Epidemiological studies suggest a role for a low glycemic index (GI) diet in the management of obesity and associated metabolic risks including diabetes. However, evidence from long-term, randomized controlled trials exploring the relationship between low GI diet, weight reduction and glycemia, particularly in children and adolescents, is lacking. Modern food-processing technology has produced many food products with high GI which may contribute to the burgeoning epidemic of obesity worldwide. Since dietary habits are shaped in early life, adolescence is a critical period to educate our young people to acquire a healthy eating habit to prevent obesity.

The investigators hypothesized that, in Chinese adolescents, low GI diet results in greater reduction in body mass index and body fat percentage, enhanced insulin sensitivity and favorable changes of cardiometabolic risk factors compared to conventional diet after 12 months of intervention.

This study is a randomized controlled trial of a low GI (<55) versus conventional Chinese diet (GI>/=70) in adolescents (12-month intervention followed by a 6-month observational period) to study; 1) the changes in body mass index and obesity associated changes in cardiometabolic profile; 2) the underlying hormonal factors associated with these changes.

This is a 12-month randomized controlled dietary interventional trial followed by a 6-month observational period. The objectives of this study include:

1. To study changes of body mass index, body fat percentage, insulin secretion, insulin resistance and obesity associated cardiometabolic factors;
2. To explore underlying neurohormonal mechanisms associated with these changes in both short- and long-term.

Inclusion Criteria:

• School children aged 15 or above
• Male or female of Chinese ethnicity
• Body mass index >/= 95th percentile of Hong Kong local age- and sex-specific reference
• No major medical illness
• Not on any chronic medications
• Post-pubertal as ascertained by clinical assessment by endocrinologist
• Willingness to give written informed consent by participants and his/her parents/guardian

Exclusion criteria:

• Concurrent participation in any clinical trial, dietary intervention or weight loss program
• Concomitant intake of weight reducing agent
• Active and uncontrolled endocrine diseases including hypo- or hyperthyroidism, Cushing's syndrome, diabetes mellitus, etc.
• Significant renal impairment, defined as serum creatinine >150 umol/L and/or estimated glomerular filtration rate <60ml/min/1.73m2
• Significant liver impairment, defined as >2 times upper limit of alanine aminotransferase and asparate aminotransferase levels
• Gastrointestinal problems that would prevent them from following the test diets
• Active malignant disease
• Pregnant or lactating
• Any medical illness or condition including known non-compliance, as judged by the investigators as ineligible to participate the study