That's utterly fascinating. I haven't experienced this, but I'll throw a hypothesis out there.
Although we think of our digestive tract as pretty much a one-way street and unconnected to our noses, gases do bubble up, most noticeably after we eat particular foods. I'm wondering whether the tiny odors from your digestive tract that you smell (but don't detect consciously) when you're hungry are so similar to aldehydes that your brain has delivered one of the cues that compel us to eat.
I'm a mediocre perfumista, but awfully smart about appetite. Our bodies have multiple “dinner bells” wired throughout our digestive tract, brain, and even our flesh (fat cells, once born on our thighs and hips, will argue to be supplied with fuel forever and ever).
Of course, I could be all wrong. Either way, I thank you for giving this hungry brain some fodder to chew on.
Wait … one more thought just came to mind: I remember taking a kind of breath test decades ago when I was on one of many diet schemes. This was to measure ketosis. {Googles} Here's a short quote:
“… there is something called 'hunger breath,' which is caused by ketosis, a medical condition in which the body starts breaking down fat if the person is not consuming enough calories. Ketones are produced in ketosis, and if there are large amounts of ketones being produced, the acidity of the blood can be increased. In turn, the body tries to lower the pH by ridding itself of the ketones in the lungs and urine, thus causing bad breath. In a low-carb diet, this is actually the main principle, since the body is tricked into thinking it is undergoing famine, even if the person is continuing the same calorie intake.”
