Most of us love potatoes in the form of crispy fries, fluffy mash, or steamed and baked. It is, in fact, the third most consumed edible crop in the world (first is wheat, second is rice). However, for years, potatoes have had a confusing reputation in nutrition science. Some studies hinted they might raise the risk of type 2 diabetes (T2D), while others found no clear link.

Finally, a new study has untangled this mystery, and its answer is surprisingly simple. It suggests that the way potatoes are cooked makes all the difference. For instance, French fries, or any other form of deep-fried potatoes, eaten regularly, do appear to raise diabetes risk. However, other forms, such as boiled, baked, or mashed, do not.

“We’re shifting the conversation from, ‘Are potatoes good or bad?’ to a more nuanced and useful question: How are they prepared, and what might we eat instead?” Seyed Mohammad Mousavi, lead study author and postdoc researcher at Harvard TH Chan School of Public Health, said.

Moreover, the study reveals that a simple dietary change, such as replacing fries with whole grains, can substantially reduce the chances of developing the disease. Here’s how the researchers arrived at these interesting findings.

Decoding the potato-diabetes dilemma

For decades, studies hinted at a link between potatoes and diabetes, but results were mixed. One reason was that earlier research often put all potatoes together — boiled, fried, or mashed — without considering that cooking method might change their effect on the body. Another issue was that most studies didn’t account for what foods people ate instead of potatoes, which can be just as important.

To tackle these issues, the study authors took a more detailed approach. They followed more than 205,000 men and women participants of three large US health studies: the Nurses’ Health Study, Nurses’ Health Study II, and the Health Professionals Follow-up Study—over nearly four decades (1984–2021).

Every four years, participants filled out food questionnaires, reporting how often they ate whole grains, rice, baked, boiled, or mashed potatoes, French fries, potato chips, and corn chips. By the end of the study, 22,299 participants had developed T2D. When the researchers compared diets, they found striking differences.

For instance, “eating three servings of French fries (or any form of deep fried potaotes) a week is associated with a 20% increased risk of developing type 2 diabetes (five servings a week pushed the risk to 27%), but eating similar amounts of potatoes cooked in other ways – boiled, baked or mashed – does not substantially increase the risk,” the study authors note.

The team also explored food swaps. Replacing French fries with whole grains such as brown rice, bulgur, or whole-grain bread cuts diabetes risk by 19 percent. Swapping any kind of potato for whole grains reduced risk by eight percent, while substituting baked, boiled, or mashed potatoes with whole grains lowered it by four percent.

On the flip side, replacing potatoes in the diet with white rice actually increased the likelihood of developing diabetes.

To strengthen their findings, the researchers also ran a meta-analysis that pulled together data from 13 previous cohort studies on potatoes and 11 on whole grains, covering over half a million people across four continents. The results matched their main study, confirming that the risks and benefits seen were not unique to the US.

What this means for our plates and policies
The findings carry a clear message. It’s not just the food we eat, but how it’s prepared and what we replace it with that shapes our health. French fries, with their high fat, salt, and calorie content, can push us toward diabetes when consumed regularly.

However, potatoes themselves are not inherently harmful and can be part of a balanced diet. The study also highlights small but powerful dietary changes, i.e., swapping fries for whole grains, could make a real difference in diabetes prevention at the population level.

“For policymakers, our findings highlight the need to move beyond broad food categories and pay closer attention to how foods are prepared and what they’re replacing. Not all carbs, or even all potatoes, are created equal, and that distinction is crucial when it comes to shaping effective dietary guidelines,” Walter Willett, one of the study authors and an expert in nutrition from the Harvard T.H. Chan School of Public Health, said.

The team hopes future studies will continue exploring how the preparation of different food items influences human health in the long term. For now, the takeaway is simple: potatoes are not the enemy, but watch out for those crispy, tempting fries.