Background :

Regular exercise is a cornerstone in the prevention and the management of comorbidities. Unfortunately, the metabolic benefit of exercise training is not universal and varies among individuals. A main factor likely to explain the exercise training variability is the lack of empirical evidence on the determinants of exercise training. A series of muscle-derived cytokines have recently been discovered that (1) are released during exercise and (2) exert positive effects on peripheral tissues. Irisin is one of these novel "myokines" and might contributes to the metabolic adaptations to exercise training.

Methods:

The investigators will perform a pilot cohort study in which obese adolescents will perform resistance exercise training for 6 weeks. The main exposure variable will be the acute release of irisin during resistance exercise. The main outcome measures will be the change in hepatic triglyceride content and glucose area under the curve during a 75g, 2-hour oral glucose tolerance test.

Study Hypothesis:

The primary overall hypothesis is that the change in plasma Irisin with a single bout of exercise will be associated with the metabolic adaptations to 6 weeks of resistance training, specifically, the reductions in hepatic triglyceride content and post-prandial glucose excursions, in obese adolescents.

Inclusion Criteria:

1. Adolescents (12-18 years)
2. Body mass index considered obese according to the International Obesity Task Force

Exclusion Criteria:

1. Adolescents with medication-induced or type 1 diabetes
2. are currently being treated with corticosteroids or atypical antipsychotics, as these agents significantly influence carbohydrate metabolism
3. have an orthopaedic injury or chronic illness that would prevent them from performing the intervention
4. have experienced >10% weight loss or enrolled in weight loss program within six months of enrolment
5. a history of alcoholism or drug abuse
6. require use of medication(s) known to affect insulin sensitivity or secretion within the last 30 days
7. medication(s) known to cause weight gain
8. anabolic steroids
9. weight loss medication(s)
10. pregnant or planning to be pregnant

A community-based convenient sample of overweight adolescents aged 12-18 years with a BMI > 95th centile for age and sex.