Influence of Physical Fitness Levels in Circulating Irisin

Abstract

Obesity has a large economic cost on the nation. As of 2008, the annual medical cost of obesity was $3.38 billion. The protein hormone irisin has been shown to increase the mitochondrial density of adipose tissue, a process known as browning. As irisin was found to improve metabolism, it is thought that it may help prevent and/or treat overweight, obesity, and other conditions associated with obesity. PURPOSE: To investigate the association of physical fitness (PF) levels with circulatory levels of irisin in both healthy males and females. METHODS: Our sample was comprised of a total of 32 unpaid male and female volunteers with 21.3 ± 2.2 years, 65.2 ± 8.8 kg of body weight (BW), 22.2 ± 1.99 kg/m2 body mass index (BMI). The subjects were taken through a series of tests: a cardiopulmonary maximal fitness test on a treadmill, a one repetition maximum (1RM) strength test, and dual-energy x-ray absorptiometry (DXA) to assess their PF levels. To study the irisin levels, blood was taken from the subject via venous puncture. RESULTS: We found large correlations between irisin levels and maximal oxygen uptake (VO2max) (r= -0.59, p=0.025) and lean body mass (LBM) (r= -0.41, p=140) in males (n=14). However, we did not obtain the same results in females. In addition, percentage of body fat was not correlated with irisin (r=0.04 and r= -0.04 in male and female, respectively). CONCLUSION: Irisin levels in males seem to be positively associated to increased fat content, negatively associated to increased PF (VO2max and LBM), and these results were different for females. Therefore, the secretion mechanisms of this hormone should be studied on a greater sample size, with more varied body composition profiles (health vs. unhealthy), and, contrary to prior studies, taking into account sex differences.