French fries may be a comfort food classic but when it comes to long-term health, they could come at a cost.
A new study from Harvard T.H. Chan School of Public Health suggests that how potatoes are prepared plays a key role in type 2 diabetes risk. The research, published in The BMJ, followed the diets and health outcomes of more than 205,000 adults over 30 years. It found that people who ate three or more servings of French fries a week had a 20% higher risk of developing type 2 diabetes. But other forms of potatoes (baked, boiled or mashed), were not associated with increased risk.
“Not all carbs — or even all potatoes — are created equal,” said Walter Willett, MD, one of the study’s senior authors and a professor of epidemiology and nutrition at Harvard. “That distinction is crucial when it comes to shaping effective dietary guidelines.”
This is the most comprehensive study to date exploring potatoes and type 2 diabetes risk. Previous research had shown mixed results, often failing to distinguish between preparation methods or account for what people ate instead.
“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?” said lead author Seyed Mohammad Mousavi, PhD, a postdoctoral fellow at Harvard.
The study pulled data from three large U.S. health cohorts: the Nurses’ Health Study, Nurses’ Health Study II and the Health Professionals Follow-up Study. Over 22,000 participants developed type 2 diabetes during the study period. Researchers controlled for major lifestyle and demographic factors, including body weight, smoking, physical activity and total calorie intake.
The standout finding: swapping fries for whole grains such as whole wheat bread, farro or brown rice could reduce diabetes risk by 19%. Replacing even baked or mashed potatoes with whole grains was linked to a 4% lower risk. Even refined grains fared better than fries in comparison.
To validate their findings, the researchers also conducted a meta-analysis of over 500,000 participants from other global cohort studies. The results were consistent: whole grains tend to lower risk, while fried potatoes do the opposite.
“The public health message here is simple and powerful,” Willett said. “Small changes in our daily diet can have an important impact on risk of type 2 diabetes.”
While potatoes are often grouped with other starchy vegetables or carbohydrates, the findings highlight the need to look more closely at cooking methods and substitutions. Frying potatoes at high heat can create compounds that affect blood sugar regulation, and the added fat and sodium may amplify health risks over time.
For individuals, the takeaway isn’t to ditch potatoes entirely. Instead, it's about thinking critically about what’s on your plate and what could be swapped in.
“Choosing healthy, whole grain sources of carbohydrate could help lower the risk of type 2 diabetes across the population,” Willett added.
This study was funded by the National Institutes of Health, the UK National Institute for Health and Care Research, the Medical Research Council Epidemiology Unit, the Kuwait Heart Foundation and Friends of FACES/Kids Connect.