Where do kilojoules and calories go?

Only one rule matters in weight loss. You need to eat 32,000 kilojoules less than your body expends for every kilogram you want to lose (7700 calories).

The science behind this rock-solid fact of nature is more than half a century old but there is growing army of intelligent people who don’t believe it because they don’t understand how it works. The reason for the growing confusion about the good old “energy in, energy out” equation is blindingly obvious; the energy in question is invisible.

The only kilojoules and calories you will ever “see” are the ones that enter your eyeballs and land on your retina. All the other kinds are imperceptible to the eye. A 100 watt light bulb radiates 2 joules per second worth of visible light, and 98 joules per second of the infrared kind. You can feel that infrared light because it warms your skin, which is why we call it heat.

The alien in the movie Predator starring Arnold Schwarzenegger could see heat but while that creature was pure fiction, animals that can “see” infrared light are not. Snakes of the viper and constrictor variety and vampire bats have pit organs that can detect this form of radiation. Fire-seeking beetles that lay their eggs in freshly burnt trees have similar organs and some butterflies have ‘thermoreceptors’ on their antennae and wings to protect them from heat damage. Humans have the same protein involved in biological infrared imaging and sensing, known as the “wasabi receptor”.

All warm animals emit infrared light, which makes the ability to detect this part of the electromagnetic spectrum very useful for hunting and for search and rescue missions. Cameras that can “see” the thermal emissions of living things were once prohibitively expensive but recent advances in technology have made these devices small and affordable enough to plug into a smartphone. This is very good news for the next generation of high school science teachers and their students, because they can use these cameras to “see” where all the energy that vanishes from their experiments and demonstrations “goes”.

The kilojoules on the “energy in” side of the weight loss mantra are conspicuous enough. It’s how the ones on the “energy out” side sneak back out that make life so difficult for the doctors and dietitians who prescribe this age-old remedy for overweight and obesity.

You typically lose about 60 joules per second as infrared light. This has been known for donkey’s years but the first person to measure it directly with a radiometer was a CSIRO scientist called Peter Funk, who reported his extraordinary achievement in a succinct letter to the journal Nature in 1964.

About 30 joules per second are carried away by water molecules that have evaporated from the lungs and skin. You lose 2257 kilojoules for every litre of water that evaporates from your body. Chemists and physicists refer to this energy as the enthalpy of vaporisation.

Another 10 joules per second are lost to the environment via conduction and convection. Those are the joules that make chairs warm when you sit on them. Douglas Adams,  who also wrote the Hitchhiker’s Guid to the Galaxy, came up with an excellent word for the energy people leave on chairs in The Meaning of Liff, which he co-authored with John Lloyd:

Shoeburyness: “The vague uncomfortable feeling you get when sitting on a seat that is still warm from somebody else’s bottom”

The convection currents rising above you carry heat away but they, too, usually remain invisible, except on really cold, rainy nights, when you sometimes see wisps of water vapour rising from hot skin. The heat that’s causing that was once in food.



Altogether, an average person loses a total of about 100 joules per second by infrared radiation, evaporation and conduction and convection. The precise figure depends on the temperature of the surroundings and physical activity but if you multiply it by the number of seconds in a day, you find that the average adult converts 8,640,000 joules of food energy to heat per day.

24 hours × 60 minutes × 60 seconds = 86,400 seconds

And what’s the recommended daily intake for adults? Well what a coincidence, it’s 8700 kilojoules! That’s the same as 2000 Calories.

About half of the food energy you eat is converted directly to heat by chemical reactions that happen inside your cells. The other half is shuttled through a series of convoluted metabolic pathways and converted to the chemical bond energy in a magnificent substance called adenosine triphosphate (ATP). This energy can then be used by cells to do things like rearrange amino acids to grow new bone, hair or muscle, make enzymes, power a muscle cell’s twitch, fire a neurone or do any of the other amazing things you can do, but here’s the thing: all of the energy that makes it into ATP is also ultimately transformed into HEAT.

It might be starting to dawn on you by now that making yourself hot or cold could potentially speed up your metabolism and lead to faster energy loss, and therefore weight loss. And you’re right! Lowering the thermostat in buildings has been touted as a potential solution to the obesity crisis, but that’s not going to do much for people living in warm cities like Mexico or Sydney.

And if you’re wondering whether or not obesity might be caused by some kind of metabolic disorder that prevents a person from radiating their heat, you’re doing a bit more wishful thinking. Any disorder of thermoregulation is potentially life-threatening.

