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Customers buy vegetables at a market in central Beijing, July 9, 2012.
Still, Dr. Barbara Rolls of Pennsylvania State University, who co-authored the study, told Reuters Health, "it's exciting to show that you can use variety to increase the intake of healthy foods, find your favorites and enjoy."
During the new four-week study, Rolls and her team looked at what people ate during a weekly lunch of vegetables and pasta.
For the first three meals, 66 study participants were offered 600 grams of pasta and tomato sauce, plus 600 grams of either broccoli, carrots or snap peas. The fourth meal included pasta, sauce and 200 grams of each of the three vegetables.
The findings, published in the Journal of the Academy of Nutrition and Dietetics, showed that people ate 48 grams more vegetables, on average, when they were offered a variety, compared to a single serving of just one type.
However, eating more vegetables didn't affect the amount of pasta that people ate, or reduce the overall amount of calories that they took in, researchers said in their report.
According to the researchers, previous studies have shown that people need to eat at least 150 grams more vegetables before they start cutting back on other foods they enjoy.
Women ate anywhere from 164 to 203 grams of a particular vegetable when they had only that vegetable. That jumped to 232 grams when they ate the varied meal.
As Roll's team wanted to encourage people to eat more vegetables without piling on the pounds, they kept a close eye on mealtime calories.
"We were giving large portions," said Rolls of the meals, which added up to as much as 1,400 calories. "We didn't want them to be limited in the amounts they were eating."
The broccoli was seasoned with a little butter, and Rolls said that adding flavoring and seasoning to vegetables is fine. But she cautions that "you can take in too many calories from vegetables if they're fried or in rich sauces."
Carrots had the lowest calorie content. So when men ate the carrot-based meal they took in 687 calories, on average. That increased to 772 calories when they ate all three vegetables.
Women ate 582 calories when offered just carrots and pasta, and 604 calories, on average, during the varied meal.
Those differences were likely due to chance, the researchers said.
Variety The Spice?
The study suggests that the same psychology that may lead obese people to eat more could also lead them to eat less, said Dr. Leonard Epstein, who studies eating behaviour at the University of Buffalo.
"Variety is a potent stimulus for eating, and a lot of research suggests that obese people eat a wider variety of foods than lean people," Epstein told Reuters Health by email.
But usually they are eating a variety of less healthy foods, and eating more calories. Replacing those with vegetables can shift the balance of the diet, said Epstein, who wasn't involved in the study.
According to Rolls, who designed the Volumetrics Eating Plan - a weight loss plan that encourages people to fill up on low calorie, nutrient-dense foods - people will eat less overall if they fill up on veggies first. "If you eat enough, you might not eat so much of the higher calorie foods."
"The main thing is that most people need to realise they're probably not eating the amount of vegetables that would be good for them so they should try to find veggies they like and keep a variety on hand," said Rolls.
The United States Department of Agriculture healthy eating campaign, "MyPlate," recommends that people fill half their plate with fruits and vegetables. They also suggest eating from a smaller plate.