New Delhi, Aug. 31: Potatoes when cooked the right way might reduce blood pressure and protect overweight people from heart disease and stroke, new research suggests, challenging health wisdom about this popular tuber vegetable.

But scientists caution that French fries are likely to cancel any such benefits from potatoes and that it is still unclear how much health virtues are loaded in the most commonly consumed white and yellow potatoes.

Researchers at Scranton University in the US have shown two servings a day of pigmented purple potatoes can lower the blood pressure readings of obese people with a history of hypertension, or high blood pressure. The study, described as the first to investigate the use of potatoes to combat hypertension, was presented today at a meeting of the American Chemical Society.

“The other remarkable result in our study is that there was no weight gain from these two servings of potato per day,” said Joe Vinson, the study’s principal investigator and professor of chemistry at Scranton University.

Nearly five years ago, US Agricultural Research Service scientists had found that the skin and flesh of potatoes contain many beneficial phytochemicals, including folic acid, vitamin C, and phenolics such as flavonoids. Some of these phytochemicals serve as anti-oxidants, helping neutralise biologically nasty molecules called free-radicals implicated in many human diseases.

In an initial experiment, Vinson and his colleagues observed that healthy people who ate six to eight small purple microwaved potatoes showed an increase in anti-oxidant levels in their blood samples. The scientists then gave two servings of potatoes to 18 obese people with a history of hypertension and found that diastolic blood pressure dropped 4mm (about 4 per cent), and the systolic blood pressure reduced by 3.5 per cent.

The study microwaved potatoes, but researchers believe boiled or baked potatoes without added fats would be nutritionally similar. A potato served without butter or margarine delivers only 110 calories. “You can microwave a potato until hot (and soft) enough for personal taste,” Vinson told The Telegraph.

The researchers now plan to launch a study comparing white and purple potatoes. Studies have shown differences in anti-oxidant levels between and purple fleshed potatoes. “Regular (white or yellow) potatoes have no anthocyanins and only one-fifth the amount of chlorogenic acid, an antioxidant responsible for coffee’s action on reducing blood pressure,” Vinson said.

“But white and yellow potatoes also have other phytochemicals,” said Roy Navarre, a geneticist at the US Agricultural Research Service who was among the scientists who had identified phytochemicals in potatoes. “White and yellow potatoes also have a compound called kukoamine that has been shown to be an ingredient of a Chinese medicinal plant used in the treatment of high blood pressure,” Navarre told The Telegraph.

Independent nutrition specialists say the findings need to be demonstrated through larger, rigorous studies. “Many fruits and vegetables have phytochemicals — but how much benefit can a specific fruit or a vegetable deliver needs to be established through large and long studies,” said Kamala Krishnaswamy, former director of India’s National Institute of Nutrition, Hyderabad.

“All we can recommend for now is that people should consume 500 grams of fruits and vegetables per day to maintain a good anti-oxidant status,” said Krishnaswamy who has herself studied the anti-oxidant benefits of turmeric.

Vinson has in the past investigated the health virtues of some of the world’s most popular food and drinks — wine, tea, even chocolates. In one study about a decade ago, Vinson showed that red wine’s benefits can be attributed to polyphenols.

In another study published in the American Journal of Clinical Nutrition 10 years ago, Vinson and his colleagues showed that dark chocolate may reduce cardiovascular risk by modestly reducing bad cholesterol oxidation and increasing good cholesterol.