There is some evidence from observational studies that intake of potatoes, particularly French fries (chips) is associated with increased risk of T2D (1-6). However, observational studies (where researchers just observe the effect of a risk factor rather than trying to change who is or isn’t exposed to it) can only show associations, not cause and effect, and it is not possible to exclude confounding by other variables. Importantly, differences in cooking methods (like frying) and what’s added at the table (e.g. added salt, fat or sauces) are not always taken into account in these studies. Eating fries is likely to be associated with other unhealthy behaviours, and these factors (rather than consumption of fries themselves) could influence any observed association between potato intake and health outcomes. For example, people who consume fries may also be less likely to eat a healthier diet overall, have a higher body weight, be more likely to smoke, drink alcohol and sugar sweetened beverages, and/or have lower physical activity levels. Such associations must therefore be interpreted with caution, and the evidence to date does not support a clear recommendation to reduce total potato intake, with the possible exception of regular consumption of fried chips.
We should also consider that potatoes are rarely eaten on their own and it is hard to fully capture the complexity of what we eat, especially over many years. Increasingly the importance of dietary patterns is recognised as important in health, rather than focussing on specific foods. Healthy dietary patterns higher in fibre, vegetables, fruits, and whole grains and lower in refined grains, and sugar-sweetened foods and beverages, help reduce the risk of developing T2D7. The evidence perhaps suggests that frying potatoes are not the healthiest way to eat potatoes, and potatoes like other starchy carbohydrates, should be eaten in a healthy, balanced diet. If you choose to eat potatoes, try to opt for healthier options such as limiting deep-fried versions, serving them in skins, limiting addition of salt or sauces high in saturated fat, and boiling, baking, mashing or roasting with only a small amount of fat or oil8.
Like any other carbohydrate-containing food or drink, potatoes increase blood sugar levels. One of the mechanisms suggested to explain the association between potatoes and T2D risk, is therefore its glycaemic index (GI) (the measurement of how quickly glucose from carbohydrate-containing foods gets into the bloodstream after consumption). Potatoes can be a high GI food and a diet containing more foods that are higher in GI is associated with a greater risk of T2D. However, using GI only can be problematic, as it does not consider the portion size. The glycaemic load, that accounts for the amount of available carbohydrate in a portion, has been suggested as a better measure. This is because the amount of carbohydrate you eat has a bigger effect on blood glucose levels than GI alone. In addition, potatoes are typically eaten as part of a meal, and what they are combined with will change the glycaemic impact overall. For example, fat lowers the GI of a food and crisps will actually have a lower GI than potatoes cooked without fat. So a lower GI alone doesn’t necessarily determine whether a food is a healthy choice. If you do want to lower your GI then use new potatoes instead of old potatoes, or try sweet potatoes for a change9.