The science of feeling peckish (part 1)

In the diet culture, it’s common to find people talking about “body weight set points”: Your body wants to stay at a particular weight, and fights your attempts to slim down by making you hungry until you top up your weight. I don’t have the heart to link to the huge number of sites claiming you can change your set point by doing things like controlling calories (sigh), but knock yourself out if you want to search for them.

The thing is, set points for body weight, blood sugar levels (a body “glucostat”), blood fats (a “lipostat”), and even body temperature have been tested in the scientific literature to try and explain how hunger works, and found lacking. What turns out to matter most for making you feel hungry vs. sated is the availability to the body of utilizable fuels, seen in toto.

On this subject, Gary Taubes makes this recommendation in GCBC:

Several variations on this hypothesis [about hunger and availability of utilizable fuels] were published from the mid-1970s onward by LeMagnen and others. The most comprehensive account was published in 1976 by Edward Stricker at the University of Pittsburgh, and Mark Friedman, then at the University of Massachusetts and now at the Monell Chemical Senses Center in Philadelphia. Their article, “The Physiological Psychology of Hunger: A Physiological Perspective,” should be required reading for anyone seriously interested in eating behavior and weight regulation. [GCBC, Ch. 24, p. 433]

Go ahead — click that link above. You can buy the article, if you like, for the low low price of US$11.95.

Or, don’t! This is where I demonstrate that carbgrrl has gone around the bend. I went and got the paper, studied it closely, and present here a layman’s summary of its arguments and conclusions. You’re welcome. :-)

(I’ll do this in two parts. Part 1 — call it Energy Metabolism 101 — is below the fold. I’ll provide Part 2, The Hunger, in a later post. As always, I will gladly accept all error corrections and pointers to research that disputes or usefully refines the information below.)

Energy Metabolism 101

Your body deals with food consumption and energy expenditure in two phases. (The article reviews the literature on hunger in rats, but this is how metabolism works for humans too.) Your body needs energy all the time, but doesn’t eat all the time. So:

  1. “Postprandial” is when your body sets about handling (both burning and storing) the food you’ve just eaten.
  2. “Postabsorptive” is when, having fully dealt with your last meal, your body begins consuming itself for longer-term energy needs.

Postprandial: To a first approximation, here’s what happens in the postprandial stage. You break down all those nutrients into their constituent parts and then distribute them among the jobs of (a) providing immediate energy to the brain, muscles, and other body tissues and (b) storing the rest as fat and muscle (plus a bit of glycogen in the liver). The organ doing most of the heavy lifting is the liver; for “liver-centric” diagrams vs. my energy-obsessive ones, do check out the original Friedman-Stricker article. [UPDATE: See the comment thread for detail on how muscles use glucose for immediate energy; this diagram is oversimplified to the point of inaccuracy.]

gcbc-postprandial

What’s the takeaway? Mainly that energy (both the immediate kind and the stored-for-later kind, as you’ll see in a moment) can be gotten from any dietary source. There’s lots of redundancy here. The article notes that “[W]ith but a few exceptions, each of the various metabolic fuels is equally capable of providing energy in all tissues.”

Postabsorptive: Okay, what happens once everything is burned or stored? You need to begin mobilizing fuel out of your tissues to keep you alive. In the postabsorptive phase, your body burns fat (and what other limited storage it has) to keep running. Here’s what that looks like, again to a first approximation.

gcbc-postabsorptive

What’s the takeaway? Mainly that there’s humongous redundancy around ensuring that the brain is well supplied with energy at all times. The article notes that “The supply of fuels to the brain is elaborately defended even during prolonged fasting” and goes into detail about the ways in which the brain is covered against all contingencies (except extreme starvation) by routings and syntheses of glucose from various sources. As a second priority, muscle tissue is spared as much as possible.

Fat storage and usage is a more dynamic process than you may think, and the liver conducts this whole energy symphony with precision. The article notes:

Adipose tissue participates actively in metabolism by removing the excess nutrients that are usually obtained during each meal and by releasing them in the subsequent postabsorptive period. This tissue thus provides a massive energy buffer that prevents dramatic shifts in nutrient supply despite the episodic nature and inconstant magnitude of food ingestion. The liver complements the function of the adipose tissue and ensures that nutrient supplies are sufficient and appropriate to the specific needs of individual tissues.

A few pointed observations: Okay, this is me talking for myself again.

  • No, you don’t need to eat carbs to make sure your brain is supplied with energy. (If that were true, I’d be dead by now.) You’d have to go to quite a bit of trouble — like getting to the advanced stages of starvation — to prevent your brain from being first in line for all food and body-tissue sources of glucose.

  • Remember the stuff about insulin “masking” body fat and making it harder to burn, putting a priority instead on clearing whatever carbs are currently flooding your bloodstream? That makes your fat less available as a utilizable fuel source.

  • If you succeed in making your fat pretty much invisible, your postabsorptive period will be less effective, you’ll need more postprandial episodes just to keep yourself running — and your supposed “body weight set point” will magically rise.

  • Exit question: How do you suppose you can get it to fall?

(Stay tuned for an eventual Part 2!)

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10 Comments to “The science of feeling peckish (part 1)”

  1. Reinout van Rees 15 April 2009 at 1:07 am #

    The muscles’ energy needs took me some time to track down in your article. So for some sanity check, is the following true?

    * Right after meal time is not the right time for your muscles to be really active. They just get some stuff from protein, but the rest of the body is stashing away energy in fat or feeding the brain.

    * Normally, your muscles get their energy via intermediary storage: mostly out of your fat.

