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MELISSA BLOCK, HOST:

This is ALL THINGS CONSIDERED from NPR News. I'm Melissa Block.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. For our series called On the Run, we've been reporting all week on how kids eat and exercise during the busy family hours between school and bedtime. It's part of NPR's ongoing coverage of childhood health and obesity. Now, the emotional angle. Anyone who's dug into a pint of ice cream after a sad or stressful day knows that food and feelings are linked.

And a poll conducted by NPR, the Robert Wood Johnson Foundation and the Harvard School of Public Health suggests that our emotional response to food is one reason for this country's obesity epidemic. NPR's Jon Hamilton explores how humans came to connect food, family and feelings.

JON HAMILTON, BYLINE: Our poll found that in more than a quarter of families, food is considered an important way to show affection. That's no surprise to Carol Cassie, who lives in Great Falls, Montana. For many years, she ran a restaurant there called Mama Cassie's.

CAROL CASSIE: Right on the front of the menu it says, Mama Cassie's Food is Love. And I wanted my customers to know as soon as they came into the restaurant that I was caring for them.

HAMILTON: She's also the author of "Mama Cassie's Food is Love! Cookbook." The editor is her close friend Erin Duffy Osswald, who used to work at the restaurant. Osswald says it was a place where customers really did feel like family.

ERIN DUFFY OSSWALD: People would get engaged at Mama Cassie's. They would satisfy their pregnancy cravings on turtle cake from Mama Cassie's. They would bring their babies into Mama Cassie's later, and Carol would walk around the restaurant with them.

HAMILTON: But looking back at her restaurant experience, Cassie says, there's something that bothers her. Some of her customers, sometimes whole families, didn't know when to stop.

CASSIE: They'll have big plates of whatever, like a big pork sandwich that was just oozing with cheese and pulled pork and the meat sauce. They would have a piece of cheesecake afterward.

HAMILTON: Just plain gluttony or did they think more food meant more love? Scientists think love is at least part of the explanation. That's because of the way humans have evolved. You can see some aspects of human food behavior in our animal ancestors, including marmoset and tamarin monkeys. Adrian Jaeggi at the University of California, Santa Barbara, says that like people, these monkeys make a big deal of providing food to youngsters in the family.

ADRIAN JAEGGI: They'll do so especially when they encounter very high quality, highly desired items. Those are usually things like insects that are hard to find, but they're big and juicy and very nutritious.

HAMILTON: The older monkeys give a special food call to announce the find, then adopt the food offering posture to present young monkeys with the treat. An ancestor that's even closer to humans, chimpanzees, takes the food-is-love concept to the next level. Chimps share food with some individuals who are not even family members.

Jaeggi says the sharing often involves a precious food: meat.

JAEGGI: It's not like a chimpanzee, you know, makes a kill and then goes around distributing food to everyone else. They'll sort of try to hog the food for themselves, but the ones that they do allow to take and sometimes do actively give are those long-term allies.

HAMILTON: The members of their group they count on. Jaeggi says sharing food appears to be a way of strengthening the alliance and ensuring future cooperation. Think business lunch. Humans have another close relative in the ape world, bonobos. Unlike chimpanzees, the bonobos live in groups run by females, and everything is about cooperation rather than competition. Like people, bonobos use food to make new friends, not just to keep old ones.

Researchers showed this in a series of experiments done at a bonobo sanctuary in the Democratic Republic of Congo. Jingzhi Tan from Duke University says the experiments involved a plate of food.

JINGZHI TAN: What we call it is the giant salad bowl. We have apples, bananas, peanuts, papaya and cucumber all mixed together.

HAMILTON: Researchers give the salad bowl to a bonobo in a locked enclosure. Tan says the bonobo has two neighbors in adjacent enclosures.

TAN: One of them is a bonobo that they know and the other one is the bonobo that they don't know.

HAMILTON: And neither of these bonobo neighbors has any food. Tan says the bonobo with the salad bowl knows they can unlock either door to let a neighbor in at any time.

TAN: We create a situation that they can eat or they can share.

HAMILTON: And most of the time, they did share, but not with the bonobo they already knew.

TAN: The majority of the time, they chose to share with the stranger.

HAMILTON: They used the salad bowl to make a new friend, rather than reward an old one. Then, something really surprising happened. Tan says in many of the experiments, the stranger who had just gotten access to the food would go let the third bonobo in.

TAN: And then three of them all eat together.

HAMILTON: And that brings us back to humans. John Allen at the University of Southern California is the author of "The Omnivorous Mind," a book about how our relationship with food has evolved. Allen says the social importance of food seemed to take a big leap forward one or two million years ago, when animals like mammoths began to appear on the menu.

JOHN ALLEN: Humans start to be able to hunt things that are fairly large and that with such a large quantity that it couldn't be consumed by one or two individuals, that, in effect, provides a little arena for sharing and social exchange.

HAMILTON: The feast is born. Then when agriculture came along, we added harvest feasts, which eventually led to Thanksgiving. Allen says somewhere along the way, our brains became wired to remember these food events and the people associated with them. It's no accident that the digestive system produces hormones like insulin, leptin and ghrelin that act on the hippocampus, a part of the brain that plays a key role in memory.

These hormones stimulate the formation of lasting memories. Allen says the gut-brain connection probably exists because our ancestors were more likely to survive if they remembered clearly where they got their last good meal. And he says this same link between gut and brain is probably why he responds the way he does to a dish called ketchup fried rice.

ALLEN: It's basically just fried rice with ketchup thrown on at the end.

HAMILTON: But he says that for him, it has a powerful emotional association.

ALLEN: Every time I make it I think of my mother.

HAMILTON: Because she used to make it for him when he was a kid. Now, Allen says he makes ketchup fried rice for his own children in hopes that they will always associate the dish with their father. And if his kids really like that ketchup fried rice, another system in the brain will kick in to help create a lasting memory. It's the brain's dopamine system, which rewards us with feelings of pleasure.

ALLEN: People in love or people looking at food they like or drug addiction, they're all working off the same sort of systems. They may use different pathways, but they're all focusing on this dopamine system.

HAMILTON: So in our brains, at least, food really is connected to love and a sense of well-being. Allen says, until recently, that was purely a good thing.

ALLEN: There's never really been any incentive ever to limit calories. Overeating, undereating, you know, when you had a feast, when the food was there, you ate it.

HAMILTON: A lot of us still think that way. In the poll NPR did with the Robert Wood Johnson Foundation and the Harvard School of Public Health, about half of families said they treat celebrations as a time to take a break from concerns about overeating. In a world where it's possible to feast every day, that can be a problem. But Carol Cassie, the cookbook author in Great Falls, says it doesn't have to be.

CASSIE: You make that one day of celebration, or a weekend if you're going away for a weekend, no, but then you go back on Monday and you say, okay, now we have to get back to real life and still make it taste good.

HAMILTON: Cassie adds that even though she'll never stop expressing her love through food, she's not asking any of her loved ones to eat every last bite. Jon Hamilton, NPR News.

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