg that is constant across the sexes, and thus efficiency is independant of sex.

Have we bottomed out if age makes a difference to energy efficiency?

Dunno the answers to Mick's questions, but I note that a Morrisons Deep Pan Meat Feast Pizza, 10" diameter, is a mere 1194 kcal so he would need nearly five every day. That's okay. Two for breakfast, two for lunch and one at tea. And no beers on top!

Numbers might be nice but not easy to produce when the answer is "it depends".

Muscle fibres are believed to be something like 18 to 26% efficient at converting the energy from respiration to movement. This is absolutely not the same as saying we are 18 to 26% efficient at converting food to movement.

There is no difference between men and women if you look at muscle fibres in isolation. However, there are differences in the distribution and the way that skeletal muscle is arranged in men versus women - it is well known that a wide pelvis is less efficient for running for example. This would apply to men just as much as women, but it is usually women that are cursed with child bearing hips. These differences are probably insignificant on a bicycle. Men trounce women when it comes to strength and speed mostly due to greater muscle mass, but women have an equal, if not greater capacity for endurance. If your wife/girlfriend/mother is knackered after following you all day on a bike, it might be because you are expecting her to have the strength of a man and pushing her too fast on a bike that is as heavy as yours, not necessarily because she is less efficient, more fat or even less fit than you.

The biochemistry of energy production is waaaay more complicated than breaking it up into anaerobic and aerobic and the two are inextricably linked. Furthermore, it is inextricably linked to the metabolic circumstances of the body as a whole, because the body will allocate resources where required.

Glycolysis is the component of energy production that is anaerobic but its metabolic by products can be, and are, recycled and fed into other aerobic energy processes once the oxygen becomes available. Glycolysis happens whether or not the exercise is anaerobic and powers all sorts of things, not just muscles. (Glycolysis could theoretically go on indefinitely but is inhibited by the build up of its by products. Fitness is all about dealing with them).

What complicates the system is that the products of glycolysis may also be routed into other bodily processes that are not linked to muscle movement, depending on circumstances. It not as simple as one molecule of glucose will lead to x amount of movement. You can only know how much it could potentially produce under ideal circumstances. Much like the mileage figures given in car adverts.

But there are two parts to human efficiency. As well as looking at the efficiency of converting stored energy to movement, you have to look at the efficiency of getting food turned into energy in the first place. This is even more complicated and subject to huge variation, as I touched on before.

Anyone is looking for a very comprehensive model of the energy (or power) requirements of cycling could do worse than use Modeling road-cycling performance, Olds et al, 1995, http://jap.physiology.org/content/78/4/ ... f_ipsecsha

It includes allowances for drafting, tyre pressure, kinetic energy of moving legs and feet, and the effect on drag of different helmets, long hair, short hair or baldness, and whether legs are shaved.

I can't find software that incorporates this model. If someone has an hour or two to spare, perhaps they could knock one up?

The model doesn't mention sex or age. But someone might simplify it by assuming that women have long hair but shaved legs, old men are bald, and so on.