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By Matthew Rees

Food and Health Fact #189:

Why we overeat and why it's hard to lose weight

I post on Twitter semi-regularly. Find me at @foodhealthfacts The full catalog of my previous posts, articles, and commentaries can be found on the Food and Health Facts website Have an idea for something I should write about? Send it along by replying to this email.

The following is an extended excerpt from a recent interview in which Ezra Klein of the New York Times talked to Stephan Guyenet, a neurobiologist and the author of The Hungry Brain: Outsmarting the Instincts that Make Us Overeat. A full transcript of the interview is available here.  

On sensory-specific satiety
STEPHAN J. GUYENET: So sensory-specific satiety is the observation that we can basically get full on a particular type of food, like a food with a particular sensory quality, like savory or sweet, and not necessarily be full for different foods with different sensory qualities. And so you can give people a particular food, let them eat as much as they want, and they might be full on that food. They have no desire to eat any more of that particular food.

But you give them a different type of food, and they’ll be prepared to eat, possibly, several calories of additional food from that or other foods. And so this explains why we tend to eat so much at buffets, because we have so many different sensory experiences to choose from. And people tend to overconsume dramatically in that situation.

And you can see the same thing in rodent studies. If you give rodents one tasty food like chocolate chip cookies, in addition to their normal food, they will eat more and gain weight. If you give them three tasty foods at the same time, they will gain even more weight. And so the variety of foods seems to have a pretty substantial impact on our intake, and ultimately, possibly, on our weight.

I think another thing that is interesting to note is that this also goes a long way toward explaining why we eat dessert. Because you can have a savory meal — let’s say you have fish and potatoes and a salad — you can be completely done with those savory foods, not interested in eating any more of those savory foods. If somebody brought another potato out, you’re not going to eat it.

But as soon as there’s ice cream or cake or a brownie or something like that, you might be prepared to eat hundreds of extra calories. Part of that is due to sensory-specific satiety. Part of it is due to the high reward value of those dessert items.

EZRA KLEIN: One, just, stat from the book that speaks to this variety, I think, in a kind of astonishing way was, you cited a report from the Food Marketing Institute that found that the average U.S. grocery store contained around 15,000 items in 1980, which is great — a lot of items — and has about 44,000 in the present. And this is a couple of years old, so it might be higher than that now.

It really — I mean, I know people know this on some level. We all know this. But the variety is totally astonishing that we have access to — and not just the variety, but the convenience. I mean, I remember a study that influenced me a lot back when I used to cover food and health policy more closely. This is by — I want to say David Cutler and some co-authors.

And they really found that people weren’t eating that much more at meals. What they were doing was eating much more during snacking periods that accounted for a lot of weight gain over time. And that, in part, that was just because it had become easy to have a variety of foods around you in between meals.

Nobody had to put any effort into cooking them or changing them. We had preservation. We had refrigeration. We had vending machines in the modern era. And as such, it’s not just that variety exists, but variety exists across time now. Right? There might have been variety at dinner in 1950 or 1930, but it was harder to get that same variety at 3:00 PM or at 11:00 AM. But now, it’s not just a lot of foods, but a lot of foods that are available to you at all times.

Your workplace — maybe if you work at a tech company or — some kinds of companies now has a kitchen with all kinds of stuff around for you all the time — that it’s really — the food environment is unbelievably, dramatically different.

STEPHAN J. GUYENET: Absolutely. And I wish I had highlighted that stat about increases in snacking in my book, because I think that’s actually one of the most compelling correlates of the so-called obesity epidemic, which is, in the U.S., the increase in obesity prevalence since about 1980. What you see is that the number of snacking occasions has increased quite a bit. And it really correlates with that period where obesity rates started to rise rapidly.

And by the way, I’m using the word “snacking” loosely, because this includes between-meal sugar-sweetened beverages. So solid foods as well as beverages — both of those things increased. And if you look at the number of calories that’s supplied by these additional snacking occasions that we didn’t have in the 1970s, it’s actually single-handedly able to explain our increase in calorie intake over the course of the obesity epidemic. . . .

