Obese teenagers will see little benefit from taking vitamin D supplements meant to reduce their risks of insulin resistance and heart disease and the practice may actually be bad for them, researchers say.

While some doctors have put their obese teenage patients on large dosages of vitamin D to attempt to slow or reverse those obesity-related health issued, there is little evidence that they do so, says Dr. Seema Kumar, a pediatric endocrinologist at the Mayo Clinic Children's Center in Rochester, Minn.

What they may be doing, she suggests, is increasing levels of cholesterol and fat-harboring triglycerides.

Dr. Kumar has been researching the effects of such supplements in children for a decade and has conducted clinical trials while authoring six studies. 

The supposed benefits of the supplements for obese teens have not been borne out in any of her studies, she says.

"After three months of having vitamin D boosted into the normal range with supplements, these teenagers showed no changes in body weight, body mass index, waistline, blood pressure or blood flow," she explains. "We're not saying the links between vitamin D deficiency and chronic diseases don't exist for children – we just haven't found any yet."

With one in every five American teens classified as obese, and several studies suggesting associations between vitamin D deficiency and a number of weight-related medical complications, Kular's latest study set out to determine if supplements could decrease any medical complications linked with obesity. Her findings were published online Aug. 14 in the journal Pediatric Obesity.

"I have been surprised that we haven't found more health benefit," she says. "We're not saying it's bad to take vitamin D supplements at reasonable doses, and we know most obese teens are vitamin D deficient. We're just saying the jury is still out on how useful it is for improving overall health in adolescents."

Her study suggests instead a possible increase in triglycerides and cholesterol with vitamin D supplementation, although she acknowledges the findings could have been the result of the small numbers of children taking part in the study and its relatively short time frame.

Larger and longer-term studies should be conducted examine the lasting effects of vitamin D supplements on obese children and teens, she says.

She says the issue is of growing concern as vitamin D supplements, sometimes given to children by parents at five to 10 times the recommended daily amount, are becoming increasingly popular as homeopathic treatments for obesity.

It is well-known that ingesting an excess of vitamin D can result in a condition, hypervitaminosis, which can result in poor appetite or nausea and vomiting, she says.

In some cases it can lead to kidney damage, she points out.