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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.