Eating a handful of walnuts could provide near-instant protection from heart disease.
Scientists found"‘significant" improvement in cholesterol levels and blood vessel flexibility, which helps blood flow smoothly, just four hours after people consumed either the shelled nuts or walnut oil.
The research suggests regular consumption would protect against cardiovascular disease in the long term.
"Just a handful could help significantly reduce the risk of heart disease," said Dr Penny Kris-Etherton, professor of nutrition at Penn State University in Pennsylvania.
"Eating shelled walnuts or some walnut oil four times a week will certainly provide very significant benefits."
The study was the first to identify which parts of the walnut provide the health boost, she explained.
The team gave 15 participants with high blood cholesterol levels four treatments – two handfuls of shelled walnuts (85g), six grams of walnut skin, 34g of the nutmeat with the fat removed, or three tablespoons of oil (51g).
They looked at their responses after 30 minutes, one hour, two hours, four hours and six hours.
The researchers found that a one-time consumption of walnut oil – also found in the shelled nuts – improved blood vessel health after four hours.
"Our study showed that the oil found in walnuts can maintain blood vessel function after a meal," said lead author Claire Berryman, a graduate student in nutritional sciences at Penn State.
The walnut oil was particularly good at preserving the function of endothelial cells. Endothelial cells, which line the blood vessels throughout the body, play an important part in blood vessel flexibility.
According to the researchers, walnuts contain omega-3 fats, plant sterols known to lower cholesterol, and vitamin E, all of which may help explain their protective effect.
Miss Berryman added: "Implications of this finding could mean improved dietary strategies to fight heart disease."
Heart and circulatory diseases cause more than a quarter of all fatalities in Britain, or more than 159,000 deaths a year.
Victoria Taylor, senior dietitian at the British Heart Foundation, said: ‘Nuts can be a nutritious choice as they provide us with protein and minerals.
However, nuts are also high in fat. Portion control is important to make sure you’re benefiting from the nutrients without adding extra calories.
"Walnuts do contain omega-3 fats. However, the best source of omega-3 fat is oily fish and we don’t yet know for certain if walnuts bring the same benefits."