Larger portions have more calories, obviously. But they also encourage people to eat more than they otherwise would and to underestimate how much food they're actually consuming.

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I've just agreed to write a Q&A column, Nutritionist's Notebook, for New York University's student newspaper, the Washington Square News (WSN). The columns will appear on Tuesday. This first one was published on February 22.

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This week, WSN welcomes professor-columnist Marion Nestle. A Paulette Goddard professor of Nutrition, Food Studies, and Public Health at NYU, Nestle also co-authored the recently published book Why Calories Count: From Science to Politics. Each week, she will answer student questions about nutrition, health, and food.

What is the importance of size in our portions? What is the best way to judge portions when going out to dinner?

Easy. Large portions make you eat more. If I could teach just one thing about nutrition, it would be this: Larger portions have more calories. Funny? Portion size is anything but obvious. Research repeatedly confirms that larger food servings not only provide more calories but also have two other effects. They encourage people to eat more and to underestimate how much they are eating.

A few years ago, I asked Lisa Young, who teaches our department's introductory nutrition course, to ask her students to guess the number of calories in an eight-ounce Coke and a 64-ounce Double Gulp -- yes, such things exist. She did not expect beginning students to know the exact numbers, but did expect them to do the math. To her surprise, the average multiplier turned out to be three, not eight. How come? Students said that 800 calories in a drink was impossible. No, it is not, as menu labels now reveal.

How to deal with the portion size problem? Use small plates and cups in the dining hall. When eating out, order appetizers, not entrees. Order the small size, or share large portions with friends.

The system is stacked against you and it's up to you to figure out how to cope with it. Small sizes, for example, usually cost relatively more.

For a long time, I've wanted restaurant owners to give a price break for smaller portions. No luck. They say this would put them out of business. We need to make it easier for people to choose smaller portions, which means changes in public policy.

Image: Andy Dean Photography/Shutterstock.

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This post originally appeared on Food Politics, an Atlantic partner site.