Eating French fries just three times a week could raise your risk of type 2 diabetes by 20%, while boiled, baked, or mashed potatoes carry no such risk, according to a new Harvard University study published in the British Medical Journal.

The research, led by Dr. Syed Muhammad Mousavi with renowned nutritionist Dr. Walter Willett among the co-authors, analyzed over 30 years of dietary data and health records from 250,000+ participants.

Frying method makes the difference

Researchers say the way potatoes are prepared is key. Frying at high temperatures in unhealthy oils produces harmful compounds, while boiling or baking preserves the nutritional value without introducing trans fats.

“Replacing French fries with whole grains can lower diabetes risk by up to 19%,” Dr. Mousavi explained. “Replacing them with another fried food won’t help.”

Fried food health risks extend beyond diabetes

The American Diabetes Association warns that fried foods—especially French fries—often contain trans fats, which can also increase the risk of heart disease and stroke. Many fries are also coated with flour or breading, raising carbohydrate content.

Nutritionist Stacy Krausik from the association advised, “Choose whole, less processed foods and use healthier cooking methods rather than frying.”

Potatoes aren’t the villain

Not all experts believe potatoes deserve a bad reputation. Dr. Hanna Kahliova from the Physicians Committee for Responsible Medicine noted that boiled potatoes may even aid in weight loss and reduce diabetes risk, citing studies from Iran, Finland, and the Netherlands showing over 50% lower risk among high boiled-potato consumers.

Public health implications

Dr. Mousavi said public health messages should focus less on labeling foods as “good” or “bad” and more on preparation methods and healthy alternatives.

The study’s two meta-analyses—covering more than 500,000 people and 43,000 diabetes cases—reinforce the idea that diet quality is as much about how you cook as what you eat.