Your metabolic mass balance:
Half of everything you eat and drink comes back out of your body as carbon dioxide and water vapour. The problem is that we can’t see these invisible gases so most people never wonder how much they weigh.
You can easily measure the invisible mass you lose every night by simply weighing yourself just before you go to bed and again first thing in the morning.
Most of the mass that vanishes while you sleep is water vapour but an average 70 kg person (154 lbs) also loses around 50 grams worth of carbon atoms in all the CO₂ they exhale overnight.
The numbers in the figure above are averages for a typical adult who consumes 400 grams of carbs, fat and protein plus 1.5 standard drinks worth of alcohol in one day.
To metabolise that much food and alcohol requires 640 grams of oxygen, which follows another fascinating pathway through your body. Every O₂ molecule you inhale gets split into two separate oxygen atoms which are welded together with hydrogen atoms obtained from food making two brand new water molecules. A typical adult makes about a cup (250ml) of this freshly minted ‘metabolic water’ every day.
The dissolved solids in urine contain all the nitrogen and sulfur atoms they consume as protein,. The solid part of their faeces consists of indigestible plant fibres, mucous and dead cells shed from the intestinal walls, plus a large number of the bacteria that inhabit the gut.
The substance that matters most for weight loss, however, is CARBON DIOXIDE because that is how all of the carbon atoms in carbohydrates, fat and alcohol come back out of the human body. Protein is converted to carbon dioxide, too, plus a tiny wisp of urea and whiff of sulfate that escapes via the bladder, but most of its mass also departs via the lungs.
An average 70 kilogram adult loses 200 grams worth of carbon atoms in the 740 grams of CO₂ they exhale every day. The exact figure depends on the duration and type of physical activities they engage in.
You can calculate how much carbon you lose on a typical day in your life by completing the 24 hour physical activity log. To figure out how much weight your body gains or loses per day, you will also need to calculate how much carbon you replaced with all the macronutrients you consumed.