The amount of exercise you need to do depends on your weight loss goal. If your goal is:

1. Weight loss

150-250 minutes of moderate intensity* physical activity per week provides modest weight loss. People with a body mass index that classifies them as overweight or obese should aim for more than 250 minutes of exercise per week (e.g. 5 sessions of one hour per week) and perform resistance training to increase the amount of metabolically active muscle tissue in their bodies as well as reduce their health risks.

2. Weight maintenance

More than 250 minutes per week of moderate intensity physical activity per week (e.g. 5 sessions of 50 minutes per week).

3. Preventing weight gain

150-250 minutes of moderate intensity physical activity per week (e.g. 5 sessions of 30 - 50 minutes per week).

*Moderate intensity physical activity is an increase in your breathing rate to a point where you are still able to talk comfortably but not sing. A more technical explanation is that it is exercising in a range of 65-75% of your maximum heart rate. Modes of exercise which are best suited to moderate intensity exercise are brisk walking, swimming, bike riding or cardio machines at the gym. Exercise should be continuous rather than in short bursts (these short bursts occur in sports such as tennis or touch football).

Remember that it's very difficult to lose weight through exercise alone. If you try to lose weight without reducing your energy intake, you'll need to exercise for at least 300 minutes a week! The best way to lose weight is to exercise AND consume less food energy. Not only will you lose weight at a faster rate, but by engaging in regular aerobic exercise you'll keep the weight off for life.

Reference Donnelly, J. et al., 2009, ' Appropriate Physical Activity Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults', Medicine & Science in Sports & Exercise Vol. 41:459-471.