Calories or Kilojoules?

Like many of us, I’m engaged in a constant struggle to maintain a healthy weight and fitness level, and being an economist, I naturally like to think about this in quantitative terms (I’m not alone in this).

The basic equation is simple[1]: Energy used – energy consumed = fat burnt. But to make sense of this equation, we need units, and that raises the immediate questions:

Calories or kilojoules? and
How much do I have to burn to lose 1kg of fat?

The short answers are: Calories and 9000 Cal[2]

More over the fold

Calories (more precisely, kilogram calories, Cal in SI) are familiar and widely used, but Kilojoules are the Standard International Unit, and are more prominently displayed on food. 1 calorie is approximately equal to 4 kilojoules, which is discouraging, because Australian food labels report kilojoules (calories in small print) while most exercise programs only give you calories. Looking at how the unit are derived, this is the wrong way around.

A calorie is a unit of heat: the amount required to raise the temperature of a kilogram (or litre) of water by 1 degree Kelvin (or Celsius). That makes sense as a measure of food energy, since we burn it. A Kilojoule is a measure of work done: it’s what’s needed to generate one kilowatt of power for one second. So, if you’re concerned with running or cycling, the question of interest is how many kilojoules you can generate in torque, overcoming resistance and so on.

So, we might say that Calories are input and Kilojoules are output. If you want to lose weight, it’s calories burnt that matter. But it turns out, quite by chance, that calories are also a pretty good output measure. That’s because, considered as engines, our bodies are only about 25 per cent efficient, which roughly cancels the ratio of kilojoules to calories. So, one calorie (= 4Kj) consumed translates to about 1Kj of useful work.

That makes sense to me. At top pace, I burn around 1000 Cals an hour. At 25 per cent efficiency, that amounts to an output of 250 watts, which is about right, given that I weigh 70kg, am travelling at 3.5m/s and would decelerate to zero in a couple of seconds if I stopped pushing. (The efficiency figure is just for conversion of food to mechanical energy – there’s a whole lot more to consider in terms of the mechanical efficiency of running. Measurement is much easier in the case of cycling).

On the second question, the energy content of fat is around 37Kj/g, or 9Cal/g. Exercise will also result in fluid loss, but since fat doesn’t contain much water, this is all temporary.

So far, I’ve talked only about the difference between calories in and out, but what about the levels? It’s surprisingly hard to get good estimates, but it appears that a moderately active man of my age should be consuming 2000-2500 Cals/day. I’d say I’m burning an extra 1000/day, but on the other hand, I’d like an energy deficit of 500/day at the moment, since I’m trying lose a few kilos I put on at the end of last year. However, I’m not measuring calorie intake with any precision, relying for the moment on more activity and cutting down on a few obvious items, like alcohol.

fn1. Exploiting it is not so simple. I’m not an expert, but it seems pretty clear that, if you have been at a given weight for a long time, your body will try to keep you there, by slowing down metabolism, sending hunger signals to your brain and so on. Still, having lost 15 kilos over a couple of years, and kept all but 2 or 3 off for a couple of years more, I don’t accept the fatalist view that there’s nothing to be done about this.

fn2. My earlier understanding was 7500, and since posting, I’ve seen some sources to support this. YMMV

66 thoughts on “Calories or Kilojoules?

  1. @Jim Birch

    A lot of what you say about willpower and yo-yo dieting is right. Last time I used the one meal a day diet I lasted about 8 months and lost about 16 kg. It was very tough mentally in the end. After getting injured and losing the exercise and diet habit I put the 16 kg and a couple more back on over about 18 to 24 mths.

    What makes me think it will be different this time? Blind optimism and stupidity I guess plus a little more experience about how I approach it. Even if I fail again, I feel that yo-yoing is better then going ever and ever upwards which people seem to do after a certain “gross failure” point in lifestyle, diet and exercise is reached and passed. Cruelly, there seems no almost hope for people after that point without major outside assistance.

  2. @Sancho I think there maybe a misconception, it can’t be standing up from sitting on the floor it must be standing up from a seat using the floor (ie use your feet not the arms of the chair).

    The study used quite old people and it would be unreasonable to expect any of them to rise from the floor without using their hand(s).

  3. @John Quiggin

    Nothing beats the empirical approach. That’s my motto. I set the problem “get up from a seated position on the floor without using hands, knees or elbows” for my wife and 18 y.o. twins and watched their attempts carefully.

