Eating for ten trillion

Adults are recommended to eat about 2000 kilocalories per day. As this is an energy divided by a time it is a power consumption, and in the proper units, it is about 100 Watts. The power consumption of our bodies is a pretty basic feature of how our bodies work, but there is not much known about why a 80 kg guy like me needs 100 W, not 10 or 1000 W. We know* our brain needs of order 10 W, and our heart about 1 W, but for example we have only a poor idea of why our brain burns through 10 Joules every second.Indeed, of the 100 W of food we burn through, I don’t think we even know how much of this food is consumed by us, and how much is burnt by the bacteria in our gut. Our body consists of about 10 trillion cells, but our gut contains about the same number of bacteria. Bacterial cells are much smaller than ours, so of my 80 kg, maybe only 1 kg is bacteria, the rest is me.

So if the power consumption per unit weight is the same for my cells and for the bacteria, then about 1 % of my 100 W is consumed not by me, but by the bacteria I host. But is the power consumption per unit weight, the same for my and for bacterial cells?

We don’t know. A rapidly growing bacterial cell can have a power consumption of 1 picoWatt, which is about 1000 W/kg, 1000 times the power consumption per kg of my body. So at least at times (after a big lunch?), more than 1% of ‘my’ power consumption might not be me at all, it may be¬† my gut bacteria. I don’t know how I feel about this. Should I be mad that bacteria are stealing the food I paid for? Or should I be happy they are burning food and helping keep me slim?

* All the numbers are from the BioNumbers site.


  1. From “The International System of Units (SI)” by Bureau International des Poids et Mesures (

    “Unit names are normally printed in roman (upright) type, and they are treated like ordinary nouns. In English, the names of units start with a lower-case letter (even when the symbol for the unit begins with a capital letter), except at the beginning of a sentence or in capitalized material such as a title.”

    In other words, watts isn’t spelt out with a capital w.

  2. Richard Sear says:

    Wikipedia agrees ( so you must be right, but when the SI people name a convenient-for-nuclear-physics unit of force, the stevenson, it will probably annoy you.

    1. I’m thinking of channelling my inner e e cummings in advance of such a move

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