And up next tonight, for americans seeking health and how to be at the top of their game we have an experiment, an experiment that shows portion control may be the key to it all. Here now is the... See More
And up next tonight, for americans seeking health and how to be at the top of their game we have an experiment, an experiment that shows portion control may be the key to it all. Here now is the newest member of the abc news team. Reporter: These college students were asked to help abc with a profile on their professor. Because we scheduled this around lunch we serve a meal and measure out 8 ounz of pasta for everyone. Listen closely to what the professor tells them. We'll be serving you a double portion of pasta. Reporter: The next group. We're giving you a half portion of pasta. Reporter: The next. A full size portion of pasta. Reporter: A good meal is had by all but then we let them in on our little secret. You guys were part of a little experiment. Reporter: Those who think they have regular portions, they eight it all up except her who said it was too cold. The others belong in the clean plate club. Reporter: The students who think they have a double portion all left food. Double portion doesn't sound like a person should eat it. If you tell them they have a small size portion, my gosh I'm going to finish it all and i don't have to feel guilty. Reporter: The professor says this explains the nation's obesity problem. We eat what we're served without thinking about how much is actually on our plate. Restaurants across the country serve huge portions and some of the choices really tack on the calories. The deep dish macaroni at uno chicago grill has almost 2,000. That's what americans should eat in a day. The lesson? Be careful, sometimes your ears can be bigger than your stomach. Bottom line is the only person you should be listening to on portion size is yourself. You should use your own palm size as a guide for your protein and your carbs at each meal. So you mean right in here? Your own individual palm which is perfectly proportioned to your size. I have big hands. That's maybe good news. But your palm size is the key, all right.
This transcript has been automatically generated and may not be 100% accurate.