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. @Andre S

    The take-away lesson here (definitely no pun intended) is that I should eat my dinner with my strongest reading glasses on. They make everything close up look quite a bit bigger. Good idea! I will try it! Nothing to lose plus the bonus that I can actually see clearly what I am putting on my fork.

    I have found other aids to dieting are;

    (a) Trumpeting the fact you are doing it. Very annoying for others, including co-bloggers, but it does lock one into a position where to fail involves a loss of face. We are often very motivated by “face”.

    (b) Shrink the stomach. Crash diet or one-meal-a-day diet for initial weeks as long as you can bear it. A shrunk stomach fills quicker and you stop eating. Admittedly the hunger can come back before the next meal.

    (c) Trick your stomach with hot water. That is, drink beverages like coffee and tea as near black and unsweetened as you can hack them.

    (d) Don’t be a total diet-fascist to yourself. Moderates say a one-meal-day diets (which I am using) with personal permission for just one snack a day in daylight hours. This snack must be either a single piece of fruit (say an apple or orange but not a banana) or say two rounds of corn/rice cakes (you know the things that are 1/2 the density of balsa wood) with a dab of vegemite spread between them. These austere snacks come to feel like generous and much enjoyed indulgences.

    (e) Allow one modest home-baked biscuit or a small slice of home baked cake (from low sugar, low salt, low fat recipes) after the evening meal.

    (f) Never, ever eat store-bought cakes, biscuits, snacks, lollies, chips etc. The same goes for fast food, pizzas, burgers etc. They are “altogether evil” as Gandalf would say. Funnily enough this kind of food increases your appetite more and more, the more you eat it. It is either the combination of fats, salts and sugars or some secret appetite stimulant (I definitely wouldn’t put it past them ie. the corporate capitalists) or both.

    (g) Graph your progress and have stage goals and final goals (weights, waist measurments, exercise performance goals etc.).

    (g) Walk (or run if you are slim and fit enough) for at least 45 mins a day and do it like Death Itself is behind you… because actually It is.

    I dont mean to be gloomy but death must be your existential adviser and motivator… personal trainer in fact. Oh, and have a nice day! 🙂

  2. Oh and I left out “keep busy”. Occupation and preoccupation, physically and mentally, can take your mind off food.

  3. Alas, in middle age we have to account for the second order effects of exercise – to whit injury. If I were still playing squash then I would still weight 80kg but my “Achilles says no” more emphatically than a Little Britain computer. Swimming is gentler but still can cause injuries. Plus it is dead BORING.

    The other side of the equation is eating. But I love food. I still wait in hope for a useful medical solution but the forces of self-righteousness rally against (

  4. I can remember a medical doctor saying, “People expect us to fix years of their laziness and self-indulgence with one little pill.”

  5. Try not to over-think the math on the energy consumption vs expenditure. Unfortunately, that equation is only a rule of thumb, and doesn’t approximate reality very well. For example, when intake is sufficiently low relative to expenditure, metabolism slows in a fasting response. Also, all intake and all expenditures are not alike. For example, the same amount of food eaten in one sitting should be a greater net intake than that food eaten in multiple smaller portions separated by time. This is because there is an energy cost to digesting food. Another example would be drinking cold water. Water contains no calories, but because the water is cold it lowers your internal temperature and to maintain homeostasis the body must consume some energy. On the expenditure side, the same amount of total work performed running or lifting heavy weight will have drastically different hormonal and adaptive responses, and so will have very different effects on total expenditures.

    The point I’m trying to make is that these types of equations (fat loss=intake-expenditure) do describe some aspect of how our body works, but only in a very general and oversimplified sense. If you’re trying to lose weight, it’s best to adhere to general rules than to try and perform intricate calculations. Want to decrease intake? Drink water instead of soda. Want to increase output? Remember to exercise a little more than normal. Is it helpful to count calories precisely? Considering the biological processes that govern fat loss are so complex that the scientific literature has trouble finding consensus in measuring many of the details, probably not. At least not until our understanding is more complete.

  6. @Jon

    I agree. I don’t count calories. Basically, on a diet, I eat a lot less and exercise a lot more. It works well believe me. But if one overdoes it as I have in the past one is storing up “willpower fatigue” for oneself. The crunch often comes when one needs to convert loss into maintenance. The relaxation of eating discipline and even exercise discipline becomes a problem at least for me.

  7. i read somewhere that as one gets older the body becomes more efficient at extracting nutrition from what we eat.

    so that (to me) explains why people gradually get heavier as they get older while complaining that they don’t eat any more than they normally do.

    and if one wishes to maintain a steady weight,one must gradually eat less.

    all the really old people i know or knew have or had tiny meals.

