Severely obese teens, including those who have had weight loss surgery, have a high risk of nutritional deficiencies, according to a recent study.

Researchers from the Cincinnati Children's Hospital Medical Center found that at least five years after undergoing gastric bypass surgery, teens and young adults maintained significant weight loss but were at risk of nutritional deficiencies, particularly low iron, mild anemia and low vitamin D. The study also found low iron and low vitamin D in severely obese teens who did not undergo weight loss surgery. Those who didn't have surgery also had low levels of protein in their blood.

"We knew there were nutritional difficulties in teens who had undergone bariatric surgery, but everyone thought it was primarily the surgery that caused these problems since gastric bypass excludes the portion of the small intestine where many nutrients, especially iron, are most absorbed," Stavra Xanthakos, co-author of the study medical director the Surgical Weight Loss Program for Teens at Cincinnati Children's, said in a statement. "What this shows us is that nutritional deficiencies occur even in teens who don't undergo surgery. Severely obese patients should be screened for nutritional deficiencies, regardless of whether they've undergone weight loss surgery."

For the study, researchers examined 79 obese teens who either received weight loss surgery or were evaluated but didn't receive surgery. The patients were evaluated between 2001 and 2007 and contacted to participate in the study between 2011 and 2014.

Eight years on average after surgery, these patients had experienced "durable and significant" weight loss, about 28 percent, Dr. Xanthakos said. Those who didn't receive surgery had not lost weight.

The study is believed to be the first study to compare the nutritional status of severely obese teens who did not undergo weight loss surgery to those who did have surgery.

The findings were presented at the annual meeting of the Pediatric Academic Societies in Vancouver, Canada on May 4.