Introduction: Obesity has become a public health problem all over the world in terms of prevalence, incidence and economic costs, and according to the available evidence, in 2014, about 1.2 billion people, that is, nearly 30% of the world's population , have been overweight or obese all over the world. In addition, 5% of all deaths in the world are attributed to obesity. In acordance with the increase in the prevalence of obesity, a significant increase in obesity-related diseases (diabetes mellitus type 2, high blood pressure, cardiovascular diseases, various types of cancer caused by obesity, etc.) has also been reported. In addition to the increase in the prevalence of overweight and obesity in adults, the prevalence of overweight and obesity in children has increased by 47% since 1980.

FGF-21 is one of the factors affecting lipid and glucose metabolism, whose levels are disturbed in obesity, and an increase or decrease in FGF-21 levels has been reported in obese people. The present study was conducted to investigate the effect of an intense interval training on serum levels of FGF-21 and body composition in obese and overweight children.

Methodology: In a semi-experimental research, the 24 overweight and obese boys (10.6±0.95 years old) randomly assigned in both control two (n=10) and high intensity interval training (HIIT) groups. The HIIT program conducted for five weeks and three session per week. Each HIIT session consist of 10 intensive intervals (with the intensity more than 80% of maximum heart rate) that each intensive interval followed by one 90 second interval with intensity less than 70% of maximum heart rate. Blood sampling conducted in pre and post-test stages and FGF-21 and lipid profile was measured. Data were analyzed by Analysis of Covariance test.
Results: The results showed that the percentage of fat decreased and the changes in FGF-21 levels were not significant (p=0.061).
Conclusion: It seems that HIIT at least in short term cannot have a significant effect on FGF-21 levels.
Keywords: obesity, overweight, intense interval training, fibroblast growth factor 21