French fries, not boiled or baked potatoes, linked to higher diabetes risk, says BMJ study

What the study found

The study analysed data from three large US cohorts: the Nurses’ Health Study, Nurses’ Health Study II, and the Health Professionals Follow up Study for up to four decades. None had diabetes, cardiovascular disease, or cancer at the start. During more than 5.1 million person years of follow up, 22,299 participants developed Type 2 diabetes.

The researchers assessed potato intake repeatedly over time, which is a strength of the study. They looked separately at total potato intake, French fries and baked, boiled, or mashed potatoes. After adjusting for body mass index, lifestyle and other dietary factors, every three additional servings of total potatoes per week was associated with a 5% higher rate of Type 2 diabetes. However, this association was mainly driven by French fries. Three weekly servings of French fries were associated with a 20% higher rate of Type 2 diabetes. In contrast, baked, boiled or mashed potatoes were not significantly associated with diabetes risk.

How potatoes impact blood sugar

One reason potatoes are often discussed in relation to diabetes is their relatively high glycaemic index (GI), a measure of how quickly a food raises blood glucose levels. However, the GI of potatoes varies depending on the variety, cooking method, serving temperature and what they are eaten with.

Boiled potatoes have a moderate-to-high GI, typically ranging from about 50 to 90. Baked potatoes often have a higher GI, frequently exceeding 80, because cooking makes the starch more readily digestible. Interestingly, potatoes that are cooked and then cooled develop some resistant starch, which can lower their glycaemic impact.

Cooking method also has a major effect on calorie density. A plain boiled potato contains approximately 80–90 kcal per 100 g, while a baked potato provides around 90–100 kcal per 100 g. In contrast, French fries typically contain 280–320 kcal per 100 g, largely because they absorb oil during frying. This means that fries can contain more than three times the calories of boiled potatoes for the same weight of food.

Deep frying at high temperature may also generate compounds such as advanced glycation end products (harmful compounds formed when proteins or fats combine with sugars in the bloodstream). These may contribute to oxidative stress and metabolic dysfunction.

The study also found that body weight appeared to explain a substantial part of the association between French fries and diabetes, suggesting that weight gain is one likely pathway.

Why substitution matters

A key strength of this study is the substitution analysis. Nutrition is not just about removing one food. It is also about what takes its place. Replacing three weekly servings of total potatoes with whole grains was associated with an 8% lower rate of Type 2 diabetes. Replacing French fries with whole grains was associated with a 19% lower rate. On the other hand, replacing potatoes with white rice was associated with a higher diabetes risk. This finding is particularly relevant for populations where polished rice and refined carbohydrates are already major staples.

The authors also performed a meta-analysis of 13 prospective cohorts involving more than 587,000 participants. The broader evidence supported the same pattern. Fried potatoes were associated with higher diabetes risk, while non-fried potatoes showed little or no association.

The practical message is simple. Potatoes need not be removed from the diet. A boiled or baked potato eaten in a balanced meal is very different from frequent servings of French fries. For diabetes prevention, the priority should be to reduce fried and ultra-processed foods, avoid replacing one refined carbohydrate with another, and increase whole grains, legumes, vegetables, and protein-rich foods.

(Dr Bhattacharya is senior consultant, endocrinology, Indraprastha Apollo Hospital, New Delhi)