Food goes in, what goes out?

Pop quiz: if you consume 1 kg of food and drink in a day, and your weight stays the same, where does this 1 kg go? How does that mass actually leave your body?

I realized on the sofa last night I had no idea, so I tried without my phone to figure it out; surely I’m educated enough to guess this one, right? Here were the options I came up with. For fun, I suggest you guess what % of your food and drink leaves your body against each one:

(a) heat / energy / burning calories
(b) poo (or poop, in the USA; apparently my kids have British accents)
(c) wee (yes, one of them is almost 3 years old)
(d) breath
(e) sweat
(f) other (tears, dandruff, skin flaking off, menstruation, semen, hair cuts, shaving, snot, saliva, etc.)

My own guesses on the sofa:

Heat/burning energy : Initially I thought 80%, as we talk about that all time — ‘burning calories’? And doesn’t E = mc^2? Then I remembered: yes, you can convert mass to energy, if you happen to be the sun. Not if you are a bag of biochemistry like me. In a (clearly memorable) middle school science experiment, we burned a piece of wood in a closed system of test tubes, collecting all the gasses, liquids and ash. After burning, they weighed the same as they did at the start, providing that mass is indeed conserved. Right, I recalled on the sofa, chemical reactions create heat, but don’t destroy mass. So actually, I guess we don’t ‘burn’ any ‘weight’ in calories at all — the weight in has to leave our body somehow else. So 0% to heat.

Poo — I knew from Giulia Enders brilliant book The Gut that poo is actually mostly water and bacteria, and some indigestible fiber. So likely it wasn’t mostly poo. Maybe max 20%.

Wee — Probably all our water is passed out as pee. Astronaut ice cream is super light, so our foods must contain a lot of water too. I’ll say 70%.

Breath — I remembered an embarrassing video of graduates from a good university being asked where trees got their matter from. All of them said, as I would have too, that trees got the materials via their roots, from the soil and water. Actually, they get their bulk of their mass from the carbon in the air; when they convert CO2 to O2, they keep hold of the C. Our mental model around photosynthesis is wrong. But surely we don’t breathe out our chicken sandwich… do we? I’ll say 2%.

Other — gosh I think we do shed a lot of skin, have some tears, I don’t know maybe another 10% there?

So that’s what I got to last night. Then today the internet told me I was wrong.

We do breathe out the chicken sandwich.

Indeed, we breathe out almost all solids as CO2. We also breathe out about a third of the liquids we consume as water vapor. The other half we pee out, which at least I got somewhat right. Indeed, water is around 3/4 of the 4.2 kg that the average American ingest every day, either in pure water, coffees and other beverages, and the water content of food.

But, what!? We breathe out our food?!?

In a great TED talk Dr Ruben Meerman explains (more accurately and in more detail) that much of what you eat is made of carbon (C), hydrogen (H) and oxygen (O), which is digested and metabolized by your body. The H and the O end up as water (H2O) which you wee, breathe, poo and sweat out, and the C ends up as CO2 which is carried to your lungs to be breathed out.

In a related article I also learned I don’t just consume food and water — I also consume oxygen. A lot. Around 1/6th of the weight I add to myself I ingest in a day is oxygen I breathe in from the air.

I read somewhere the body is not a fixed entity, but a ‘system through which matter passes’. How beautiful to think that the way this matter passes through us so heavily involves our breath. We breathe out our food. We breathe out the waste products from energy. We breathe out the fat that has accumulated over the festive period, and the New Year’s bubbly too.

So I guess in sum : if you are planning to lose those holiday love handles this January, don’t hold your breath.

Happy 2022,

Dr Ruben Meerman does emphasize that you need to exercise in order to turn the fat into CO2 in the first place; you can’t just breathe more, or you will end up hyperventilating. Nice try though.

I did poke around the Other categories, and they can be small, but thought it was interesting anyway. Menstruation comes out to an average of only 0.003 kg/day. While we produce 0.100-0.300 kg/day of tears, they are 90% reabsorbed, so we evaporate around 0.010kg/day. We lose a similar 0.010 kg/day in shed skin. And of course beards are very heavy, so I recommend all men shave them off to start 2022.

If anyone can find the source of the ‘body is a system through which matters passes’ quote please let me know.

Liz Aab