On the human body’s “set point” when it comes to weightSTEPHAN J. GUYENET: So the first concept to understand is that there is a regulatory system for body fat in the brain. And it’s in a part of the brain called the hypothalamus that specializes in what’s called homeostatic regulation. And so that means keeping certain quantities in the body within a particular desired range.

So for example, there is a thermostat in your brain that regulates your body temperature. So that’s one of the things that is regulated, homeostatically, in a manner that’s analogous to how your home thermostat regulates the temperature in your home. And similarly, there is a system for regulating body fatness that some, including me, called the lipo stat. So lipo is fat, and stat is the same.

And the system responds to a variety of signals. Some of the signals it responds to have actually been uncovered since I wrote my book. But the main one, the most powerful one that it responds to, is a hormone called leptin that is secreted by fat tissue.

And the way leptin works is that it is secreted by fat tissue in proportion to its mass. So the more fat you have, the more leptin you have in circulation, and that is a key signal that the brain uses to measure the amount of fat your body has, and to determine whether that is an appropriate amount of fat. So essentially, your hypothalamus has an idea of how much fat it wants you to have, and it compares your circulating leptin level to that idea.

So that idea is the set point, that term that you mentioned, or defended level of body fat — just like we have a defended level of body temperature, we have a defended level of body fat that the hypothalamus prefers not to depart from. And this system — I want to note that it’s asymmetric in that it’s a lot better at protecting against weight loss. I guess I should put “protecting” in air quotes. It’s a lot better at protecting against weight loss than it is at protecting against weight gain. Obviously, people tend to gain weight over the course of their lives. But what we see is that the set point tends to creep up as people gain weight.

So it’s like, it’s not just that people are gaining weight. It’s that the heavier weight is the new normal, the new defended level, for the brain. It’s the new set point. And the way we know that is that when you put people on a weight loss diet, you tend to see rebound.

So a person — just to give you an example to highlight this, if you have two people at the same body mass index, the same body fatness — let’s say, B.M.I. of 25 — one of them used to have obesity and dieted down, through calorie restriction, to that level, the other person has always been at that level, and that’s their comfortable weight.

Then, if you follow those two people up for the next few years, you’re going to see that the person who previously lost weight is much more susceptible to weight gain than the person who has always been at that weight. So those two people are not the same physiologically. One of them is below their set point, and their brain is fighting to regain that fat.

And so this is a key reason why weight loss is difficult. Your brain — once the body fat starts to decline and the leptin starts to decline, your brain detects that, and initiates what I call a starvation response, which I believe that phrase is literally accurate. And that engages a suite of behavioral and physiological responses that are intended to increase your calorie intake and sometimes curtail your metabolic rate in order to get fat back into that fat tissue and bring you back to the set point, the place where your brain instinctively prefers you to be.

EZRA KLEIN: You have a nice analogy on this, where you say, imagine someone turning up the temperature on the thermostat in your house by 10 degrees. And you try to cool the house down by opening the windows, but the thermostat reacts by turning on the furnace. So even with the windows open, you’re still hotter than you’d rather be. You’re struggling against the regulatory system.

And then, my colleague, Roge Karma, I thought, had a nice addition to this. It’s really like a thermostat with a broken A.C. system. Because on the one hand, if you go low, it’ll turn on the heat to try to get you higher again, right?

If you lose 15 or 20 or 25 or 30 pounds — or, in this analogy, go down that many degrees, the heat turns on, and you really, really have to fight it. Whereas if you go up by that much, the A.C. doesn’t turn on. Nothing happens, or much less happens, maybe I should say, to bring you back down.

STEPHAN J. GUYENET: Yes, absolutely. And you know, I think that highlights this evolutionary mismatch — that this system was not designed for the modern environment. It wasn’t designed for the challenges that we face today. I do want to say, though, that the A.C. does work for some people.

So you can see this in overfeeding trials, where they have people in a controlled environment, eating a specific number of excess calories. Some people gain a lot more fat than others in that scenario. And that resistance to fat gain is something that has a tremendous amount of individual variability. And so I think some people do have good A.C.

Maybe people’s A.C. gets weaker and weaker with age. I think that would be a compelling hypothesis to test. But I think also, how good your A.C. is, is one of the things that determines whether a person is susceptible to obesity or not.