French fries raise type 2 diabetes risk, but whole grains may help reduce it, according to a large Harvard study.
A new study from the Harvard T.H. Chan School of Public Health has found that eating French fries is linked to a higher likelihood of developing type 2 diabetes (T2D). Other potato dishes, such as baked, boiled, or mashed varieties, did not show the same connection. Researchers also discovered that replacing potatoes of any kind with whole grains could help lower the risk of T2D.
The findings were recently published in The BMJ.
Scientists explained that earlier research suggested a possible relationship between potato consumption and T2D, but the results were often inconsistent. Many of those studies did not carefully examine how potatoes were prepared or what happened when other foods were eaten in their place.
“Our study offers deeper, more comprehensive insights by looking at different types of potatoes, tracking diet over decades, and exploring the effects of swapping potatoes for other foods,” said lead author Seyed Mohammad Mousavi, postdoctoral research fellow in the Department of Nutrition. “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?”
Long-Term Dietary Tracking
The researchers examined the diets and diabetes outcomes of 205,107 men and women enrolled in the Nurses’ Health Study, Nurses’ Health Study II, and Health Professionals Follow-up Study. For more than 30 years, participants regularly responded to dietary questionnaires, detailing the frequency with which they consumed certain foods, including French fries; baked, boiled, or mashed potatoes; and whole grains. They also reported on new health diagnoses, including T2D, and various other health, lifestyle, and demographic factors, which the researchers controlled for. Over the course of the study period, 22,299 participants reported that they developed T2D.
The study found that three servings weekly of French fries increased the risk of developing T2D by 20%. Baked, boiled, and mashed potatoes were not significantly associated with T2D risk. The researchers calculated, however, that eating whole grains—such as whole grain pasta, bread, or farro—in place of baked, boiled, or mashed potatoes could reduce the risk of T2D by 4%. Replacing French fries with whole grains could bring T2D risk down by 19%. Even swapping refined grains for French fries was estimated to lower T2D risk.
Meta-Analysis Strengthens Results
The researchers complemented their study with a novel meta-analytic approach to estimate how swapping potatoes for whole grains could affect the risk of T2D, using data from previously published cohort studies.
This involved two separate meta-analyses: one based on data from 13 cohorts examining potato intake and the other from 11 cohorts on whole grain intake, each encompassing over 500,000 participants and 43,000 T2D diagnoses across four continents. The results were closely consistent with those of the new study.
“The public health message here is simple and powerful: Small changes in our daily diet can have an important impact on risk of type 2 diabetes. Limiting potatoes—especially limiting French fries—and choosing healthy, whole grain sources of carbohydrate could help lower the risk of type 2 diabetes across the population,” said corresponding author Walter Willett, professor of epidemiology and nutrition. “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.”
Reference: “Total and specific potato intake and risk of type 2 diabetes: results from three US cohort studies and a substitution meta-analysis of prospective cohorts” by Seyed Mohammad Mousavi, Xiao Gu, Fumiaki Imamura, Hala B AlEssa, Orrin Devinsky, Qi Sun, Frank B Hu, JoAnn E Manson, Eric B Rimm, Nita G Forouhi and Walter C Willett, 6 August 2025, BMJ.
DOI: 10.1136/bmj-2024-082121
Other Harvard Chan co-authors included Xiao Gu, Hala AlEssa, Qi Sun, Frank Hu, JoAnn Manson, and Eric Rimm.
The study was supported by the National Institutes of Health (grants UM1 CA186107, U01 CA176726, and U01 CA167552); the Friends of FACES/Kids Connect; the Medical Research Council Epidemiology Unit (grant MC_UU_00006/3); the UK National Institute for Health and Care Research Biomedical Research Centre Cambridge Theme on Nutrition, Obesity, Metabolism and Endocrinology (grants NIHR203312, NIHR202397); and the Kuwait Heart Foundation.
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12 Comments
Hmmmmmmmmmmm
Seed oils dont even get a mention in this?
My thoughts exactly. They go so far as to identify deep fried potato is more harmful than a boiled one, yet don’t bother to question what’s fundamentally different about the deep fried potato?
Such as …
1. that it’s cooked at a much higher temp (which commonly causes the formation of carcinogenic compounds such as Acrylamide in otherwise healthy foods), and 2. it’s deep fried in toxic ultra-refined, highly oxidised industrial vegetable oils.
Curiously, a “food-frequency questionnaire” was added to the Nurse’s Health Study in 1980 (https://nurseshealthstudy.org/about-nhs/history), the same year the FDA approved the expanded use of added MSG as an alleged “flavor enhancer.” The US obesity and diabetes epidemics presented by 1990 and 1994, respectively (CDC data). Still, with no indication of practically harmless brief (about six to twelve hours duration) individual nearly subclinical (sub-acute) non-IgE-mediated food allergy reactions (https://www.foodallergy.org/fare-blog/why-we-need-stop-referring-ige-mediated-allergies-true-food-allergies) aggravated (extended and/or intensified) (or not) with added MSG turning them chronic and deadly dangerous, long-term (months to decades, highly individual, many individual variables) being factored-in. Consequently, the studies reported on are incomplete, inaccurate and invalid, as are all such studies. Still, too, the question remains: why french fries? Perhaps “tennisguy” is right or, as I have previously posted, perhaps it’s the preservative (e.g., TBHQ [since 1972], minimally) in the cooking oil?
Potato
The Japanese and Chinese use tons of MSG and have been for over a century and they’re among the healthier peoples on this planet. It wasn’t until the introduction of fast food and colas in the 1970s that certain segments of the Japanese population began to show signs of obesity and not until the early 2000s that those same groups began to develop diabetes. Now
Good points, Ashwin. However, a little extra online research this morning revealed what I already suspected, people in China and Japan consume a lot less beef and dairy (common allergens) than much of the rest of the world. Fish and rice, in particular, are less common allergens. And, as best as I have been able to determine on my own, my kind of food allergies are the underlying cause and added artificially cultured “free” (can cross the blood-brain barrier) is an aggravating (extending and/or intensifying) factor in the otherwise brief and nearly harmless individual allergy reactions. Also, despite common food allergies being around for millennia, infertility (especially in Japan) has become an area of concern among more allegedly ‘advanced’ nations embracing the expanding use of added MSG.
“Just 3 servings per week”
Hey Americans, I think I see your problem…
French fries aren’t a food group. They’re a treat. Geezus.
The common white potato has the highest glycemic index of all foods. It turns to sugar in the bloodstream faster than anything.
Faster than a speeding bullet?
So, eating at places that serve fries is bad for you. Tell me more.
I’m diabetic for many years and had noticed that whenever I had french fries my blood sugar was very high up to 350 ! ! So I switched to baked sweet potatoes with better results!
None of this is revelatory. Besides the science, generational knowledge that starches fried in oil and fat are bad for you, and while grains are good. Harvard hard at work proving what most already knew. But as an American in the South, while french fries are a common side dish, I don’t know anyone who eats them 3 or more times per week. And pretty common here to bake them in the oven at home, not fry.