Queensland University of Technology appetite regulation and energy balance researcher Dr Neil King, from the Institute of Health and Biomedical Innovation (IHBI), said our bodies have strong mechanisms to defend attempts to lose weight but very weak mechanisms to prevent weight gain.
Dr King's weight loss intervention studies demonstrate the "plateau effect", whereby weight loss from exercise and calorie restrictions stops at a certain point.
He conducted two studies on weight loss following induced energy deficits in two different groups of overweight and obese people.
"The "plateau effect" has been known about for some time and weight management consultants recommend longer exercise times, higher intensity or cross training to combat it," Dr King said.
"But these studies show that a plateau in body weight occurs even in the face of a continued negative energy balance."
In the first study, 30 obese men and women took part in a 12-week, laboratory-based exercise program in which they exercised five times a week.
The second study looked at weight loss in 200 males on a commercial weight loss program comprising exercise and dietary advice.
"In the first study, the subjects' energy deficit was caused by exercise only which was fixed and imposed in contrast to the second study where subjects used diet and exercise to lose weight but chose how much they did of each."
Dr King said the first group's weight loss during the first eight weeks averaged 3kg but it "plateaued" at week eight and weight loss for the next four weeks was markedly reduced (.7kg).
The second group had a variable pattern of weight loss but it, too, showed a plateau.
"There appears to be little at this stage to predict the onset , duration and frequency of the plateau," Dr King said.
"My research now aims to identify and characterise mechanisms responsible for our inbuilt weight loss resistance."
Dr King said our energy balance system was programmed to cope with famine, "not the current obesogenic environment which enforces inactivity and a plentiful food supply".
He will present his findings at the Physical Activity and Obesity Satellite Conference of the International Congress on Obesity (ICO2006) from August 31 to September 2 in Brisbane and at the International Conference on Obesity from September 3 to 8 in Sydney.