Dr. Marion Nestle Explains Why Calories Count

The humble calorie can strike fear in even the most thoughtful eater. We know food contains calories and that we should keep an eye on them to maintain our weight, but few of us truly understand what they are or how they work.

As a professor in New York University's Department of Nutrition, Food Studies, and Public Health, Dr. Marion Nestle has encountered her fair share of calorie questions. Still, it wasn't until her screen debut in the 2004 film SuperSize Me! that Dr. Nestle decided a definitive guide was in order.

The result is Why Calories Count: From Science to Politics, Dr. Nestle's seventh book on the subject of food and nutrition. Why Calories Counttackles everything the average person might want to know about calories and then some: from the unit of measure's history and scientific definition, to its impact on the food industry and current topics in public health. For those concerned about their waistline, the book also covers issues of calorie expenditure, metabolism and effective dieting.

We had the pleasure of speaking with Dr. Nestle about the essence of the calorie, its relationship to government policy, and the surprising facts she discovered despite decades of nutrition expertise.

Why Calories Count covers everything from the history and science of calories to current obesity trends and food marketing tactics. What compelled you to create such a definitive guide?

When my editor at University of California Press suggested the idea of this book, I knew at once that I wanted to do it. Three reasons: First, I knew that people were completely confused about calories from surveys but also from Morgan Spurlock's SuperSize Me! (my screen debut). In the film, he asked people on the street to define calories. They responded like deer caught in headlights. Even I had trouble remembering the definition. No wonder. One calorie on food labels is really a kilocalorie—the amount of energy needed to raise the temperature of a kilogram of water one degree centigrade between 14.5 and 15.5 degrees centigrade at one atmosphere of pressure. In our book, we translate that into something someone might actually understand.

Reasons #2 and #3 : one billion people in the world don't get enough calories to meet daily needs and suffer from hunger and malnutrition. Another billion eat so much that they become overweight and have higher risks for chronic disease. Calories are worth attention.

The foundations of modern "calorie counting" were established over 100 years ago, yet we’re still at a loss to explain them. So, simply put, what is a calorie? Are all calories created equal?

I've just defined them. In plain English, they are units of heat. One hundred calories—two Oreo cookies—is enough to raise the temperature of a quart of water to the boiling point. Food calories are released when we metabolize proteins, fats, and carbohydrates, and let's not forget alcohol. Calorie numbers on food labels account for losses in digestion, but they don't account for differences in heat losses during metabolism. Some differences exist but they are too small to matter much.

You talk a lot in the book about current calorie counts being "good enough" both for individual diets and government policy. Why aren't we getting a precise number when looking at the information on nutrition labels and menu boards?

Calories on food labels have to be approximations because foods vary in composition. A calorie value for an apple is an average. Calorie tables also depend on portion size. A bigger apple will have more calories than a smaller one, sometime many more. So I think of calories on food labels as ballpark figures.

The book touches a bit on New York City's "Pouring on the Pounds" campaign. More recently, Mayor Bloomberg has created a firestorm with his attempts to eliminate supersized sodas. Why have sugar sweetened beverages become such a hot topic in the fight against obesity?

Sugary sodas contain sugars and, therefore, calories but nothing else of nutritional value. Nutritionists call their calories “empty.” Think of them as liquid candy. And there is some evidence that the body does not recognize or regulate intake of liquid calories in the same way it handles calories in foods. Nobody needs sodas. And evidence increasingly suggests that people who habitually consume sugary drinks have worse diets and are fatter than people who don’t. All of this makes sodas a convenient target. It’s harder to set policy for foods that contain nutrients, even if they are "junk" foods.

The Farm Bill that passed the Senate this month provides funding to get more fresh fruits and vegetables into needy communities, but also shells out billions in commodity crop subsidies. Given this, what do you think our collective calorie consumption will look like over the next 5 years?

That depends on whether we can get a grip on our current environment of food choice. Right now, we live in an environment that greatly encourages overeating. If we can find ways to change that so the environment makes it easier for people to eat more healthfully, we might be able to reduce the increase in average calorie intake that has occurred in recent years.

You're a longstanding professor in the Department of Nutrition, Food Studies, and Public Health at NYU, with a Ph.D. in molecular biology and an M.P.H. in public health nutrition. Still, did you learn anything new while researching this book?

I certainly did. I was surprised that calories were not "discovered" until the 18th century and that Wilbur Atwater, a USDA scientist, learned just about everything we now know about calories in the late 1890s. I did not know that calories in alcohol count just as much as those in food. And I had not fully understood the difference in results when calories in food and in the body are measured (difficult to do), as opposed to estimated (usually off by 30 to 40 percent). The book was lots of fun to do and well worth the effort. I hope readers have as much fun with it as we did.