    * In emergencies, muscles can burn themselves up into acids to provide themselves with energy. So what happens to muscles when athletes temporarily overtax their muscles is called “souring” (if I translate the Dutch word correctly) for a very real “acid” reason?

  2. Eve 15 April 2009 at 7:15 am #

    Hello Reinout– The article focuses primarily on energy metabolism as it applies to hunger, not muscle tissue specifically. Nevertheless, to answer your first two questions, I realize I should correct one oversimplification in my diagram here: Muscles do get immediate energy from glucose, despite the priority placed on brain utilization. Here’s a quote from the article:

    The circulating glucose not removed by the liver is used for energy production in brain, muscle, and other tissues or else is stored in adipose tissue as triglycerides.

    (Muscle also stores glycogen; this isn’t covered in the article, but a million and one bodybuilding websites talk about how to increase these stores!)

    Also, there’s one subtlety I left out of my post because it wasn’t directly on-topic, but it’s a phenomenon I do find interesting:

    [T]he postprandial delivery of glucose into the hepatic [liver] portal system, together with other meal-related events, stimulates the secretion of insulin, which promotes the formation of lipids and glycogen (while inhibiting their mobilization) as well as the uptake of glucose into muscle and its utilization of glucose into muscle and its utilization there.

    My personal trainer has showed me bodybuilding books that argue for eating a 4:1 ratio of carbs to protein in the 20-45 minutes right after a hard weight-lifting workout, specifically to spike insulin postprandially for its muscle growth effects. I’m thinking this passage is suggestive of these effects, while also pointing out that they put the body at risk for fattening effects. (I’ve tried the 4:1 thing and found that the hunger stimulation from the insulin is generally too great for me as an insulin-sensitive person, making a mockery of any muscle-growth optimization. :-)

    My best guess about “souring” is that this is about training to “muscle failure”, when there’s such a large buildup of lactic acid in the tissues that the muscle gets sort of paralyzed. Here’s one article I found on a bodybuilding site about this phenomenon. Part of the body’s defense of brain energy, and secondarily muscle tissue, involves resynthesis of lactic acid, as this Friedman-Stricker article passage notes:

    The pyruvate [pyruvic acid] and lactate [lactic acid] formed from glucose in nonneural [e.g., muscle] tissues are returned to the liver for resynthesis into glucose to be used by the brain…. minimizing the need for gluconeogenesis from amino acid substrates … produced by muscle catabolism.

    This suggests, I think, that muscle training doesn’t tear down the tissue for energy, though it is being “used up” in a sense (while giving the body a signal that it should build up the muscle even more next time!). Muscle doesn’t cannibalize itself for the brain’s sake if it can help it.

    I hope this helps. Please do share any sources you come across that shed accurate light on this picture.

  3. Robin Wilton 21 April 2009 at 7:23 am #

    Nice piece of analysis, Eve – great work. Two comments:

    1 – ref. Reinout’s comment, if I remember correctly (from about 30 years ago), lactic acid build-up in muscles is a product of anaerobic respiration:
    C6H12O6 → 2C3H6O3 + energy. The 2C3H6O3 is the lactic acid…

    2 – I absolutely understand why the vast majority of work in this area looks at the problem of how to lose weight, but I would be so grateful if articles like Taubes’ helped me understand why I seem to have a ‘set weight’ which is off the bottom of the BMI scale. Don’t get me wrong – I’m not fishing for sympathy from those who have the converse problem – just trying to figure out why the only way I appear able to put on weight is to build muscle… which disappears again if I stop the muscle-building activity.

  4. Eve M. 21 April 2009 at 7:37 am #

    Hi Robin– Yes, you’re right about lactic acid buildup. (Hey, throwing around chemical names for lactic acid, maybe you should be blogging on this stuff too!)

    Taubes does actually spend a fair amount of time in GCBC talking about the implications of the “alternate hypothesis” beyond unwanted weight gain. I don’t have my book with me ’cause I’m on the road, but here’s a precis from memory.

    Mostly the problems he discusses are around metabolic syndrome and diabetes (easily understood connection) and even cancer and Alzheimer’s disease (potential connection that deserves further research), but he also talks about ways we may be able to understand anorexia and chronic underweight: Just as insulin resistance and hyper insulinemia may contribute in the one case, perhaps insulin oversensitivity etc. could lead to the other. Definitely worth real research, and perhaps even worth personal experimentation with “insulin-loading” on your part if you feel it’s safe.

    Of course, given our diet culture, most people wouldn’t look askance at you if you wanted to try and eat more carbs — though they’d be mistaken about your intent! Heck, you could try adding one “100-calorie low-fat snak pak” of Oreo cookies a day, and see if it works to pack on the pounds. :-)

  5. Robin Wilton 27 April 2009 at 3:09 am #

    I think my daily carb intake is quite high. Speaking of Oreos, have just been finishing off some Limited Edition Candy Cane Creme ones I picked up in Orlando :^P. That puts me at 225 calories already, just from those…

  6. Eve M. 27 April 2009 at 6:48 am #

    Okay, now you’re just taunting me. :-)

  7. john 3 August 2009 at 3:16 pm #

    Whatever happened to part 2 of this post?

  8. Eve 3 August 2009 at 3:18 pm #

    It’s nice to know someone is paying attention! :-) Thanks for the reminder. I’ve got another post in the works (think recipes rather than chemical reactions) and then I’ll try and make Part 2 the next one after that.

  9. […] is part 2 of “The science of feeling peckish”, promised way back in April. Thanks for the encouragement/prodding in the original comments thread […]

  10. […] feeds our brains and bodies can be made from practically any old thing lying around, as I’ve discussed before. And in GCBC, (The Great) Gary Taubes discusses the pernicious effects of eating fructose […]