Gastric bypass surgery helps "severely obese" teenagers shed weight and keep it off for years, according to research published on Friday.
In two studies, one in the United States and the other in Sweden, young people who underwent the procedure were about thirty percent lighter five to 12 years after the operation, scientists found.
Negative side effects, including vitamin deficiences and the need in some cases for follow-up surgery, were minor compared to health gains, scientists reported in The Lancet Diabetes & Endocrinology, a medical journal.
There were 58 patients in the US cohort, and 81 in the Swedish one.
In the US study, led by Thomas Inge of Cincinnati Children's Hospital Medical Center, eight years after the operation the number of teenagers with diabetes had dropped from 16 to two percent, and those with high cholesterol fell from 86 to 38 percent.
The share with chronic high blood pressure also declined, by two thirds.
Yet most of the patients remained at least "very obese", and only one stabilised at a normal weight.
Healthy weight is determined with the body-mass index (BMI), one's weight in kilogrammes divided by the square of one's height in meters.
A BMI of 25 to 30 indicates being overweight, while above 30 means one is obese. The range of normal weight is 18.5 to 24.9.
"Very obese" corresponds to a BMI over 35, while "severe obesity" is 40 or over.
Gastric bypass involves reducing the stomach to below three percent of its natural volume, then connecting a new gastric pouch that bypasses the stomach and goes straight to the intestine.
In the Swedish study, led by Torsten Olbers of the University of Gothenburg, young patients undergoing the surgery were compared with obese teenagers who did not.
Five years later, the gastric bypass group were on average 28 percent lighter, while the other group had only shed a couple of percentage points from their body mass.
In the United States, there are about 4.6 million children and teenagers classified as "severely obese".
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