"The underlying cause is genetic, but there are many differences between people," says Dr Jenkins.
"That is a very unwelcome fact for people who would like to think an easy solution is close at hand."
In a previous month-long over-eating test, Prof Campbell and her team found relatives of people with type 2 diabetes gained more weight than people with no family history.
"The dice are loaded against relatives of people with type 2 diabetes," says Prof Campbell, Director of Diabetes Services at St Vincent's Hospital in Sydney and a senior member of Garvan Institute's diabetes and obesity group.
"Fatness is probably due to quite strong but rare genes. Probably different families have different genes.
"Just like breast cancers are not all the same, neither is obesity. There is not a single cause and not one treatment.
"There are drugs coming. But they are going to have to be specific."
She says people can can help themselves down to a lower weight, "but most people want to be models" and end up failing.
She recommends more emphasis on exercise than on dieting.
"The reason we see so many people getting fat is they carry strong hunger genes.
"People no longer have to go fishing or hunting and gathering to eat. They just go to McDonald's, or KFC, or the freezer.
"People don't have to expend energy to get an abundance of food.
"This does not mean they are greedy.
"The same genes would serve them well in times of famine. They would survive while their leaner neighbours would perish," says Prof Campbell.
- AAP
