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RENEE MONTAGNE, host:

This is MORNING EDITION from NPR News. I'm Renee Montagne.

STEVE INSKEEP, host:

I'm Steve Inskeep, and this is the waitress.

IDA: My name is Ida. I'll be your server and I'll be right with you to start you out with something to drink and some appetizers. OK?

INSKEEP: Ida serves up heaping platters of tacos and fajitas at a Mexican restaurant in Philadelphia. On this afternoon, she's serving Lisa Plummer(ph) and her four girls.

Ms. LISA PLUMMER: Do you know what you want? What do you want?

Unidentified Girl #1: I want chicken fingers with french fries.

Unidentified Girl #2: Same here, please.

INSKEEP: It's a typical request, but why don't these kids choose the grilled chicken salad or even a bowl of chili? Most children and many adults, for that matter, relish salty, high-fat foods in large portions, the kind that doctors and new government guidelines tell us to avoid. As part of a series on what Americans eat, NPR's Patricia Neighmond reports on the science of why we choose the foods we do.

PATRICIA NEIGHMOND reporting:

Whether she's taking her girls to lunch or cooking for them at home, Lisa Plummer says mealtimes are often a battle. And over the years, she's asked her pediatrician for advice.

Ms. PLUMMER: And one in particular said don't battle with them over what they should eat when they're little. She said kids will gravitate towards what they need. If they're reaching for pretzels, it's maybe because their body needs salt right now. Just let them eat it.

NEIGHMOND: To figure out why we gravitate towards certain foods like salt, we come to a building not far from the restaurant.

(Soundbite of door closing)

NEIGHMOND: This is the Monell Chemical Senses Center. In front sits a giant sculpture of a 12-foot gold leaf nose. Gary Beauchamp directs the center.

Mr. GARY BEAUCHAMP (Director, Monell Chemical Senses Center): Evolution holds the gun to your head. I mean, that's the beginning reason.

NEIGHMOND: And Beauchamp says it started literally millions of years ago. For example, take our desire for things that are sweet.

Mr. BEAUCHAMP: For animals that consume plants, which is really what the human ancestors were, they have a couple of very important things that they have to make sure about. One is getting enough calories; that's fundamental. And sweet things tend to be calorie-rich. They also actually tend to be high in vitamins in the real world. So most plant-eating animals have evolved a ability to taste sweet compounds, particularly sweet carbohydrates, and a liking for them.

NEIGHMOND: And we like salt, he says, for similar reasons. It's a signal for nutrients like sodium. But when it comes to fat, Beauchamp says why we like it is much more speculative.

Mr. BEAUCHAMP: Fat, of course, has, on a gram-for-gram basis, twice as many calories as sugar. It's the richest source of calories. And most people seem to think that the way we detect and recognize fat is because of the mouth feel, the texture. So a presumption is that people and animals learn to associate this feel with the feeling of the calories in the gut. That association is what makes these things so attractive.

NEIGHMOND: Beauchamp says something about fat may also enhance the sweetness of sugar and the saltiness of salt.

Mr. BEAUCHAMP: The third thing that we're sort of built with in terms of taste is the opposite thing, which is the avoidance of just what many mothers think they should be feeding their kids, namely compounds that are bitter, which, of course, characterizes many vegetables.

NEIGHMOND: In the early days of human development, he says bitter was a signal for danger. Poisonous plants often tasted bitter. And rather than gamble on potential benefit, early humans may have avoided bitter as much as possible.

Unidentified Woman #1: OK. So now we're going to play a game with things to taste.

NEIGHMOND: The Monell Center is a basic research institute dedicated to the study of taste, smell and chemical irritation. On this day, a scientist examines individual reactions to extremes of bitter taste. She gives volunteers liquid samples of bitter compounds typically found in foods.

Unidentified Woman #1: OK. And taste this one, please?

Unidentified Woman #2: It doesn't taste that good. It kind of tasted disgusting.

NEIGHMOND: While avoidance of bitter may be innate, of course, as many mothers will tell you, it's possible to overcome. But, says Gary Beauchamp, it's not easy.

