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Eating three servings of French fries per week could raise your risk of developing type 2 diabetes by 20 per cent, a new study has found.

But potato lovers need not despair. People who ate similar amounts of baked, boiled, or mashed potatoes were not at significantly higher risk of developing the chronic condition, according to the study published in The BMJ.

“The association between higher potato intake and increased [type 2 diabetes] risk is primarily driven by intake of French fries,” the Harvard University-led team said.

Potatoes contain nutrients such as fibre, potassium, and vitamin C – but they also have a lot of starch, which is a type of carbohydrate that breaks down into energy-providing glucose.

Potatoes have been linked to a higher risk of type 2 diabetes, high blood pressure, and weight gain.

Yet the latest findings underscore that how we prepare our food matters quite a lot when it comes to our health.

“In nutrition it is not only what we eat that matters, but also what we stop eating when we do so,” Diana Díaz Rizzolo, a lecturer at the Open University of Catalonia – Barcelona, said in a statement.

The study included more than 205,000 people in the United States, with researchers tracking their diets over more than three decades. Over that time, more than 22,000 people were diagnosed with type 2 diabetes.

Compared with people who rarely ate potatoes, those who ate at least seven servings of potatoes per week had a 12 per cent higher risk of developing type 2 diabetes, the study found.

But people who ate French fries three times per week were at a 20 per cent higher risk of diabetes compared with people who rarely ate them. At seven servings per week, the risk rose to 27 per cent.

The findings suggest people should prioritise whole grains and limit the amount of fried foods they eat, “even when it comes to options that may initially seem ‘healthy’ such as potatoes,” said Cristóbal Morales, a specialist in endocrinology and nutrition at Vithas Sevilla Hospital in Spain who was not involved with the study.

What the study means for healthy diets

The study has some limitations. Overall, people who ate more potatoes tended to be less active, have less nutritious diets, and to eat more calories. Their diets were heavy in red meat, eggs, refined grains, and sugary drinks.

The researchers tried to take lifestyle and demographics into account, but the results could still be skewed as a result of these factors.

The report also does not establish that eating French fries actually causes type 2 diabetes – just that the two are related.

The findings add to the scientific debate over potatoes’ impact on our health. A similarly designed study from 2015 found that eating more potatoes was tied to a higher diabetes risk, but it did not differentiate between French fries and boiled, baked, or mashed potatoes.

And in 2019, an expert panel recommended that people limit their potato intake to around 350 grams per week – roughly one large Russet potato.

But other researchers have noted that potatoes are typically just one item on the plate – meaning they shouldn’t get all the blame. Americans, for example, tend to pair potatoes with red or processed meat, which have also been linked to type 2 diabetes and other health issues.

The latest findings reinforce the idea “that we should not demonise whole foods without considering how they are prepared, what they are accompanied by, or what they are replaced with,” said Díaz Rizzolo, who was not involved with the study.

Independent experts said people looking to maintain a healthy diet should consider both how potatoes are cooked and the alternatives on the table.

For example, French fries are often ultra-processed, and these types of foods have been linked to poor health. Meanwhile, baked, boiled, or mashed potatoes are typically minimally processed.

Overall, Morales said the study has “a clear message: preventing type 2 diabetes starts with adopting and maintaining healthy lifestyles and nutrition”.