Researchers at Harvard University say that the potato, an all-time American favorite, contributes to our national obesity crisis much more significantly than previously thought.

Our love for French fries, chips and baked potatoes gives cause for concern, according to a report issued by the Harvard School of Public Health in Boston, Mass. and published in the New England Journal of Medicine.

Based on a study that tracked more than 120,000 participants over 20 years, the research team calculated an average weight gain of more than 3 pounds every four years.

Surprisingly, potato consumption was singled out as one of the worst culprits, causing about 0.8 pounds weight gain per year.

While this may not sound particularly alarming, the compounding effects over time are not to be discarded.

Gaining 16-20 pounds extra weight over two decades just from one item on your dinner plate is enough to pay closer attention, says Dr. Walter Willett, chairman of the Nutrition Department of Harvard School of Public Health and co-author of the study.

According to the U.S. Department of Agriculture (USDA), the typical American eats about 117 pounds of potatoes per year and almost a third of that in the form of French fries.

One of the reasons why potatoes contribute more easily to weight gain than other vegetables, says Dr. Willett, is that we don’t eat potatoes raw but cook, bake or fry them.

This way it is easier for the body to transform the starch to glucose. This can prompt sudden spikes in blood sugar, causing the pancreas to release additional insulin to bring the levels back down to normal.

The combined burst of blood glucose and insulin secretion has the unfortunate side effect of making us feel hungry again and wanting to eat more.

If this cycle continues over long periods of time, weight gain is inevitable.

What’s worse is that the pancreatic functions fatigue, which increases the risk of developing type 2 diabetes.

Also, unlike most other vegetables, potatoes are quite caloric. A large baked potato without any fixings contains about 275 calories.

A medium-size bag of potato chips has 450 calories or more. And a large serving of French fries can deliver 500-600 calories, close to a third of a healthy daily calorie intake for adults.

The report names plenty of other causes for weight gain among the participants in the study as well: Drinking sugary sodas accounted for about one quarter of a pound per year.

Similar effects had the consumption of both processed and unprocessed meats. Alcohol was also seen as a factor.

“Our findings indicate that small dietary and other lifestyle changes can together make a big difference, for bad or good,” says Dr. Dariush Mozzafarian, professor of epidemiology at the Harvard School of Public Health and the study’s lead author.

By consuming fewer starches and refined foods, like potatoes, white bread, low-fiber breakfast cereals, processed meats, sweets and sodas, and by adding more fresh fruits and vegetables to their meals, most people could manage to stay within their healthy weight range over long periods of time, preferably for life.

The U.S. Potato Board (USPB), a marketing organization for the potato industry, has challenged the Harvard study, citing “unfortunate myths and misconceptions.”

“Singling out the potato as a cause of weight gain is misleading and contrary to existing research,” said Tim O’Connor, president and CEO of the Potato Board.

He also complained that the participants’ calorie intake was not controlled in the study and that the data were largely based on self-reports of portion sizes, which may often be understated.

Potato defenders have pointed out that there may be more correlation than causation at play. People who like potato chips and French fries often eat them with other less-than-healthy items, like hamburgers or hotdogs as well as sugary sodas and desserts.

Since many of these contain high amounts of calories and fat, it would be unfair to put all the blame on the potatoes.

Nutrition experts generally acknowledge the health benefits of potatoes. They are good providers of potassium, fiber and vitamin C.

Therefore, potatoes can be part of a healthy diet and successful weight management.

In its response to the Harvard study, however, even the USPB admits that “the overall diet quality is better when adults and children consume non-fried white potatoes.”

Timi Gustafson R.D. is a clinical dietitian and author of the book “The Healthy Diner – How to Eat Right and Still Have Fun,” available in bookstores, at timigustafson.com and at Amazon.com.