Compared to those who consumed two servings of fruit and vegetables per day, participants who consumed five servings a day of fruits and vegetable had a 13 per cent lower risk of death from all causes; a 12 per cent lower risk of death from cardiovascular disease, including heart disease and stroke; a 10 per cent lower risk of death from cancer; and a 35 per cent lower risk of death from respiratory disease, such as chronic obstructive pulmonary disease (COPD).

The study identifies an optimal intake level of fruits and vegetables and supports the evidence-based, succinct public health message of '5-a-day,' meaning people should ideally consume five servings of fruit and vegetable each day.

"While groups like the American Heart Association recommend four to five servings each of fruits and vegetables daily, consumers likely get inconsistent messages about what defines optimal daily intake of fruits and vegetables such as the recommended amount, and which foods to include and avoid," said lead study author Dong D Wang, a member of the medical faculty at Harvard Medical School.

Wang and colleagues analysed data from two studies including more than 100,000 adults who were followed for up to 30 years.

For this analysis, researchers also pooled data on fruit and vegetable intake and death from 26 studies that included about 1.9 million participants from 29 countries and territories in North and South America, Europe, Asia, Africa and Australia.

Analysis of all studies revealed that intake of about five servings of fruits and vegetables daily was associated with the lowest risk of death.

"Eating more than five servings was not associated with additional benefit. Eating about two servings daily of fruits and three servings daily of vegetables was associated with the greatest longevity," said the study that appeared in American Heart Association's flagship journal Circulation.

Not all foods that one might consider to be fruits and vegetables offered the same benefits.

For example: Starchy vegetables, such as peas and corn, fruit juices and potatoes were not associated with reduced risk of death from all causes or specific chronic diseases.

On the other hand, green leafy vegetables, including spinach, lettuce and kale, and fruit and vegetables rich in beta carotene and vitamin C, such as citrus fruits, berries and carrots, showed benefits.

"Our analysis in the two cohorts of US men and women yielded results similar to those from 26 cohorts around the world, which supports the biological plausibility of our findings and suggests these findings can be applied to broader populations," Wang said.