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Since the 1970's, when the Happy Meal was developed by a Midwestern advertising executive, McDonald's has made incentivizing food choices something of an art form. The little box of fast-food calories with a little toy surprise has proven to be so appealing to children that in 2010 San Francisco passed a law prohibiting toys from being sold with meals that fail to meet certain nutritional standards.

Now, researchers are looking at ways of using a similar model to get kids to choose fruits and vegetables with their school lunches.

Childhood obesity is a growing problem that currently affects about one third of American kids, and for years, public health officials -- first lady Michelle Obama among them -- have been arguing in favor of making school lunches healthier.

In partnership with Cincinnati Public Schools, researchers at the Cincinnati Children's Hospital Medical Center have piloted a program that made a relatively small adjustment to the lunch program at one low-income public elementary school.

"The components are the entree with whole grain, fruits, vegetables, a salad bar and milk, which could either be flavored or plain low fat milk," said Robert Siegel, a professor of pediatrics at Cincinnati Children's Hospital Medical Center. "The problem is the students don't necessarily select the best combination of those, and they often select chocolate milk."

Chocolate milk is often high in sugar and can be no better than soda in terms of healthfulness.

"We came up with the idea of the power plate: plain milk, main dish or entree, fresh fruit, fresh vegetables or salad," said Siegel, who will present the program's findings at the European Child Obesity Group Congress in Austria on Saturday.

Students had the choice of between three and five options, and if they chose the options that constituted the "power plate," they received a small prize. It could be a pencil or a water-based tattoo or a sticker -- all very inexpensive items.

The of the 297 children at the school, almost everybody -- about 99 percent -- participated in the free and reduced lunch program; very few brought lunches from home.

At the onset of the program, fewer than 10 percent of kids voluntarily chose the components of the "power plate."

But when they were offered prizes, about 42 percent picked up the healthier options. Even when the prizes were removed altogether, 36 percent continued to choose healthier options.

High-quality prizes were more effective at incentivizing kids to make the healthier choices. But "low quality," less expensive prizes like tattoos and stickers still worked -- even when they weren't offered every day.

"From a cost-benefit perspective, our conclusion is that the best way to carry this on is to do tattoos and stickers twice a week if you're on a budget," said Siegel, who was the lead author on the program's accompanying study.

It's important to note that the study didn't look at whether the children actually ate the food they picked up. And as most parents know, kids can be crafty in pursuit of rewards.

Students also weren't asked to choose between unhealthy foods and healthy foods -- they simply were asked to take additional healthy foods that they might have otherwise skipped.

The study's limited duration -- two months -- doesn't tell us anything about whether the effect of the prizes lasts.

In subsequent programs, Siegel said, the researchers hope to look more closely at those issues. But the results of the pilot program suggest that children can receive a critical introduction to healthy foods at school at a very low cost to the system.

"We look at this as the first step," Siegel said. "Getting the kids to select. Even at home, getting them to look at it or maybe even touch it is the first step. We are going to get consumption data in our follow up studies."

"You can make the argument that by making the kids make a selection, you're teaching them to select healthy foods," he added.