Your metabolic mass balance:
Half of everything you eat and drink comes back out of your body as carbon dioxide and water vapour. We cannot see these invisible gases so most people never wonder what any of these substances weigh. The answer is a lot more than you might guess.
You can measure the invisible mass you lose by simply weighing yourself just before you go to bed and again first thing in the morning.
Most of the mass that goes missing while you sleep is water vapour but an average 70 kg (154 lbs) person also loses around 50 grams of carbon atoms in all the carbon dioxide they exhale overnight.
The numbers in the figure above are averages for adults. The dissolved solids in urine contain almost all the nitrogen and sulfur atoms you consume in protein,. The solid part of poo consists of indigestible plant fibres, dead cells and mucous from your intestinal walls, plus large numbers of the bacteria that inhabit your gut.
The substance that matters most for weight loss is CARBON DIOXIDE because that’s how all of the carbon atoms in carbohydrates, fat and alcohol come back out of your body. Protein is converted to carbon dioxide too, plus a tiny wisp of urea and a whiff of sulfate.
An average 70 kilogram adult exhales around 250 grams of carbon atoms per day – the exact figure depends on the physical activity 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.