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.]

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.

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!)