Five San Antonio elementary schools are placing cameras in cafeteria lines, hoping to learn whether those green beans are going uneaten.

Alarmed by the growing waistlines of children and unsure of how much they actually eat, the U.S. Department of Agriculture is spending $2 million over the next four years to develop a scientific way of determining what ends up in those young bellies.

And although the technology isn't quite ready for prime time, the lead researcher on the project thinks it eventually will be possible not only to know how many bites of mystery meat kids are eating, but get a sophisticated breakdown of calories, nutrients, fats and sugar.

“Diabetes and obesity in children is an enormous problem,” said Dr. Roberto Treviño, inventor of the Bienestar school health program, who was awarded the grant. “If we're going to be able to impact those diseases, we're going to have to understand them better. And nutrition is a big component.”

Treviño demonstrated the technology Wednesday at W.W. White Elementary, one of the San Antonio Independent School District campuses taking part. The others are Brewer, Franklin, Japhet and Woodlawn Hills.

“Healthy diets and lifestyles for children is an issue of great importance to SAISD,” said Superintendent Robert Durón. “We know that poor health can often affect student school attendance, their grades, their test scores and their ability to pay attention in class and stay focused.”

For decades, the USDA has surveyed Americans about what they eat, and shared that information with researchers and policymakers. It would be helpful to have a better method, said Treviño, director of the Social and Health Research Center here.

“Everything is self-reported, and self-reported information is very inaccurate,” Treviño said. “People tell you what you want to hear. Parents aren't going to tell us the truth, that they're feeding the children high (levels of) sugars or fatty food.”

Thus the idea to use sophisticated cameras — two at the cash register, two more at the disposal window — to take before-and-after photos of trays, and develop equally sophisticated software to analyze the results.

Each set of cameras captures an overhead and side view, to better determine food volume. Trays also are weighed to help with the calculations.

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Children who have permission to participate — they'll be anonymous — will have bar codes stuck on their trays to match the before-and-after pictures. Those pictures will be shared with cafeteria mangers, who might use them to tweak the menu.

A few children shyly demonstrated the system, picking up lunches and dropping them off. They were quickly ushered out afterward, making it unclear whether they minded the watchful eye of big brother on their midday meals.

If the technology proves viable, Treviño sees a day when a cellphone app can accurately tell a parent what's in their kid's fast food meal, or a dieter how many calories he or she consumed from the blue plate special.

Mayor Julián Castro, who attended SAISD schools, recalled the joy of enchilada Wednesdays, and the frequent appearance of hamburgers and French fries on the lunch menu.

“Now, as a parent of a 2-year-old child who will be in SAISD in a couple of years, sometimes I think that we as parents think the school cafeteria is taken care of,” said Castro, who has made fighting obesity a priority as mayor. “They're going to have their lunch. But we often don't think enough about what they're actually eating, and what effect what they're eating is having on their health.”