    My daughter was the one who crossed her legs just above the ankles. As she is light also but not as athletic as my son I figured that it was an intuitive, instinctive or personally learned method from a younger age. As soon as I saw it, I thought there might be a lever advantage in it so I tested it on myself. I think there is all of a levering advantage, the advantage of bringing more muscles into play and a compensating geometry for lack of range and mobility in joints.

  4. @rog

    I think in the original post/link it was made clear that at least one hand could be used from the floor. Some of us just wanted to take it further and do it without hands or knees.

  5. @Ikonoclast As a kid everything I did was physical and all work was by hand. We even used to shovel sand soil etc onto the truck. I marvel at old industrial chimneys and buildings because I know how much effort went into making the bricks, hauling them up and then laying them. Not an engine in sight. Same with old split post fences and slab huts, made with elbow grease.

  6. I’d like to state that using calories instead of joules, or worse calories instead of kilojoules, or even calories instead of kilocalouries makes Jesus cry. Mind you, this particular Jesus has a physics degree and is a bit of a pedant.

  7. @pjm

    I didn’t say any cultures were vegetarian actually. There is undoubtedly a strong correlation btw slimness and health with low proportions of animal products as seen in Asian diets (compared to Western ones) and within Chinese populations (see the China Study). And why should we take dietary habits (there wasn’t one pre-agricultural diet) as the perfect template for eating? We also evolved to bash neighbours to death because they weren’t in our tribe. Yes, we’ve evolved to survive on a massive range of dietary intakes. Move on.

    Gary Taubes and his followers had better get on to every Tour de France rider and marathon runner with the high protein/paleo/fasting fad ASAP. They’re ALL doing it wrong! Right now, what do the pro teams in the Tour Down Under in Adelaide munch on all day? Bananas and coke. Not steak. Five hours in the saddle, burning 5000-8000 calories a day eating pasta, vegies, lean meat, pasta, rice, coke, energy bars, and more pasta. Oh and Taubes has also said that exercise won’t slim you down, it’ll just increase your appetite and if you’re slim you’re just born that way. He’s a scam artist.

    Carb up and I’ll see you out on the road running and cycling. Not in the gym looking like a puffy unfit Michelin man with high cholesterol.

  8. my first job was in a bank. standing up all days does amazing things for your fitness.

    after 4 weeks annual leave, I was exhausted by the end of my first day back.

  9. I think physics Jesus was traumatized by a calorie when he was little. I have to admit I don’t like them myself. Anything that isn’t SI makes me feel like I should have 12 fingers.

  10. @rog

    Hang on rog, how old are you? Nearly 300 or maybe 1000 or more? Steam engine invented in 1712, the Newcomen. First practical application about the same year for pumping water. Don’t give me this no engines stuff. 🙂 Even you said you shovelled sand onto a truck. An internal combustion engine truck I dare say.

    Cranes existed in the middle ages, the treadwheel crane (a machine tho powered by human or animal muscle) not to mention good old block and tackle. Also, windlasses, windmills, water wheels etc etc. All machines, and many in fact used renewable energy.

  11. Oh dear… Both are units of energy, and as such interchangeable with the appropriate constant (if you can work out exactly which Calorie you’re using), so forget the units of heat and units of work – same thing. Apart from that, as Ronald pointed out, the calorie is simply an evil unit.

    As I remember 1Cal ? 4.2J, which combined with it’s definition (the amount of energy needed to raise the temperature of 1g of water by 1°C (ignoring the details)) leads to its only advantage: idly working out in your head how much water you could heat from room temperature to 100°C with a Mars Bar (?250kCal). About 3L. And a circa factor 4 in there isn’t too much extra work to do it in Joules…

    So if you’re not into mental combustion of food, we’re left with a pointless unit with a dubious definition 🙂

  12. @Jim Rose
    Arranging a standing desk is a priority for me this year. Elevating desks are expensive, but there’s a range of hydraulic arms that can mount monitors and workspaces, which are useful for transferring between desks.

  13. @John Quiggin Why settle for less?

    Like most economic equations, there is always a gap between the pen and reality

    Energy used – energy consumed = fat burnt.

    Or is it:

    Energy used – energy consumed = change in muscle + fat burnt.

    Then someone may like to point out that energy used = energy consumed, and that you only get more muscle and fat if you import surplus food – an externality – and run into deficit when the supermarket is closed.

