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In Your Health today, the struggle to stay slim even as your metabolism slows. We look at how age alters the calculation calories in, calories out. But before we get to that story, let's consider why it's so hard at any age to keep weight off after losing it. NPR's Allison Aubrey begins her report on the streets of Manhattan, where she's surrounded by food vendors.
ALLISON AUBREY: Looks like you've got hot peanuts.
Unidentified Man #1: Peanuts.
AUBREY: And what else? Almonds.
Unidentified Man #1: Almonds.
AUBREY: Wow, the smell is really good.
You know how the smell of street food can make you hungry? Just the smell, even if you didn't have much of an appetite? But what happens inside the brain when you see food? Well, I'm about to find out what happens in my brain. I'm at the Columbia University Medical Center, just outside the Neurological Institute, and I'm getting ready to have an MRI.
ANDREW KOGAN (Research Associate, Columbia University Medical Center): OK. So Allison, this first scan is going to be a localizer scan. It's just going to give me a few rough images of your brain.
AUBREY: What Kogan and neuroscientist Joy Hirsch are doing here is pretty simple. They're going to put me in the MRI and watch my brain react to food that they show me. Joy Hirsch rolls in a cart filled with all kinds of foods. There's a bowl of nuts, crackers, slices of apples, red pepper, grapes and broccoli.
Nothing fancy here.
Dr. JOY HIRSCH (Neuroscientist): Nothing fancy. I think - oh, no. This is just ordinary, garden-variety cheese. Here. Hershey's kisses.
AUBREY: Now, I imagine the brain will respond to chocolate, right?
Dr. HIRSCH: Oh, indeed.
AUBREY: The scanner will detect exactly which parts of my brain respond by measuring increased blood flow.
Dr. HIRSCH: And your job is just to look at the food, appreciate it.
AUBREY: In their regular experiments, they'd also measure the amount of leptin in their subject's blood, because they want to understand the relationship between this hormone that helps regulate appetite and how it affects the brain's response to food.
Mr. KOGAN: Allison, are you ready?
AUBREY: Yes, I'm ready.
After a bunch of awful beeps that reminded me of a test of the Emergency Broadcast System, the food parade began. For about 20 minutes, the researchers showed me all the edible goodies.
Dr. HIRSCH: And you will see a very specific circuit in your brain that's associated with the appreciation of food.
AUBREY: And the interesting part is that these circuits or networks of brain activity tend to vary, given a person's weight status. You see, what Hirsch and her colleague Michael Rosenbaum have found in their research is that when they scan people who've been on diets, lost 10 percent of their body weight and have depleted levels of the hormone leptin due to the weight loss, their brains tend to show an emotional pattern of response. There's more activity in areas related to reward seeking.
Dr. MICHAEL ROSENBAUM (Associate Director, Clinical�Research�Center, Columbia University Medical Center): After you've lost weight, you have an increase in the emotional response to food, but a decrease in the activity of brain systems that might be more involved in restraint.
AUBREY: A combination that Rosenbaum says helps explain the yo-yo effect: when people repeatedly lose, then gain weight.
What's fascinating about their research is what happens when they replenish levels of leptin in their subjects, basically giving them intramuscular injections of the hormone. They find that the brain responds to food changes significantly. Hirsch says with restored leptin, her test subjects' brain responses looked something like mine.
Dr. HIRSCH: Well, this is very cool, very cool. This is your brain in action.
AUBREY: Analyzing the images of my brain response to food, Hirsch says she sees increased activity in areas of my parietal and frontal lobes.
Dr. HIRSCH: This is the executive part of the brain. This is...
AUBREY: So I'm using executive function.
Dr. HIRSCH: You are. I mean, in this case. I mean, you're responding like somebody who is in the, what I call, homeostatic state. This is your natural way of processing food.
AUBREY: Of course, this could change. It's just a snapshot in time. But it was fascinating to see that I didn't have a very emotional response. Hirsch says that her research is a work in progress. But what she thinks it's showing is that our physiology tends to set the brain in one of two modes: the regain mode, nudging our emotional brains to seek food, or what she calls the retain mode, which helps us maintain a steady weight.
Allison Aubrey, NPR News. Transcript provided by NPR, Copyright NPR.
