Experts say the key to weight control is keeping energy intake (food) and energy output (physical activity) in balance so if only as many calories as the body needs, are consumed - weight will usually remain constant - however if a person takes in more calories than their body needs, the result will be excess fat.

Research has shown that exercise can play an important role in weight control and by increasing the energy output, more calories are used up to provide extra fuel; not only does exercise increase the metabolism during a workout, but it causes the metabolism to remain revved up for a period of time after exercising, allowing more calories to be burned off.

Therefore how much exercise is needed to make a difference in a person's weight depends on the amount and type of activity, and on how much they eat.

In a study led by Dr. Tracy Oliver, a specialist at Temple University's Center for Obesity Research and Education, the researchers found that obese women reported their activity levels most accurately, when compared to overweight and normal weight women.

By using an accelerometer, a device that measures movement and acceleration over the course of a year, the researchers were able to compare the amount of activity reported by the women to the actual amount of activity recorded.

It was found at the first measurement session and a second three months later, obese women most accurately reported the amount of physical activity they were getting - but at 12 months, all three groups were reporting on their activity levels more accurately.

Dr. Oliver says the ability to estimate the amount of minutes engaged in physical activity is a difficult task for many, however the accuracy of this estimate can increase with education and participation in physical activity over time.

Aerobic exercise, such as running, swimming and cycling, burns body fat - it is brisk physical activity that makes the heart and lungs work harder to meet the body's increased oxygen demand and promotes the circulation of oxygen through the blood.

A medium-sized adult would have to walk more than 30 miles to burn up 3,500 calories, the equivalent of one pound of fat and if a person consumes 100 calories a day more than their body needs, they will gain approximately 5 kilos in a year.

By doing 30 minutes of moderate exercise daily a person could take that weight off and keep it off and the combination of exercise and diet offers the most flexible and effective approach to weight control.

The research is to be presented at The Obesity Society's annual meeting.