The human body may only be able to effectively process a certain amount of fruits and vegetables every day, limiting its ability to absorb important nutrients from extra helpings, said the review's senior author Dr. Frank Hu, a professor of nutrition and epidemiology at the Harvard School of Public Health in Boston.
"It is possible that the digestibility of fruits and vegetables and the availability of nutrients and other bioactive compounds of these foods may have reached a plateau at five servings per day for most people," Hu said. "More research is clearly needed in this area."
Before you reach that five servings a day recommendation, however, the review suggests that the risk of death from any cause drops 5 percent for each additional daily serving of fruits or vegetables consumed. And, the risk of death from heart disease seems to decrease 4 percent for each additional daily serving of fruits and vegetables, according to Hu's research.
However, the study is only designed to show an association between fruit and vegetable consumption and the risk of death during the study periods. It isn't able to prove that eating fruits and vegetables is the cause of the reduced risk.
Still, the research supports dietary guidelines that recommend five servings of fruit and vegetables a day, said Joy Dubost, a registered dietitian in the Washington, D.C., area and a spokeswoman for the Academy of Nutrition and Dietetics.
Dubost said it's pointless to worry about an upper limit at which people gain little or no benefit from fruits or vegetables, given that it's so hard to get people to eat their veggies anyway.
She noted that adults eating an average 2,000-calorie daily diet should eat 2.5 cups of vegetables every day, but instead eat an average 1.6 cups. The same goes for fruit -- adults should eat 2 cups a day, but on average can manage only 1 cup.
"People in general don't even consume the recommended amount of fruits and vegetables," she said. "Instead of focusing on 'is consuming too much not providing additional benefit,' we should be talking about how to get people to eat the recommended amount, given that there are proven health benefits shown in this very study."
For the review, a team of researchers in China and the United States analyzed the results of 16 studies to gain better understanding of the association between fruits and veggies and a person's risk of death. These studies involved more than 830,000 people in total, and more than 56,000 deaths.
The investigators found a link between eating fruits and vegetables and a lower risk of death overall during the study follow-up periods, as well as a reduced risk of death from heart disease. However, fruits and vegetables did not seem to have any effect on a person's specific risk of death from cancer.
Fruits and vegetables contain a broad variety of essential vitamins, minerals and nutrients but not a load of calories, Hu and Dubost said. Chowing down on them allows a person to get much of the nutrition they need without risking weight gain, they noted.
The nutrients found in fruits and vegetables have been associated with reduced risk of heart disease, stroke, diabetes, high blood pressure and other chronic diseases, Hu said.
Fruits and vegetables also are a major source of dietary fiber. "Fiber obviously helps with a healthier gut and moves things through the system," Dubost said. "It also can help with weight loss and reduce heart disease."
What about vegetarians? Even if the benefits of fruits and vegetables level off after five daily servings, that does not necessarily mean that vegetarians are less healthy than people who eat meat, both Hu and Dubost said.
Hu noted that in reviewing the benefits of a vegetarian diet, you also have to consider the health benefits of decreased consumption of meat -- something his study did not review.
"The potential of vegetarian diets could be due to a combination of both," he said.
The report was published online July 29 in BMJ.
For more information on daily nutrition, visit the U.S. Department of Agriculture.