The potato chip may be the biggest demon behind that pound-a-year weight creep that many of us incur, says a major diet study.

If you’re fighting off pounds and counting calories, watch out for the dreaded potato chip. It’s the top enemy in America’s battle of the bulge-not cupcakes, soda or double bacon cheeseburgers. The simple potato, according to Harvard University researchers, can be a major pound packer.

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

Unfortunately, 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 lb. a year, close to the U.S. average, according to a report published in the New England Journal of Medicine.

The Harvard Medical School team and the Harvard School of Public Health in Boston examined the potato’s role in the modern diet, and found that people who ate an extra serving of French fries each day gained an average of 3.4 lb. within a four-year period.

The problem, says study co-author Walter Willett, chairman of the nutrition department at the Harvard School of Public Health, is that “we don’t eat potatoes raw, so it’s easier (for the body) to transform the starch to glucose.” Potatoes encourage a speedy spike in blood sugar levels, causing the pancreas to move into overdrive, trying to bring levels back down to normal. As blood sugar spirals down, people usually experience hunger, which leads to snacking. Making matters worse, potatoes pack a lot of calories into a relatively small package.