The Negative Calorie Diet

Every afternoon, a former colleague of mine would unwrap a small package of celery she'd bought from the company cafeteria. "Fills me up," she once explained, munching away, "and burns off my lunch, too."

My co-worker believed in "negative calorie foods." The theory goes like this: Your body burns some of the calories in your food to run the chemical and mechanical processes that digest the food. But some foods contain fewer calories than are needed to digest them—so by eating such foods, you actually lose weight. Say, for example, you're eating a 1-1/2-ounce stalk of celery. It contains 7 calories, but maybe your body needs 30 calories to break it down. If that's true, eating a stalk will burn 23 calories—and if you eat five stalks a day, you'll lose a pound a month. (A pound is equivalent to 3500 calories.)

Other foods that supposedly have negative calories include asparagus, lettuce, broccoli, beets, onions, cabbage, carrots, cucumber, zucchini, apples, oranges, pineapple, grapefruit, raspberries, pineapple, strawberries, lemons, limes, chocolate truffles, and potato chips. (Sorry, just kidding about those last two.)

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The theory was the basis of the 1990s book Foods that Cause You to Lose Weight: the Negative Calorie Effect, co-authored by Neal D. Barnard, M.D., founder and president of the Physicians Committee for Responsible Medicine (PCRM). According to ActivistCash.com, a food-industry-backed watchdog, the chief funder of PCRM is People for the Ethical Treatment of Animals (PETA), the famous animal-rights-activist group. Dr. Barnard also serves on the advisory boards of Earth Save International, Veggie Life magazine, and Vegetarian Times; and is president of the Foundation to Support Animal Protection (FSAP), which shares a mailing address with PETA. We were unable to reach Dr. Barnard, but we did speak with someone else at PCRM—see below.

"Faster Than Fasting"

You can find plenty of information about negative-calorie foods online. Most of it is in the form of advertisements for an e-book called The Negative Calorie Diet. Here's how one ad describes the science behind the diet:

The process starts by chewing. Then your esophagus moves everything down to your stomach. This usually takes anywhere from a few minutes to a half hour. For about four hours, your stomach mixes the food up with acid and sends everything down to your small intestine. For approximately another four hours, your small intestine receives very strong alkaline (digestive) juices from your gallbladder and pancreas. These digestive juices mix with the now liquefied food and your body starts to absorb it. This process continues on down to your large intestine where the rest of any food and fluids are absorbed into your body. This may take up to 12 hours! Any residue that is left over is eventually eliminated, but here's the GOOD NEWS! This entire process BURNS CALORIES and results in weight loss!

The e-book, according to the ad, identifies over 100 negative-calorie foods that safely force your body to work harder during digestion, thus turning your body into a "fat-burning machine." Moreover, it "reveals the secrets of consuming negative-calorie foods" so you can lose 14 pounds in seven days. It promises you can shed weight "three times faster than FASTING itself!" (But first, of course, you have to shed $19.95 to download the book.)

"Defies Science"

The idea of eating your way to weight loss is very appealing. I hear it frequently in conversations about dieting, and you probably do, too. But is it really possible?

To begin with, no scientific studies have been done to test the concept of a negative calorie diet. We can only rely on expert opinions.

Since Dr. Barnard was not available for an interview, PCRM arranged for me to talk with its nutrition director, Dr. Amy Lanou, who has worked with Barnard for four years. The theory of negative-calorie foods, she said, "is a little elusive. I don't think there is much information about the energy cost for digesting specific foods. I don't think the research exists to say that if you eat these many raspberries, you'll burn these many calories."

She added that she herself wouldn't call this a negative calorie diet. "The foods have calories," she said, "but you'll burn more energy than you'll gain from them." Lanou thinks the theory is "substantiatable"—but she knew of no studies supporting it.

Robert Eckel, M.D., a professor of medicine at the University of Colorado Health and Sciences Center and the American Heart Association's spokesman on nutrition, calls the idea of a negative calorie diet "ridiculous. It defies the principles of science. There's no food I can envision that can be associated with energy loss."

Try a Low-Fat Salad...

Calories are expended in three ways, Eckel explained: 70 percent through basal metabolic rate (basic activities of living), 25 percent through exercise, and 5 percent through thermogenesis (the cost of absorbing and storing calories). "Typically, a five-calorie carrot would need about 0.25 calories to absorb and store," says Eckel. "Thus, the carrot still has about 4.75 calories. The only way one burns more than one eats is if you eat less or exercise more. A calorie never changes."

Barbara Rolls, Ph.D., professor of nutritional sciences at Pennsylvania State University, finds the theory of negative-calorie foods "interesting, but very hypothetical. I don't think we have the metabolic tools to determine whether this is possible." Rolls thinks people lose weight eating so-called negative-calorie foods not because the foods burn calories, but because they displace higher-calorie foods. Co-author of the book Volumetrics: Feel Full on Fewer Calories, Rolls says that "clinical trials show that eating high-water-content fruits and vegetables helps with weight management."

In recent studies, Rolls and other researchers found that people who were given a large, low-calorie salad as a first course ate fewer calories in their entire meal. Before serving them pasta, the researchers gave 33 women a 100-calorie salad of lettuce, tomatoes, celery, carrots, and cucumbers (most of which are considered good candidtes for a negative calorie diet), fat-free dressing, and light mozzarella. The researchers found that the women ate 12 percent fewer calories in their entire meal than when they started without a salad. In contrast, when they ate a small, high-calorie salad (200 calories), they ended up consuming 8 percent more calories overall; and when they started with a 400-calorie salad, they ate 17 percent more overall.

"You eat fewer calories through satiety," Rolls concludes, "not the metabolic effect."

...Or a Helping of Shoe Leather

When asked about the negative calorie diet, Ann Coulston, a former president of the American Dietetic Association, said, "It's folklore. People have made these calculations themselves with the information that is available. There are no studies to support the theory. "

Very low-calorie foods are good to eat, says Coulston, an authority on carbohydrate and fat metabolism, "but you shouldn't get mathematical about it. Eating is not just the science of calories and nutrients. It also deals with behavior. The act of eating stimulates eating other foods as well. If you're eating celery, you'll put a spread on the celery. You open the fridge and look for other things to munch on.

"People should expend extra calories by adding ten minutes to their exercise routine," she suggests, "not by chewing or playing games with their metabolism."

So there you have it. If you want to shed unwanted pounds, a negative calorie diet isn't the answer. The only real source of negative calories is a brisk walk, a daily jog, or some other form of exercise.

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