Mr. BEAUCHAMP: So we've got this biological, built-in, push-pull system, liking for sweet and perhaps salt and disliking for bitter. And the food industry is responding to, at least in part, how we're built. Many foods are very sweet, many foods are salty and very few foods are bitter. That's what you see in the grocery carts.

Unidentified Woman #3: Today's menu is blueberry muffins and chilled pineapple for breakfast. For lunch, we're having cheese pizza with carrot and celery sticks.

NEIGHMOND: There may be another biological pull, variety, a need for foods that taste different; a bit of sweet, a bit of salt, some sour, some savory, tastes that tell our body it's getting the range of nutrients it needs.

Unidentified Woman #4: OK. Here we go. Here's your lunch.

(Soundbite of tray being placed down)

NEIGHMOND: This dietitian at Pennsylvania State University's food lab hands a tray of food to a research volunteer.

Unidentified Woman #4: All right. Just like yesterday, the first thing you're going to do is you're going to rate your appearance of the entrees.

NEIGHMOND: For the next two weeks, all of the food eaten by volunteers will be prepared here. Nutritionist Barbara Rolls runs this Laboratory for the Study of Human Ingestive Behavior.

Dr. BARBARA ROLLS (Laboratory for the Study of Human Ingestive Behavior): In our abundant society, where we have a huge variety of very calorie-dense foods that tastes really good, we can keep eating way beyond when we should stop.

NEIGHMOND: Rolls wants to figure out the environmental triggers for appetite. How strong is our need for variety and, for this volunteer in the food lab, how much food is too much?

"BLUE": What I'm looking at is three slices of cheese pizza, which looks really good. I'm supposed to take a bite of it.

NEIGHMOND: This anonymous subject is code-named Blue. Blue sits in a small cubicle shielded by curtains. She doesn't know it, but each portion of her meal has been meticulously measured down to the gram. And before each meal, she fills out a questionnaire.

"BLUE": Finally the last question is: `How does this size or this serving of food compare to the usual portion?' It's a lot larger than what I would normally eat at lunch.

NEIGHMOND: Today's lunch is pizza, carrots, celery sticks and ranch dressing. For dessert, two chocolate chip cookies.

"BLUE": These have a lot of chocolate in it. This is very good.

NEIGHMOND: Blue can eat as much or as little as she wants. In all, 30 men and women will take part in this study. They eat alone, isolated from other influences like friends, family, ambience. Blue's portions will eventually be increased, and Barbara Rolls wants to know if Blue will eat more as a result. Rolls says previous research showed many people didn't notice when portions were increased and they ate more.

Dr. ROLLS: Even when we're doubling portions, often people are very unaware. And that's really surprising because we need to be aware. When we're eating out, we need to be aware of it and we need to take action if it's too much.

NEIGHMOND: So is there hope for success when humans have basic biological drives for certain flavors? Barbara Rolls says yes, it can be done, starting in your own kitchen. Rolls has written a book with recipes that offer ample servings of a variety of foods. Take a casserole, for example. Rolls says you can lower the caloric content not only by using less cheese and reduced-fat milk but by bulking the casserole up with other foods.

Dr. ROLLS: If you add vegetables to a casserole, you lower the calorie density because the vegetables are adding water, which has volume but no calories. When you do that and let people serve themselves from family style dishes, they end up serving themselves the same amount of food regardless of the calorie density, and they don't feel any hungrier.

NEIGHMOND: Up until about age five, we have a built-in ability to regulate appetite and stop eating when we're full. After that, other influences kick in: pushy parents, big portions, peer pressure and food advertising. As adults, of course, we can learn to redesign our diets, but it does seem to boil down to discipline. After all, if we were really responding to our biological prompts, we'd probably just have to reach for that extra potato chip. Patricia Neighmond, NPR News.

INSKEEP: One other note this morning. Dr. Barbara Rolls has recipes designed to help you lose weight by eating more. You can find them at our Web site, npr.org.

(Soundbite of music)

INSKEEP: You're listening to MORNING EDITION from NPR News.

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