There’s so much more to all of this that you can do a whole three year university degree about it, called a Bachelor of Science majoring in biochemistry. And it would be very helpful if the folks who reckon the good old “energy in, energy out” mantra doesn’t work went and did precisely that!

12 thoughts on “Where do kilojoules and calories go?

  1. Great article, Ruben. Random question – I read somewhere (I know that’s not a very helpful source) that 90% of our energy comes from oxygen and only 10% comes from food. Is that correct? And if so, does having more oxygen in our food (like whipped cream vs cream or meringue vs a raw egg white) help at all with increasing our energy levels and/or reducing appetite or losing weight in any way?

    1. Hi Shaz, apologies for the slightly delayed (5 years) reply… the 90/10 rule is sort of correct but it’s not quite what you’re thinking. The oxygen you inhale is converted to H₂O, ie, water. You produce around a cup of water inside your cells every day. The hydrogen atoms in all that “metabolic water” come from the food you eat. Around 90% of the energy we produce comes from marrying hydrogen atoms to inhaled oxygen atoms. The other 10% comes from anaerobic respiration that does not consume oxygen. Whipping cream or egg whites doesn’t change the number of atoms or the energy content. It does do cool stuff to the protein but the number of atoms remains the same. Hope that helps.

  2. Hi Ruben, recently watched your very informative TEDx video; The mathematics of weight loss & had a question. I’m a fan of carbonated sparkling mineral water so by drinking CO2 infused water could I be hampering weight loss by consuming CO2 & should I just stick to plain water for weight loss? Thanks Stamos.

    1. Hi Stamos, and apologies for the looooooooong delay replying to your email. This is a very common question so I wrote a blog post about it. I hope this helps.

  3. There’s another energy out that is never mentioned- urine urea. Amino acids can be turned into glucose by gluconeogenesis (not nearly as energy efficient as digesting starch). The excess nitrogen though has to be excreted and has energy you can make bombs out of. When the Lewis and Clark expedition was working hard they had to eat 4kg of game to keep up their weight and strength. I took classes to get a BS in biology and a MD degree but none of the professors mentioned this.
    David Laurentz MD(ret)
    Marble Falls, Tx

  4. Hello,
    I have a question regarding the delay of weight loss. If in a calorie deficit, would you expect the loss to show up on scales immediately? I am curious as I’ve noticed if I’ve been very compliant during the week with calories and being in a deficit, I tend to notice the weight loss a week later. I’ve also had a female friend note a lag also. Or rather is due to other factors eg salt intake/fluid retention, building muscle etc
    Many thanks!

    1. Dear Rachel, apologies for the looooooooong delay replying to your email. The unfortunate thing about weighing yourself everyday is that we eat, drink and inhale around 4 kilograms of atoms every day and, if we are weight stable, we also exhale and excrete 4 kilograms of atoms every day. However, if you have some more salt one day, you’ll retain a bit more water. If you weight yourself directly after urinating, you’ll have lost a few hundred grams. On a typical weight loss diet, most people usually lose less than 100 grams of fat per day. That’s less than your bathroom scales can detect. The trick is to weigh yourself first thing in the morning, every day, after your first visit to the toilet, in the same clothes (or better, none)… and then graph your weight. You need to do this for at least a month to clearly see a trend. I hope that helps.

  5. Hola…thank you for the insights!
    Shouldn’t the above quote “you find that the average adult converts 86,400,000 joules of food energy to heat per day” ….be 8,640,000 joules and not 86,400,000? hmmm
    no big deal…just wondered

    1. G-day Steve,

      Apologies for the delayed reply and YES!!! Spot on, 8640 kilojoules is indeed 8,640,000 joules

      I have corrected my error so that the (millions of😉) other people reading this post won’t be confused. Thank you for letting me know!

      Kind regards,


  6. Mr. Meerman, What are your thoughts on this controversial study with the Hadza tribe by Pontzer in Tanzania. It doesn’t seem to make sense or be possible, but it is quoted often and seems to indicate that our “breathing out carbon dioxide” (and thus carbon and oxygen atoms” doesn’t affect our weight, or more accurately, the tribe doesn’t breath out any more carbon and oxygen tgan a sedentary person.

    1. Hi Christian, apologies for the delayed reply. I am fascinated by Herman Pontzer’s work and loved his book. And I am very much looking forward to the results of future work either verifying or challenging his model. This perspective article by Gonzalez et al is worth a read and argues that reality may lie somewhere between the traditional additive model and the constrained energy model: https://www.sciencedirect.com/science/article/pii/S216183132300217X

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