  14. A calorie is a unit of heat: the amount required to raise the temperature of a kilogram (or litre) of water by 1 degree Kelvin (or Celsius)??????????????

  15. IIRC, a degree is the same unit in celsius or kelvin. They just index from different temparatures: celsius from the boiling point of water and kelvin from true zero.

  16. This a.m. I went to Woolies and Subway. A sign in the Woolies delicatessen said our energy intake should be 8,700 kJ and Subway said 8,750 kJ. I presume that’s for a semi-sedentary adult.

  17. @aidee

    It can be a bit hit and miss with it’s calorie counting, but generally it keeps me thinking more about food, which is my primary burden. I can run a half marathon no problem, but it’s the food that the toughest part of staying fit!

  18. I’ve now lost 4 kg in 23 days. The scary thing is I can see no difference although my belt has easily gone in one notch. My BMI is now 26.5 – still way too high – and my waist is 103 cm when a healthy waist for me would be about 93 cm down to 91 cm.

    I am suffering some continuous leg weariness from outdoor work and long walks each day (50 minutes plus) combined with my obvious calorie deficiency. (Some of this leg weariness I put down to being 58.5 years.)

    Another scary thing is that dressed casually I don’t really look fat compared to a lot of other people my age. I think the general large expansion of waistlines has given many of us a false idea of what constitutes being overweight. I noticed this in the past when I dropped 10 to 15 kg. People I had used to think were quite trim started to look chubby and overweight to me. It’s an interesting distortion of perception that occurs. Whatever our weight we tend to regard our own appearance as the norm, at least for our own age group.

    I am thinking my goal should be 70 kg. That’s the only way I could break even 60 minutes for a 10 k run. Not sure of my chances in reaching and maintaining it though. 75 kg would be acceptable if I can increase muscle mass. A bit of mass is handy for yard work, cutting and lifting logs, leaning on crowbars etc.

  19. It seems I am about 178.5 cm. 70 kg would give me a BI of about 22 which is healthy but not low by any means. My wife starts to complain that I look “gaunt” below 76 kg. Without the subcutaneous fat in the face the wrinkles show more, ’tis all.

    If I want to actually run again, 70 kg is the first benchmark. 65 kg might be the benchmark if I get fanatical. I don’t call jogging running, I call it plodding. My theory is that there is a genuine difference between running and plodding. There is a distinct (qualitative) difference in the feeling. In plodding one runs “on” the ground. In running one runs “over” the ground.

    In quantitative terms, I think proper running involves (apart from just going faster than mere jogging);

    (a) significant glide time mid-stride when both feet are off the ground.
    (b) significant storage of energy in elastic tissues (mainly tendons) such that push-off is only partly muscle powered as it is also signicantly powered by stored energy in the elastic tendons.
    (c) toe landing on each stride rather than heel-toe landing or flat foot landing.

    I think (b) and (c) are difficult for older runners. Elasticity is lower the older you get. I was never a natural toe-lander even as a young runner. I got away with it at that time as I was extremely light. However, studies show (I believe) that toe-landers are several times less likely to get running injuries.

    I used to sail a bit once and there is an analogy I make between sailing and running. A small boat with a correctly shaped hull can proceed slowly by displacement movement or more rapidly by planing. A large plodder is working like a displacement boat in a sense. A large proportion of his energy goes straight down in the plod to arrest and prevent his whole bulk falling to earth. A lesser proportion of his energy is used to propel him forward. He would be better off walking briskly for his exercise at least until he gets to true running weight.

    A proper running gait mean you are “up on the plane” with a greater proportion of your energy now pushing you forward. Tendon elasticity (if you are still blessed with it) is used to store part of the the energy of falling and then it is employed it to push you back up and forward for the next stride. This bounciness and energy saving constitutes the feeling of gliding over the ground rather than just plodding on it. Such is true running.

  20. Footnote:

    More accurately walking is analogously equal to displacement movement and running is analogously equal to planing. Jog-plodding is the transition phase and a jog-plodder is stuck in the transition phase. A boat that does not have quite enough energy to rise to a proper plane will go a little faster but drag a larger and larger displacement wave. With a feel for sailing you can feel this displacement wave holding you back and preventing the boat rising on to a true plane.

    A jog-plodder has an analogous power-drag ratio or power-weight ratio problem and he/she can’t rise to a true running position and gait and is thus stuck in what it is really a transition gait.

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