The breakdown of sugars in your body is a natural biochemical process.
It’s really a two step process: glycolosis and the Kreb’s Cycle. In glycolysis, sugar is split into two smaller molecules, and two molecules of Adenosine TriPhosphate (also known as ATP.) In the Krebs cycle, the smaller molecules (called pyruvate) are further broken down, releasing carbon dioxide and capturing the energy of the hydrogens of the pyruvate in molecules called NADH and FADH.
The key phrase in that paragraph is “releasing carbon dioxide." Remember that... it will become important later.
In the world of chemistry, there are certain laws that govern the behavior of gasses. Most of these laws are derivatives of the “Ideal Gas Law,” which is written PV=nrT, where P is Pressure, V is Volume, n is the number of moles of the gas (essentially, how many molecules of that gas are involved,) r is a constant number, and T is Temperature. Boyle’s Law states that the Volume of a gas will vary inversely in proportion to the pressure applied to that gas. That is, if the pressure exerted on a container of gas increases, and the temperature remains the same, the volume of gas will decrease. Conversely, if the pressure decreases, the volume will increase (this is why helium filled balloons will eventually burst as they rise into the sky.)
Boyle’s Law is important.
Commercial aircraft are pressurized. If you flew in a 737 at a cruising altitude of 37,500 feet, the air would be so “thin” that you would not have enough oxygen available with each breath to last more than, oh, an hour (assuming you were in peak physical condition.) Therefore the cabin is pressurized to an equivalent barometric pressure of about 8000 feet, roughly. That is, inside a commercial jet, the air is as “thin” as it would be if you were in the mountains of Colorado, at 8000 feet above sea level.
See, now here’s the thing: When you get that high up, the pressure of the atmosphere is much lower than at lower altitudes. 8000 feet is roughly 22.2 inches of mercury, while 5000 feet is about 26 inches of mercury. Remember Boyle’s Law? When pressure decreases, gas expands? Good.
Now, consider the following: A man is sitting in his house, around 5000 feet elevation, on a brisk Sunday. It’s just chilly enough to justify staying inside, watching Formula 1 racing, eating a pizza, and drinking a couple beers while the fireplace sends forth it’s warmth into the room. For the sake of argument, let’s say he drinks five of these crisp, cold, extra hoppy, wonderfully delicious, and clean-finishing 5 Barrel Pale Ales (made by O’Dell’s Brewing Company, Fort Collins, Colorado.)
He wakes the next morning and boards a plane heading to Texas for a week of work. The Kreb’s cycle does what it does – remember the key phrase? – and Boyle’s Law does what it does, and gravity pulls the whole result downward, until nature takes over and forces an equilibrium.
To the lady sitting on American Airlines #5202, Row 12, Seat C… I’m very sorry.