Anyone who's lost weight knows it can be really hard to keep the pounds off. Scientists think this has to do with a hormone called leptin. Researchers are exploring the role leptin may play in how the brain sees food — either from an emotional, food-seeking mindset; or from a more rational, decision-making approach.
Willpower plays a role in dieting. But keeping the weight off after you've lost it? This is where our physiology can get in the way. Research suggests that hormone shifts that follow weight loss play a role in changing the way our brain responds to food.
"After you've lost weight, you have an increase in the emotional response to food," says Columbia University Medical Center researcher Michael Rosenbaum, who studies the body's response to weight loss. He says you also see "a decrease in the activity of brain systems that might be more involved in restraint."
And there's another factor making weight loss maintenance tough, too: a slower metabolism. When you lose weight, the body adapts to conserve energy, so it just doesn't need as many calories.
One of the hormones that play a role in controlling appetite in the body is called leptin. After significant weight loss, leptin levels drop. This seems to signal to the brain a need to seek more food.
Rosenbaum and his colleague Joy Hirsch, a neuroscientist at Columbia University Medical Center, designed an experiment to better understand the relationship between the brain, leptin and weight-loss maintenance.
They recruited overweight volunteers who agreed to a calorie-restricted diet aimed at shedding 10 percent of body weight. Using fMRI scans, the researchers looked at how the volunteers' brain responses to seeing food changed after weight loss.
Still Emotionally Attached
During their study, Hirsch and her colleagues found some interesting patterns of neural activity in their volunteers after they'd lost weight.
For instance, there was more blood flow to areas of the brain known to be involved in the emotional control of food intake, such as the brainstem and parahippocampal gyrus.
But here's the fascinating part: When they restored leptin to these volunteers by giving them injections of the hormone, the brain response changed. When they saw food, there was more activity in brain areas associated with conscious decisions.
"It's a feedback mechanism," says Rexford Ahima of the University of Pennsylvania. Leptin signals the brain; when there's a deficiency of the hormone, the areas of the brain associated with reward-seeking become more active.
This evolutionary programming is out of sync with what's healthiest for our bodies. The signal evolved over thousands of years when food was scarce. It was the brain's way of telling the body to seek food and protect fat stores. Many people — particularly those who are prone to gain weight easily — have retained more genes that program us to seek food.
As for the role of leptin, researchers say it's clear that leptin is not an anti-obesity hormone — it won't help you lose weight.
But Ahima says the most recent research suggests that leptin — or drugs that would stimulate leptin signaling — could potentially facilitate the maintenance of weight loss. So far, this has only been tested in experimental trials.
My Brain's Response To 'The Food Parade'
The researchers invited me to their lab at the Neurological Institute at Columbia to see exactly how the experiment works. Curious about how my brain would respond to food, I agreed to an fMRI scan.
As I lay in the scanner, I watched through a mirror as research assistants passed all kinds of foods — from carrot sticks and apples to Hershey's Kisses and cookies — through my line of sight.
"Think of it as a food parade," explained Hirsch. After 20 minutes of watching food, the researchers began analyzing my brain responses.
"You will see a very specific circuit in your brain that's associated with the appreciation of foods," explained Hirsch.
Hirsch says the patterns in my brain images were similar to those of test subjects with restored leptin. She pointed to areas in my parietal and frontal lobes that had activated as I watched the "food parade."
"This is the executive part of the brain," says Hirsch. "You're responding like somebody in a homeostatic [stable] state." This means that when I saw the images of food, my brain activated decision-making areas, and there wasn't nearly as much activity in the emotional, reward-seeking parts of the brain. Hirsch also pointed out that my brain showed lots of stimulation in areas related to visual processing.
Of course my brain response could change. The brain images captured just a snapshot in time. But it was fascinating to see that I didn't have a very emotional response to food. By comparison, images they'd shown me of mundane household objects — such as a cell phone — didn't evoke nearly as much activity in the areas associated with executive function or visual processing.
Hirsch and Rosenbaum's findings were published in the Journal of Clinical Investigation. They're now working on follow-up studies to figure out if people's behavior maps with what they're seeing in brain scans.
"It's a work in progress," says Hirsch. But she thinks this research is showing that our physiology tends to set the brain in one of two modes:
The "regain" mode, which nudges us, emotionally, to seek food. Or the "retain" mode, which helps us maintain a steady weight. Researchers are following up with more studies to see if people's eating behaviors mirror their brain response to food.