Understanding the mechanism.

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:

 

"It takes your stomach about 20 minutes to produce the hormones that tell your brain that you are full. This process doesn’t start until your stomach begins to stretch."

"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."

"Remember that it takes about 20 minutes for your stomach to tell your brain that you're full."

 

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.

Comments

[Kate]

Interesting! I've been thinking a lot about appetite lately because I'm on a medication that suppresses appetite, and it's really an odd feeling. I've lost enough weight that I've consciously started trying to eat larger portions - but it is really hard to argue with that satiety signal! I end up feeling a bit disgusted at the thought of taking another bite, even of something I really like. Fortunately the medication is out of my system by night and so right now I've just gotten into the habit of snacking before bed.

I've never really had to think too much about weight regulation before. Paying attention to hunger cues has always been enough. So this is a strange experience (and not one that gets much sympathy anywhere).

I know that obese people can feel hungry far after they should be sated, and some anorexics describe that physiological disgust at eating that I've described here - do you suppose the signalling system could become dysfunctional in cases like that?

[Amber]

Very interesting - I knew there had to be more to the 20 minutes thing! I had to laugh though, my favorite gotta eat something now snack is a small handful of animal crackers (love that big container from Costco!) about 6 chocolate chips, a smattering of raisins, and a about a quarter cup of almonds. I guess I'm feeding the simple carb craving and providing some longer term food too. I suppose it is ok so long as calories aren't an issue, which they aren't in my case.

[Melanie B]

Very interesting. I've been thinking about the satiety feeling lately especially after Dom's diabetic nurse tried him on a drug that she said helps to correct a lack of a particular hormone that makes you feel full. To drastically oversimplfy, when he tried this drug for the first time in his life he said he understood what people mean when they say they feel full-- not "my stomach is stretched so I can't fit any more in" but "I have eaten enough and am satisfied". In fact at first "I am satisfied" almost felt like being queasy, he said. Sadly, the drug costs $200 a month and our insurance won't cover it. But during the month he was on it he started eating much less at a meal, had much less between meal snacking, lost a significant amount of weight, and had his blood sugars more in control than he has since he started checking his levels.

Now I'm even more curious about the actual mechanism involved.

[Dorian Speed]

Very interesting post and I especially like this comment:

I for one find it easier to adhere to suggested behaviors if I have a mental model of what's really going on.

[GretchenJoanna]

I don't remember what it was that got me started following your blog, but I'm pretty sure it wasn't this gluttony/weight-loss thread. Just today I noticed the tag and found all these fascinating posts, just when I need them, during (Orthodox) Lent. I put the scale away for these weeks so I could focus on behavior instead of stats, but I couldn't say exactly why I had done it until today when I read one of your previous posts with this tag.
Thanks for all the deliberateness and hard work you have put into the project, and into the part where you share it here!