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Study design and long-term dietary tracking
Researchers analyzed data from 205,107 participants enrolled in three established U.S. prospective cohorts: the Nurses’ Health Study, Nurses’ Health Study II, and the Health Professionals Follow-up Study. Participants answered repeated dietary questionnaires over more than three decades, reporting frequency of consumption for food items including French fries; baked, boiled, or mashed potatoes; and whole grains. The investigators also tracked new diagnoses of T2D and collected information about lifestyle, medical history, and demographic factors to adjust for potential confounding.
Over the study period, 22,299 participants reported new diagnoses of type 2 diabetes. The large sample size and long follow-up enabled the team to examine associations by potato type and to model the effects of substituting different carbohydrate sources.
Key findings and substitution analysis
The primary finding: consuming three servings per week of French fries was associated with a 20% higher risk of developing type 2 diabetes compared with lower intake. By contrast, intake of baked, boiled, or mashed potatoes showed no significant association with T2D after adjustment for known risk factors.
Substitution models offer a practical perspective. Replacing baked, boiled, or mashed potatoes with whole-grain foods (for example, whole-grain pasta, bread, or farro) was associated with an estimated 4% reduction in T2D risk. Swapping French fries for whole grains was associated with a roughly 19% lower risk. Even replacing French fries with refined grains produced an estimated reduction in risk, though the benefit was larger when whole grains were the substitute.
To strengthen these findings, the authors performed a complementary meta-analytic approach. They combined data from 13 cohorts on potato intake and 11 cohorts on whole-grain intake — together covering more than 500,000 participants and roughly 43,000 T2D cases across four continents. Those pooled estimates were consistent with the cohort-specific results.
Why French fries may be different
Several mechanisms could explain why French fries stand out. Deep frying increases caloric density and often introduces unhealthy fats and sodium. Frying and high-temperature cooking can also generate compounds such as acrylamide. Texture and added fats influence post-meal blood glucose and insulin responses, and typical portion sizes for fries are often larger than for other potato preparations. These factors, combined with the high glycemic load of starchy foods like potatoes, can contribute to metabolic stress over time.
Public health implications and recommendations
The authors emphasize a nuanced message: not all carbohydrate sources, and not even all forms of the same food, carry equal risk. "Small changes in our daily diet can have an important impact on risk of type 2 diabetes," said Walter Willett, corresponding author and professor of epidemiology and nutrition. The study recommends limiting intake of fried potato products, especially French fries, and favoring whole-grain carbohydrate sources.
For policymakers and dietary guideline developers, the results argue for recommending preparation-specific guidance rather than broad food categories alone. Practical consumer advice includes choosing whole grains, prioritizing baking or boiling over frying, controlling portion sizes, and reducing added fats and salt.
Limitations and caveats
This is an observational study, which can identify associations but cannot definitively prove causation. Diet was self-reported and therefore subject to measurement error. Residual confounding is possible despite adjustment for many lifestyle and clinical variables. Finally, preparation details (for example, oil type or restaurant portion size) vary and may influence outcomes.
Funding, publication and authorship
The analysis was published in The BMJ and led by Seyed Mohammad Mousavi with co-authors including Xiao Gu, Hala AlEssa, Qi Sun, Frank Hu, JoAnn Manson, Eric Rimm and Walter C. Willett. Funding sources included the U.S. National Institutes of Health and several international research organizations.
Expert Insight
Commentary from a nutrition expert
Dr. Elena Martínez, clinical nutrition scientist and adjunct professor (fictional expert), notes: "These findings reinforce a fundamental principle of nutritional epidemiology — how a food is prepared and what it replaces in the diet matter as much as the food itself. Swapping fried potato dishes for whole grains can reduce glycemic load and improve dietary fiber intake, both of which help lower diabetes risk over time." She adds, "At the population level, small habitual changes — like choosing whole-grain bread or a serving of brown rice instead of fries — can sum to meaningful public-health gains."
Conclusion
This 30-year analysis from Harvard and a complementary meta-analysis indicate that French fries, unlike baked, boiled, or mashed potatoes, are associated with a higher risk of type 2 diabetes. Replacing French fries with whole-grain alternatives could substantially lower that risk. While observational limitations apply, the study provides actionable guidance: limit fried potato products, prioritize whole grains, and consider preparation and substitutions when evaluating carbohydrate choices in dietary guidance.
Reference: Mousavi SM, Gu X, Imamura F, et al. 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. BMJ. 6 August 2025. DOI: 10.1136/bmj-2024-082121.
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