Small studies have found higher levels of HDL cholesterol after people drank cranberry juice. The new study however examines blood vessel health in pigs that are bred to develop high blood cholesterol and then atherosclerosis, the thickening and hardening of arteries, by eight months of age.

This chain of events is similar to disease progression in humans. The study results suggest that the fruit not only reduces the risk of developing heart disease by improving cholesterol levels but may also help people who already have atherosclerosis and therefore at greater risk of stroke and heart attack.

"Since the abnormal functioning of blood vessels is an important component of heart disease, finding ways to improve vascular function in patients with high cholesterol and atherosclerosis is critical to helping protect these patients from consequences such as heart attack or stroke," said lead researcher Kris Kruse-Elliott from the University of Wisconsin-Madison School of Veterinary Medicine.

Speaking at the annual congress of the International Union of Physiological Sciences, running concurrently with Experimental Biology in San Diego this week, she described the effects of feeding familial hypercholesterolemic (FH) pigs with a daily supplement of 150g cranberry juice powder per kg weight. The blood vessels in these pigs do not function normally.

By the end of six months, these pigs' blood vessels acted more like normal pigs, said Kruse-Elliott. Another group that did not get cranberry juice powder had "significantly less vascular relaxation" than either normal or cranberry-fed pigs.

Kruse-Elliott said the next steps are "to determine what specific components of cranberries are most important to the improvements in vascular function that we observed, exactly how they modify blood vessel relaxation, and how they can be most easily consumed as part of the diet."

Cranberries are increasingly consumed for health reasons as they have been shown to prevent urinary tract infections. They are also thought to play a role in gum disease, ulcers and even cancer.

It is thought that polyphenols in the fruit are responsible for their benefit to the heart.

The researchers noted that humans would need to eat four to eight servings of cranberry powder, or 10-20 servings of cranberry juice, in order to achieve the levels in the current study.

However co-author Jess Reed said: “The point to be emphasized is that total polyphenol intake is very low in western diets and a diet rich in polyphenols would in fact give a daily intake that is equivalent to the levels in our cranberry feeding experiments."

Atherosclerosis is responsible for more than half the deaths in developed countries. Nearly one in three people in the world die from heart disease each year, according to the World Health Organisation (WHO).

The scientists are planning to carry out tests used on humans in the pigs to further explore the cranberry-vessel function link. These include flow-mediated vasodilation tests using ultrasound to measure the change in size of the blood vessels and in flow rate and measurement of C-reactive protein, considered a predictor of cardiovascular disease.

"Furthermore, we want to correlate all those findings with LDL (levels), which should yield important physiological results as well as further validating the FH model," added Kruse-Elliott.