I think by now everybody's heard the canard: "It takes 20 minutes to feel full after eating, so wait that long before you serve up a second helping." I saw it again today, but with a twist on it I've rarely seen.
As far as I can tell, the advice to eat slowly and wait between servings in order to prevent eating until overfull is backed up by research --- I don't think anyone disputes that it probably helps and won't hurt. What I think is interesting is the rationale.
Usually, this advice is accompanied by a folk-theory explanation of why it supposedly takes 20 minutes to feel full after eating. Here are a few I managed to scrape up from Google:
"Eating quickly doesn’t give the stomach enough time to communicate with the brain to send up the “I’m full” message, and missing that cue can cause overeating. Waiting 20 minutes gives the brain enough time to register the “full” or “still hungry” signal."
There's more like that out there. The details vary but the gist of it is commonly "stomach talks to brain, 20 minutes required."
Today, for the first time, I saw deviation from this folk tale in a soundbite-type article --- the kind that takes you through a slideshow of stock images, one per "tip". Tip number 6 in a WaPo slide show that starts here:
Wait 20 minutes for seconds: Food must move 22 feet through your intestines from your stomach to peptide YY cells — the switch that says you are sated.
What's this? Peptide YY cells? The intestines are involved in the conversation? How novel!
The mechanism is apparently a bit more complex, though you wouldn't know it from the folktales. I'm always particularly annoyed when the advice columnist, or whoever, suggests that satiety is primarily the feeling of your stomach "stretching" or some such thing, and that hunger is the feeling of the walls of the stomach touching each other or some such thing. That's a theory from the early 1900s that's been well disproven. (Have your stomach surgically removed and you'll still feel hunger and satiety.)
Mostly, hunger and satiety are brought on by hormonal signals (even that is a vast oversimplification!), and if your stomach is straining to contain the food you've gobbled, chances are you've overshot it.
Peptide YY hormones (abbreviated PYY) are just one of several factors implicated in appetite regulation. I'll borrow this brief explanation of PYY action in one set of mouse experiments from an abstract:
In response to food ingestion plasma [PYY] concentrations rise within 15 min and plateau by approximately 90 min. The peak [PYY] level achieved is proportional to the calories ingested, suggesting that [PYY] may signal food ingestion from the gut to appetite-regulating circuits within the brain.
(I caution against taking single statements in reports of experiments, wherever published, as gospel. I'm just quoting this here in order to borrow the wording, and emphasize that active research has moved beyond the "stomach walls rubbing together" thing.)
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So why does it matter? If the advice is good, who cares whether we understand the real mechanism or not?
Maybe you can chalk it up to different types of people. I for one find it easier to adhere to suggested behaviors if I have a mental model of what's really going on.
Here's one example. Like many folks, I get cravings for carbohydrates when I've had a busy morning dealing with the kids or a long shopping trip or a stressful afternoon. I have had an easier time riding them out and solving them in better ways now that I understand a few things (the following does not apply to diabetics and prediabetics):
(1) they are physiological, not a sign of mental weakness;
(2) they really do signal low blood glucose, and I really will be sluggish if I don't get fed;
(3) eating carbs will feel great and deliver immediate relief, but might not last long;
(4) eating protein-and-fat will deliver the same relief from the craving, because a protein snack really will raise your blood glucose; but the relief won't show up for 30 minutes or so, and sadly it will not deliver the rush of "oh thank God this cookie is the best thing I ever ate." Still, it will solve the problem.
How does this help with the carb cravings?
(1) I know better than to try to ignore this signal. It's a real message saying, "Either eat something, or quit doing anything mentally taxing until your next meal."
(2) Even if the idea of a Coke and french fries and pie is making me go yes yes yes, and the thought of a hardboiled egg or a handful of almonds is making me go meh, I know that --- in the end --- the egg or the almonds will fix this problem.
(3) If I know this, I cannot fool myself into thinking, "But I really need a hit of sugar." I don't. But before I really understood the slower response to protein, I easily tricked myself into thinking that I did need the Coke or the cookies.
(4) If I can't stop what I'm doing and wait for the slower protein response, having a little bit of each might be a good compromise.
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I can't say for sure whether I'll be more or less likely to wait that 20 minutes if I know that one of the "you've eaten enough" signals is triggered by caloric food reaching the ileum. (Or if I know that gastrointestinal transit is not simple either: "Materials do not leave segments of the digestive tube in the same order that they arrive."
One thing that does help me is knowing from experience how I've accidentally become overfull. Can't count the times where "hm, I think I've got room for dessert!" has turned into "ooof, I guess I didn't really have room..." after, oh, twenty minutes or so. I really have to almost feel like I've undereaten for a whole dessert not to make me all stuffed.
We have a locally owned, family-style restaurant near our house that charges only a dollar for pie after 7 p.m. I am a great fan of pie, and so this restaurant (kid's meals under $3 that include drinks and dessert! No kidding!) has been a good training ground for "saving room for dessert." I almost always take the kids there after swimming lessons when Mark goes out of town for a few days, and I almost always arrive thinking "At the end of this meal, I will have pie."
Keeping in mind the feeling of having eaten too much before the pie, and then added the pie onto that, really does help me remember not to order something big and then polish it off. It's entirely mental -- another kind of knowledge that makes a real difference.