Just a couple of servings of potato a day can reduce blood pressure in obese or overweight people with high blood pressure, calling into question the lowly spud’s current reputation as a fattening, unhealthy food, according to new research presented at a conference in Denver, Colorado, USA, on Wednesday.

“The potato, more than perhaps any other vegetable, has an undeserved bad reputation that has led many health-conscious people to ban them from their diet,” said Dr Joe Vinson, a professor in the Department of Chemistry at the University of Scranton in Pennsylvania, who led the research that was presented at the 242nd National Meeting & Exposition of the American Chemical Society (ACS).

But before you go out and order French fries, the preferred way of eating this most consumed of all vegetables in the US, consider this: the research was done with purple potatoes cooked in the microwave oven, with no oil or fat whatsoever.

Vinson said when you mention “potato” what generally comes to mind is “fattening, high-carbs, empty calories“. But that is because of the way this humble vegetable is usually prepared.

“In reality, when prepared without frying and served without butter, margarine or sour cream, one potato has only 110 calories and dozens of healthful phytochemicals and vitamins,” said Vinson, adding that he and his colleagues hope their study will help to remake the potato’s popular nutritional image.

For their study, Vinson and colleagues first did before and after tests of blood and urine of a group of healthy participants who ate either 6 to 8 small purple potatoes cooked in a microwave oven or the equivalent amount of starch in the form of biscuits. The results showed that the antioxidant capacity of their blood and urine was increased by the potatoes and lowered by the biscuits.

Then they recruited 18 mostly overweight and obese patients with high blood pressure (their average BMI was 29) and invited them to take part in a crossover study. For 4 weeks, they ate either 6 to 8 purple potatoes twice a day or no potatoes in their normal diet, and then crossed over to the other regimen for another 4 weeks. The potatoes were each about the size of a golf ball and eaten with skins on.

They found that eating potatoes was not accompanied by changes in body weight, blood fats (lipids) or glucose levels (HbA1c), but resulted in lower blood pressure: the diastolic (the lower reading of a conventional blood pressure reading such as 120/80) went down by a significant 4.3%, and systolic by 3.5%. The blood pressure went down in spite of the fact 14 of the 18 participants were also on blood pressure medication.

The researchers concluded that purple potatoes are an effective agent for lowering blood pressure and thereby lower the risk of heart disease and stroke in patients with high blood pressure.

Although the participants in this study ate purple potatoes, which contain high amounts of antioxidants, such as phenolic acids, anthocyanins and carotenoids, Vinson and colleagues believe red-skin and white potatoes would confer similar benefits and are already planning a study that uses them.

Vinson said previous studies have found potatoes contain phytochemicals (chemicals that occur naturally in plants) that have similar effects to ACE-inhibitors, medications used to treat high blood pressure.

He said potatoes contain other phytochemicals in similar amounts as found in broccoli spinach and Brussels sprouts, and these could also be helping to lower blood pressure.

But deep frying potatoes subjects them to such high temperatures that it seems to destroy these healthy chemicals, leaving the consumer with only starch, fat and minerals. Vinson said that just microwaving the raw potatoes seems to be the best way to preserve the antioxidants.

The United States Department of Agriculture Agricultural Research Service (USDA-ARS) State Cooperative Potato Research Program helped pay for the study.

Written by Catharine Paddock PhD