  8. @may

    I think it is that energy use declines. One stops growing overall at maturity so there is much less energy needed for growth purposes (some things still grow like hair, ears and skin to cover extra flab and of course the flab cells themselves) and less needed for repair purposes too if people become less active. Thus anabolic needs (metabolism for growth and repair) decline markedly. Old people (in our culture anyway) usually become inactive and very sedentary, thus have low energy needs. Muscles also become smaller, weaker and need less energy to function and can do less work. Thus catabolic needs (metabolism for energy production) also decline. I suspect the gut become less efficient at absorbing nutrient not more efficient with ageing but this effect is relatively minor.

    That’s a lot of declining.

  9. I’ve been mulling over similar thoughts. It must be the time if year and the need to ‘recover’.
    While you can treat it like an engine – you can’t ignore the various control systems in place to maintain enough energy in storage. So essentially treat this part of youself like a brat: you have to trick it, it doesn’t have a long memory, but is determined to ensure you survive. Having a calorie deficit every day may get you there but it will be a battle – with the control system turning down the metabolism, turning up the cravings etc. I think this is why the fasting diet may work (500cal/day just 2 non consecutive days per week, else eat normally). You also need to have ‘big’ low cal meals – eg fluffy eggs, cucumber & tomato salad etc – well spaced illusions.

  10. If you want to lose weight, you’ve just got to go hungry: there’s no way round it. You can modify your reactions to hunger with practice and motivation.

    I’m lacking something in the motivation dept, but heigh ho…

  11. @AJS

    I don’t think the “control system” turns down the metabolism in the case where;

    (a) the calorie deficit is moderate;
    (b) the body has plenty of spare fat to use for energy; and
    (c) one makes the effort to exercise and perform all other normal tasks.

    Cravings is an interesting topic though. I find that if one stops eating junk food altogether then the junk food cravings soon stop. The danger is after 6 months or so (in my case) when one thinks “I’ve been so good I can have a bit of junk food X.” This kicks off the cravings again. I have become strongly suspicous about what is put in junk food (additives) to make us crave it. I think one’s mindset has to be like that of an ex-alcoholic or “non-practising alcoholic” if one abides by AA jargon. The mindset has to be “I never eat junk food again.”

    I’ve tried several different diets over the years and they all work provided calories in are less than calories out. (Which is axiomatic of course.) So I would suggest people use the diet that works for them. The key trick is constant maintenance after goal weight is reached. I have not achieved that bit yet. I think the twin dangers are illness / injury (disrupting the exercise habit) and re-introducing foods after goal weight is reached. I say this from past experience. So all the junk foods and all excessively farinaceous, sugary and fatty foods that you cut out to diet down, you must keep out after you reach goal weight.

    Finally, as I am now telling myself, dont be discouraged by cycles of yo-yo dieting. If you are yo-yoing you are not going ever upwards. That’s something. Keep trying.

  12. @Ikonoclast

    OK but you miss my point. If the control/feedback system wasn’t important then we would have so many struggling dieters. If the calorie deficit is predictable, maintained (at least attempted day after day), the control system or the two yr old brat in our head in charge of basic energy managment is going to get all the messages and leverage – what more could it need – to turn dials that counter the diet and maintain the system, including with cravings. If you mess up and binge you’re set back a long way.

    The conjecture is that it is a more sustainable strategy to trick that brat: very low cals for 2 non-consecutive days a week delivering a large deficit disguised visually and with strong flavours, slow eating. There’s a 2yr old driving the car with an obsession about how much fuel is in the tank. You need to be smart trying to get to the destination you want – sure you can fight and abuse it, but the brat is pretty relentless – basic survival is its job.

    On days off the ‘fast’ the 2500 or so maintenance intake is important, ie keep the brat and you satisfied with good food without the need to binge. For me it’s not the junk food – a bunch of plenty of nutrition reasons to avoid the worst of that – it’s the restaurant bowls of seafood linguine etc. I’m not keen on eliminating such meals. So the strategy is 5 days a week of enjoyable calorie maintenance (or small surplus) and 2 days a week with a precise set menu – with all the tricks and some nice foods, just not high cal. I don’t have these at other times so even look forward to the chance to have such items (eg a portion of grilled salmon). For me, it’s early days (one month) but this strategy is delivering 0.5kg plus reduction per week (and is consistent with 4000 calorie deficit needed – allowing for some water mixed in with the fat storage).

  13. “Energy used – energy consumed = fat burnt” should read “Energy used – energy consumed = tissue burnt”. Fat isn’t the only type of tissue that you can metabolise.

  14. A bit off topic but did anyone else notice with all those flood reporters mentioning the amount of rainfall as mls, meaning millilitres, when really they should be talking millimetres. I guess that doesn’t abbreviate as well as mls.

  15. has any one see an interview with one of those people lucky enoogh to live to a 100 where the centurion was in anyway overweight?

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