A large study published in The BMJ found that eating three servings of French fries per week was associated with a 20% higher risk of developing type 2 diabetes. In contrast, consuming the same amount of potatoes prepared in other ways, such as boiled, baked, or mashed, was not linked to a significant increase in diabetes risk.

The research also found that what replaces potatoes in a person's diet matters. Swapping potatoes for whole grains was associated with a lower risk of type 2 diabetes, while replacing them with white rice was linked to a higher risk.

Looking Beyond Potatoes Alone

Potatoes provide important nutrients, including fiber, vitamin C, and magnesium. However, they are also rich in starch and have a relatively high glycemic index, meaning they can cause blood sugar levels to rise quickly. Because of this, previous studies have often connected potato consumption to a greater risk of type 2 diabetes.

Yet researchers noted that two important factors have often been overlooked. First, potatoes can be prepared in very different ways. Second, the health effects of potatoes may depend on which foods people eat instead.

To explore those questions, scientists examined whether diabetes risk differed between French fries and potatoes prepared by boiling, baking, or mashing. They also evaluated the potential effects of replacing potatoes with other common carbohydrate-rich foods, including whole grains and rice.

Four Decades of Health Data

The study drew on data from more than 205,000 U.S. health professionals who participated in three major long-term studies conducted between 1984 and 2021.

At the start of the research, participants did not have diabetes, heart disease, or cancer. Every four years, they completed detailed dietary questionnaires that allowed researchers to track eating habits over time.

During nearly 40 years of follow-up, 22,299 participants developed type 2 diabetes.

After accounting for lifestyle habits and dietary factors that could influence diabetes risk, researchers found that every three weekly servings of potatoes overall were associated with a 5% increase in the rate of type 2 diabetes.

The strongest association, however, involved French fries. Every three servings per week were linked to a 20% increase in the rate of type 2 diabetes. Similar consumption of baked, boiled, or mashed potatoes was not associated with a statistically significant increase.

Whole Grains Show a Benefit

Researchers also looked at what happened when potatoes were replaced with other foods.

Replacing three weekly servings of potatoes with whole grains was associated with an 8% lower rate of type 2 diabetes. When baked, boiled, or mashed potatoes were replaced with whole grains, the rate was 4% lower. Replacing French fries with whole grains was associated with a 19% lower rate.

The results were different when potatoes were replaced with white rice. Substituting either total potato intake or baked, boiled, or mashed potatoes with white rice was associated with a higher rate of type 2 diabetes.

Important Caveats

Because this was an observational study, it cannot prove that French fries directly cause diabetes. The researchers acknowledge that other factors not measured in the study may have contributed to the results.

The participants were also predominantly health professionals of European ancestry, which means the findings may not apply equally to all populations.

Even so, the researchers wrote: "Our findings underscore that the association between potato intake and type 2 diabetes risk depends on the specific foods used as replacement. The findings also align with current dietary recommendations that promote the inclusion of whole grains as part of a healthy diet for the prevention of type 2 diabetes."

Are Potatoes Back on the Menu?

In an accompanying editorial, researchers argued that potatoes should not be viewed as a single category when considering health effects.

They emphasized that both preparation methods and replacement foods are important factors when making dietary recommendations or shaping public policy.

According to the editorial, baked, boiled, and mashed potatoes can fit into a healthy and environmentally sustainable diet because of their relatively low environmental impact and overall nutritional value. However, the authors noted that whole grains should remain a priority food choice for reducing diabetes risk.

They also called for future studies involving more diverse populations and analyses that continue to examine both cooking methods and food substitutions.