Snacking on one sweet fruit could prevent a heart attack or stroke, according to research. The 'superfood' is rich in chemicals that boost gut bacteria and lower cholesterol.

A study found just four weeks of eating grapes did the trick for people who were eating few vegetables. The simple lifestyle change may reduce the risk of cardiovascular disease - the world's number one killer.

Grapes are rich in powerful antioxidants that dampen inflammation. Lead author Professor Zhaoping Li, of the University of California, Los Angeles, said: "Grapes increase diversity of gut bacteria and reduce cholesterol."

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His team tracked people aged 21 to 55 who consumed the equivalent of about 40 grapes, or two servings, a day. They were on a diet otherwise low in fibre and plant chemicals, or polyphenols, before and during the trial. Within a month, the diversity of their microbiome was higher. This is the community of trillions of bugs essential for good health.

They had almost eight per cent less 'bad' cholesterol. Bile acids, which fuel them, fell by more than 40 per cent. The harmful fats can lead to clots that block vessels, cutting off blood to the heart or brain.

Among the beneficial bacteria that increased was Akkermansia. It burns up sugar and cholesterol, and strengthens the lining of the intestines. Prof Li said: "We found grapes have a beneficial effect on gut bacteria, which is great news, since a healthy gut is critical to good health."

Prof Li said: "This study deepens our knowledge and expands the range of health benefits for grapes. It reinforces the heart health benefits of grapes, which include lowering cholesterol."

Previous research has linked eating grapes with combating middle age spread - and boosting libido. Prof Li added: "Grapes provide a rich source of polyphenols and fibres. In summary, our results provide novel preliminary information about the effects of grape intake on the gut microbiome, host cholesterol and bile acid metabolism.

"These data will assist in the future design of studies to explore the health benefits of grape consumption."

Cardiovascular disease kills an estimated 17 million people across the world each year - accounting for a third of all deaths. It claims more than 160,000 lives in the UK, annually. The study was published in Nutrients.

Another study found grapes 'flush out' refined fats and sugars in processed foods and regular consumption combats the effects of a junk food diet. Dr John Pezzuto, of Western New England University in the US, said: "It adds an entirely new dimension to the old saying 'you are what you eat.'"

In a series of experiments, mice gorged on a high fat diet typically consumed in western countries . They also received a daily powdered grape supplement equivalent to a punnet. The lab rodents had less fatty liver - and lived longer than peers.

Dr Pezzuto said: "The change observed in the study would correspond to an additional four to five years in the life of a human."

The team also found grapes improve the function of neurons - protecting against Alzheimer's. They had positive effects on behaviour and cognition that were impaired in mice fed high fat diets without the extract. A third study showed in addition to the other qualities, grapes also help burn up calories by increasing metabolism.

In particular, they may help prevent Alzheimer's and non alcoholic fatty liver disease (NAFLD), both of which have been linked to eating junk foods. Dr Pezzuto said: "The adverse response was diminished by grape intervention. Life-long survival was also enhanced.

"These results suggest the potential of dietary grapes to modulate gene expression, prevent oxidative damage, induce fatty acid metabolism, ameliorate NAFLD and increase longevity when co-administered with a high-fat diet."

Liver disease is a growing problem across the world, because of unhealthy eating habits. Added Dr Pezzuto: "The life-long addition of grapes to a high-fat diet increases longevity. These data illustrate the extraordinary influence of nutrigenomics, a burgeoning field of investigation that will augment our appreciation of diet and health."

The findings were published in the journals Foods, Antioxidants and Food & Nutrition.