Eating French fries raises type 2 diabetes risk, major study finds

Not all potatoes are equal: New research reveals that French fries increase diabetes risk, while whole grains can help protect against it.

Study: 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. Image Credit: LarysaPol / Shutterstock

French fries were associated with an increased risk of developing type 2 diabetes (T2D), while other forms of potatoes, including baked, boiled, and mashed, were not, according to a new study led by Harvard T.H. Chan School of Public Health.

The study published in the British Medical Journal also found that swapping any form of potato for whole grains may lower the risk of T2D, though substituting them with white rice increased the risk. Replacing all potatoes with whole grains was associated with an 8% lower risk of T2D, and this risk reduction was statistically significant and consistent across analyses.

The study also found that the lack of association for baked, boiled, or mashed potatoes was robust in multiple sensitivity analyses.

Clarifying the potato-diabetes connection

According to the researchers, while previous studies hinted at a link between potatoes and T2D, the evidence was inconsistent and often lacked detail on cooking methods and the potential effects of substituting other foods for potatoes.

"Our study offers deeper, more comprehensive insights by looking at different types of potatoes, tracking diet over decades, accounting for long-term weight changes, 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?"

Decades of dietary data from large US cohorts

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; potato or corn chips; 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. Notably, the association between French fries and diabetes risk was strongest when consumption occurred 12-20 years before diagnosis, suggesting long-term dietary patterns matter.

The increased risk from French fries was most pronounced in individuals with higher body mass index (BMI) and among White participants, and this effect modification was statistically significant.

French fries drive diabetes risk, but other potatoes do not

The study found that three servings weekly of French fries increased the risk of developing T2D by 20%, with about half of this risk explained by weight gain over time. The hazard ratio for three servings per week of French fries was 1.20 (95% CI 1.12 to 1.28). Baked, boiled, and mashed potatoes and potato/corn chips were not significantly associated with T2D risk.

For baked, boiled, or mashed potatoes, the hazard ratio for three servings per week was 1.01 (95% CI 0.98 to 1.05), indicating no significant association. 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. Conversely, replacing potatoes, especially baked, boiled, or mashed, with white rice was associated with higher T2D risk. No significant association was found between potato or corn chips and T2D risk.

The lack of association for chips was consistent across all analytical models.

Meta-analysis confirms findings across populations

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.

Implications for dietary guidance and diabetes prevention

"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."

Other Harvard Chan co-authors included Xiao Gu, Hala AlEssa, Qi Sun, Frank Hu, JoAnn Manson, and Eric Rimm.

Study limitations and funding sources

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. As an observational study, the authors note the findings cannot prove causality, and the substitution analyses are modeled rather than randomized. The robustness of the findings was confirmed across sensitivity and subgroup analyses.