The most appropriate and effective exercise interventions for weight loss remain a matter of continued discussion and there is a need to identify sustainable exercise programs which successfully promote weight loss and benefit health.

The primary objective of this study is to determine if high-intensity intermittent sprinting (HIIS) produces better results in terms of reducing metabolic and cardiovascular risk factors, with special emphasis to fat mass loss and insulin sensitivity in "healthy" obese volunteers compared with an iso-caloric program of moderate-intensity continuous cycling (MICC) (control condition following the international recommendations). We will also assess the impact of short duration sprints (SDS) in the above factors and to understand the potential mechanisms behind different outcomes among training programs.

The overall hypothesis is that HIIS will lead to a greater reduction in metabolic and cardiovascular risk factors compared with MICC in healthy obese volunteers and that SDS will produce similar improvements in cardiovascular risk factors as the longer duration HIIS

Participants will be randomized to one of three groups: HIIS (8s sprint:12s rest) or MICC (control group) iso-caloric protocols (for the duration needed to induce a 250 kcal energy deficit) or SDS (8s sprint:12s rest) for 10 minutes for 12 weeks. The intervention groups will be matched for age, BMI and male/females ratio and participants will be asked not to change their normal diet throughout the study.

Before and after the exercise intervention (12 weeks), participants will perform the following assessments:

1. Three-day food diaries
2. Anthropometric measurements (weight, height, waist and hips) using standard procedures;
3. Body composition using dual energy x-ray absorptiometry (DEXA)
4. RMR and resting respiratory exchange ratio (RER) using indirect calorimetry;
5. Endothelial function assessed by flow-mediated dilation (FMD) of the brachial artery using vascular ultrasound according to current guidelines;
6. Maximal oxygen uptake (VO2max) and fat oxidation, cardiac output and left ventricular function during exercise using a cycle ergometer;
7. Muscle and fat biopsies
8. Fasting and postprandial release of appetite related hormones (and subjective feelings of hunger/fullness using visual analogue scales - VAS).

Inclusion Criteria:

• weight stable on the last three months,
• not currently dieting to lose weight
• inactive lifestyle.

Exclusion Criteria:

• History of endocrine/cardiovascular/pulmonary/kidney disease,
• anaemia,
• gout,
• depression or other psychological disorders,
• eating disorders,
• drug or alcohol abuse within the last two years
• current medication known to affect appetite or induce weight loss.
• planned surgery during the study period
• participation in another research study
• restraint score derived from the TFEQ>12
• post-menopausal women