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Matt:

In my ponderings, accompanied by snacks, I've often wondered why our stomachs can digest everything from cake to goat meat, but it doesn't digest itself. How come?

-- Eating and Wondering, San Marcos

Matmail:

Why does eating a big meal at Thanksgiving just leave you very full, while eating lots and lots of candy leaves you very, very sick?

-- Paul, the Net

Dear Source of All Knowledge:

Usually when you eat something you expect to feel full, or at least no longer feel the pangs of hunger, right? Well, this seems to be true for all but one food: apples. For as long as I can remember, any time I eat an apple, afterwards, I actually feel hungry. Could it be the acids in the fruit, or am I just fruity?

-- Overruled, San Diego

Dear Matthew Alice:

How long does it take for a glass of water I drink to come out my armpits as sweat?

-- Judith, Up North

Dear Matt:

Admit it. Just before we flush, everybody looks. So -- why do some float and some sink?

-- Bob B., San Diego

Suit up for a trip down the alimentary canal. We'll be following that clump of chewed apple, so keep your arms inside the car, please, to avoid deadly crushing injury from the molars.

Ladies and gentlemen, today's gastrointestinal experience is brought to you by the tenth cranial nerve, the vagus nerve, in charge of most processes from the mouth to the small intestine. Irritate this nerve and signals will be sent to the brain's vomit center in the medulla oblongata. We'll be gripped by nausea, and it is possible the car will automatically begin moving backwards until we're ejected from the park. So no irritants, toxins, poisons, or the like. And please, no smoking, since a branch of the vagus also serves the lungs.

Ladies and gentlemen, as we enter the stomach, we ask you to dip yourselves in this vat of special mucus, a product of certain cells in the stomach lining. This keeps you from being eaten away and digested during our tour.

On your left, you'll note how the apple has triggered secretions of hydrochloric acid, mucus, and enzymes. The stomach's job is to mix your food and your gastric juices, reduce it to a semiliquid, and begin splitting food molecules to make it absorbable. There's a lot of noise and activity here but little actual digestion. If the gastric activity is interpreted as "hunger pangs," you may think you haven't eaten at all.

Over there on the right, you can see some water being absorbed through the stomach wall. It will be in the bloodstream in half an hour and could be in your pits as early as an hour later. And on your far right, near the exit sign, note how your carbs are already lining up to leave the stomach, they'll be followed by your proteins, followed by your fats. Those fats can hang around here for 10 or 15 hours.

Oh, look out, ladies and gentlemen, I think we have Snickers bars at ten o'clock. Yep, about a case of Snickers bars. Please put on those hard hats, and watch for falling peanuts. Whoa, our guy's in trouble now. This big wad of sugars and fats is going to throw off his pH and chemical action, and some of that load could even be dumped unprocessed into the small intestine. His blood glucose will be through the roof, and we're likely to get that vagus nerve irritation I mentioned earlier. If there are no questions, I think we'd better stay clear of the candy bars. Please watch your head as we squirt through the pyloric valve, and I'll meet you all in the small intestine. The area's a little cramped, so please keep an eye on your wallets and purses.

Those worm-like projections on the intestine wall are villi. You can hear the faint sucking sound as they absorb nutrients. And before I forget, the stomach has asked us to mention that a considerable amount of nausea actually originates from disruptions in the small intestine. Now, as we move to the large intestine, please note the presence of gases and undigested roughage.

For the rest of the trip, we'll just be compacted and dried off a little. Intestinal gas content is low today. That means we'll be sinkers, not floaters, so put on your scuba gear and prepare to land. Please remain seated through this final downhill ride. If you stand up and yell Whee! we'll have to ask you to leave. And don't forget to stop by our gift shop before you go home.

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