New U.S. research has found that the number of vegetables included in children's packed school lunches increases when children are involved in deciding which foods to pack.

Carried out by researchers at the University of Illinois, the new study looked at 90 pairs of parents and children, who all took home-packed lunches to school every day during the study.

Every day before the school period the researchers met with each child individually, unpacked their lunch, and made a note of the contents.

They found that nearly half of the lunches rarely or never included vegetables, and even when vegetables were included they usually made up just one-third to one-half of the recommended half-cup serving.

However, the good news is there may be an easy way of increasing the amount of veggies children take to school, and it's simply to get them more involved in packing the lunches.

"When the child was more involved in deciding what to pack, their lunches contained more fruits and vegetables across the week and additional servings of vegetables on Mondays," said lead author Carolyn Sutter. "Having the child help decide what they'll eat for lunch may allow the parent and child to work together to choose fruits and vegetables the child is interested in eating."

"Parenting practices that provide structure and support the child's growing autonomy in dietary behavior have been found to be the most beneficial for promoting healthy habit development," Sutter said. "These parents may create guidelines and limits around what their child can pack in their lunch, such as requiring them to include a vegetable some days, but also be responsive to their child's needs and dietary preferences."

The findings, published in the journal Appetite, also showed that children and their parents were also significantly better at including fruits, with kids taking around a full half-cup serving nearly every day.

About one-third of the families packed a fruit in the child's lunch every day during the study.

Parents who were more knowledgeable about nutrition also included more fruit servings during the entire week and more servings of vegetables on Mondays. However, the number of vegetable servings declined across the week, possibly due to financial constraints and work stressors. For parents with limited resources, offering "nutrition education and suggestions on affordable vegetable options" could "help increase the number of vegetable servings children consume across the week," noted Sutter.