Studies show almost one-third of American kids take nutritional supplements, the most popular of which include multivitamins, fish oil, melatonin and probiotics. But that concerns some experts, who cite a lack of evidence about the benefits and even the possible risk of damage to vital organs like the heart and liver.

"There is yet to be a study showing a benefit in a normal child to multivitamins" and other supplements, said CBS News medical contributor Dr. David Agus. "People take them to build muscle, people take them for brain development, people take them because they think they're nutritionally deficient. Parents say, 'My child isn't eating that well, let me give them a multivitamin.' Yet there is no data to support that, and in large concentrations these pills can be harmful."

There are a number of potential consequences, Agus said, including damage to the heart and the liver. Taking probiotics can impact normal gut bacteria, he added, and muscle stimulants can lead to cardiac arrhythmias (heartbeat irregularities).

"Parents [and] doctors have to stand up and say, 'Hey, eat real food.' A pill isn't going to make you do better on a test. ... A pill won't help you make the varsity team. Food will, exercise will, studying will," he said.

For kids nervous about sports tryouts as school approaches, Agus said to avoid supplements for muscle building, energy, and weight gain. "They don't work!" he said. "Clearly, taking a pill isn't going to help them."

What will help is consistent sleep. "Going to bed [at] the same time and getting up [at] the same time actually improves athletic performance," he said.

"You have to push your child," he added. "There's not a pill to get around the work it takes to succeed."

If parents want to seek medical advice about nutritional supplements for their child, Agus advised that they "be really open and honest with the doctor about what supplements their child is taking. ... Ask the doctor — say, 'Is there a benefit to my child?' In almost all cases, the answer is going to be no."

He also cautioned against children taking supplements like melatonin to help them sleep. "Melatonin hasn't been studied long-term in children," he said. "It affects the endocrine system, it affects muscles, it affects the immune system. We have to stop."

The Council for Responsible Nutrition, which represents the supplement industry, told CBS News that it "recommends parents talk to their children's pediatrician about their children's diets and whether a multivitamin or other dietary supplement can help."