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!

2 thoughts on “Where do kilojoules and calories go?

  1. 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,


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