By Daniela Hernandez, Los Angeles Times

Public Enemy No. 1 in America’s battle of the bulge isn’t cupcakes, soda or double bacon cheeseburgers. It’s the simple potato, according to Harvard University researchers.

Daily consumption of an extra serving of spuds — French fries, crispy chips, mashed with butter and garlic, or simply boiled or baked — was found to cause more weight gain than downing an additional 12-ounce can of a sugary drink or taking an extra helping of red or processed meats.

Altogether, after tracking the good and bad diet and lifestyle choices of more than 120,000 health professionals from around the country for at least 12 years, the research team calculated that participants gained an average of 0.8 of a pound a year, close to the U.S. average, according to a report published in Thursday’s edition of the New England Journal of Medicine.

It may not sound like much, but as the years go by “it becomes like compounded interest,” adding up to 16 pounds over 20 years, said Dr. Jeffrey Schwimmer, director of the weight and wellness program at Rady Children’s Hospital in San Diego, who wasn’t involved in the study.

Potatoes have a long pedigree in the human diet. They were once hailed as history’s most important vegetable, and the Incas — whose ancestors are credited with domesticating spuds in South America — worshiped a potato god.

They are still certified as a “heart healthy” food by the American Heart Assn. And the United Nations declared 2008 the International Year of the Potato, praising the tuber for being a good source of vitamin C, several B vitamins, and minerals including iron, potassium, phosphorus and magnesium.

But when the team from Harvard Medical School and the Harvard School of Public Health in Boston examined the potato’s role in the modern diet, they found that people who ate an extra serving of French fries every day gained an average of 3.4 pounds over a four-year period. On top of that, those who munched on an extra serving of potato chips daily gained an average of 1.7 pounds every four years. Overall, an extra serving of potatoes prepared in any non-chip form was found to contribute an average of 1.3 pounds to total weight over four years.

The typical American consumes 117 pounds of potatoes each year, including 41 pounds in the form of previously frozen French fries, according to data from the U.S. Department of Agriculture. “Fresh” spuds account for only 28 percent of the total, the USDA said.

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