In the BBC News article CES 2019: Tech preview of the expo's hottest new gadgets there is a new product that one can use to measure the hydrogen in ones breath, and this is supposed to have some connection to diet:
By contrast, FoodMarble's Aire measures the hydrogen in your breath and combines this with a log of what you have eaten, to warn you of foods you should avoid and possible substitutions.
Aire is designed to discover which foods are being passed to the large intestine without being fully digested
There also exist air quality monitor chips that measure both TVOC (total volatile organic compounds) and "eCO2", where "eCO2" is really a measurement of H2 in the air, and via some electronic algorithm uses it to calculate the expected rate that people are exhaling CO2.
Question: Both of these are examples of interpreting the presence of molecular hydrogen as having some indication that we are alive, breathing and eating. Here I would like to understand the sources of hydrogen in human breath, and how that hydrogen makes it from source to breath.