Director of the school, Julio Licinio, said two studies had commenced, one investigating the genetic factors in obesity and another delving into possible links between depression and obesity.

''What we have discovered, which is completely new, is that just having exposure to anti-depressants, even for a short time like two weeks and later in life - perhaps the following year - having several months of high fat food you will gain a lot of weight,'' he said.

''It would be a lot more weight than if you just ate the junk food.''

Professor Licinio said laboratory rats given anti-depressants and then given a junk food diet became ''really huge'' and much fatter than the rats that had never had anti-depressants.

''We don't know why this occurs,'' he said.

''We do have a theory that the anti-depressants stimulate growth factors in the brain and it may be that it is occurring all over the body.''

Professor Licinio said the clinical studies which began a few months ago would continue for several years, hopefully delivering more insights to how hereditary traits affected obesity and how depression could lead to weight gain.

''Some of the key symptoms of depression are a lack of energy and enthusiasm, fatigue and a feeling that everything is overwhelming - so you are not going to go out and run a marathon. You don't want to go out and buy nutritious food so you buy the easiest food and that's the junk food route. And then, without doubt, anti-depressants contribute to weight gain.

''For the research on genetic factors we are looking at obesity in children. We are trying to get people who suffer from early onset obesity to see if there is a genetic factor.''

He said the research would probe a link between obesity and the way the brain dealt with thinking described as ''executive decision making''.

''So you might tell people 'just make better eating choices' but what if they can't make that choice? These are the things we want to know.''