Kidney stones — a painful condition usually thought to plague people in middle age — are on the rise in children, due in part to a poor diet, physical inactivity, and rising rates of childhood obesity.

"I am seeing more and more children who have kidney stones," Dr. Gary J. Faerber, a urologist with the University of Michigan Health System, Ann Arbor, noted in a university-issued statement.

"The best way to prevent kidney stones is to encourage drinking plenty of water," Faerber told Reuters Health. "I encourage that children drink enough water to keep the urine clear. If the urine is yellow, that is an indication of the urine being concentrated, which increases the risk of precipitation of mineral crystals, which then form into stones."

It's especially important, Faerber said, to stay hydrated during the hot summer months, "and have a glass of water before bedtime, as nighttime is also a time of relative dehydration."

"You can add lemonade or orange juice because they contain citrate, which is a chemical that helps to prevent stone formation," Faerber said. "Just make sure they see their dentist on a regular basis since lemonade and orange juice can affect teeth enamel."

Skip the soft drinks, though, Faerber advises. Kids don't need the sugar and, in the case of dark colas, they don't need the extra oxalate, which can fuel stone formation. In some cases, children may need to reduce intake of other foods high in oxalate like teas, chocolate, nuts and dark green leafy vegetables, he added.

It's also wise, Faerber said, to avoid high-sodium, processed or fast foods and get plenty of exercise. "Childhood obesity is most likely a strong risk factor for stone formation just as obesity is in adults."

There is probably not a single primary cause for kidney stones in children. "Like adults, it's multifactorial," Faerber noted. "Certainly there are risk factors that are well known." For example, 50 percent to 60 percent of children with kidney stones have a family history of kidney stones, and up to 75 percent of them have urine abnormalities.

Children with kidney stones often have different signs and symptoms of kidney stones than adults. Adults typically have severe flank pain, which can radiate into the groin region. In children, the symptoms are less clear — " vague abdominal pain, belly button or back pain"— and they often have nausea and vomiting.

"It's not at all uncommon that children are misdiagnosed as having gastroenteritis, or appendicitis," Faerber explained.

No one really knows what the long-term effects of kidney stones in children are, other than the fact that kidney stones can be painful, Faerber noted. "Research is greatly needed to further understand why certain children are at risk, what treatments are the most effective in treating stones in children, and what medical and dietary modifications are most effective in stone prevention," he said.