The rats given coffee fortified with olive leaf extract for a further eight weeks showed improved cardiovascular, liver and metabolic signs compared with rats given normal coffee.
Lindsay Brown of University of Southern Queensland's said the olive leaf extract led to weight loss because of its anti-inflammatory properties, reports the Sydney Morning Herald.
The role of inflammation in causing fat storage is the subject of studies around the world.
Green tea, turmeric, aspirin derivatives and ibuprofen are thought to have a similar weight-loss effect.
Greg Jardine, a biochemist said he had observed that olive leaf extract products encouraged weight loss.
Yet the results of the study on rats exceeded expectations.
The study was published in the American Journal of Nutrition.