Researchers have come up with a new way of getting kids to follow a healthy diet: putting more vegetables on their plate.

Larger portions of veggies resulted in kids chomping down 68 percent more of them on average (an extra 21 grams, or 0.74 ounces per day), in a 4-week experiment involving 67 children aged from 3 to 5.

The research team used broccoli and corn as their test vegetables, doubling the amount served – from 60 grams to 120 grams – to see how this would change the eating behavior of the children.

"The increase we observed is equal to about one-third of a serving or 12 percent of the daily recommended intake for young children," says nutritional scientist Hanim Diktas from Pennsylvania State University.

"Using this strategy may be useful to parents, caregivers, and teachers who are trying to encourage kids to eat the recommended amount of vegetables throughout the day."

Similar effects have also been noticed in adults, but it's not quite as simple as shoveling more veg on top of every meal.

The types of vegetables, the foods they're served with, and whether or not the portion sizes for other foods are increased seem to make a difference.

In this study, the portion sizes of the other food on the plates remained unchanged.

In this case, the researchers also tried adding butter and salt in some of the tests. However, this didn't make a significant difference to how many vegetables the kids ate, irrespective of portion size. Portion size was a much more effective strategy for improving their diet.

This may be because the veggies were pretty well-liked even without butter and salt: 76 percent of the kids rated the vegetables as "just okay" or even "yummy", and the researchers say food choice is important. Fish sticks, rice, applesauce, and milk were served alongside the broccoli and corn.

"We chose foods that were generally well-liked but also not the kids' favorite foods," says nutritional scientist Barbara Rolls, also from Pennsylvania State University.

"If you offer vegetables alongside, say, chicken nuggets you might be disappointed."

"Food pairings are something you need to be conscious of, because how palpable the vegetables are compared to the other foods on the plate is going to affect the response to portion size. You need to make sure your vegetables taste pretty good compared to the other foods."

In other words, there would probably be different results with different vegetables, perhaps ones that weren't quite as well-liked.

The researchers are keen to run further tests on how different portion sizes and even food substitutes might work.

There's also going to be a tipping point where larger portions end up leaving more leftover food; the trick can't just be extended up to an unlimited portion size.

This is something else the team behind the study wants to look at in more detail.

With most kids not getting enough vegetables – US administrators recommend about 1.5 cups per day – any potential strategy to improve this is going to be welcome.

The researchers say there are lots of variables that parents and schools can play around with.

"It's important to serve your kids a lot of vegetables, but it's also important to serve them ones they like because they have to compete with the other foods on the plate," says Rolls.

"Parents can ease into this by gradually exposing kids to new vegetables, cooking them in a way their child enjoys, and experimenting with different flavors and seasonings as you familiarize them."

The research has been published in Appetite.