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Men who are slim and healthy when their children are conceived could be helping several generations of offspring avoid obesity, a new University of New South Wales study suggests.

The study found rats with one fat grandfather who ate a high-fat diet were more likely to become obese than those with four slim grandparents.

Team leader Margaret Morris said scientists already knew mothers who were obese during pregnancy could pass the condition on to their children, but this new data showed that paternal and even grand-paternal diet could have an impact.

She said the results were likely relevant to humans and signalled that the obesity epidemic today could have repercussions for decades to come.

''The way we're exercising and what we're eating is changing a lot and we may be unwittingly effecting the next generation,'' she said.

Professor Morris will present her team's findings on Thursday at an Obesity Summit in Canberra, which will bring together scientists, industry groups, advocates and policy makers to discuss Australia's weight problem.

An Organisation for Economic Co-operation and Development last week found Australia was the fourth fattest nation in the world, with 28.3 per cent of the adult population obese.

Professor Morris said when male rats were fed a poor diet, those with obese paternal grandfathers became about 8 per cent heavier than rats whose grandparents were all thin.

Female rats with obese grandfathers were affected by obesity at an earlier age but the impact on their body weight was not as great, she said.

Rats which ate a healthy diet stayed slim, regardless of the weight of their grandparents.

''It is important that people know that if you yourself have a healthy lifestyle you're going to help your disease risk, regardless of what your ancestry is,'' she said.

Professor Morris said the impacts of poor diet were passed on to offspring in part through epigenetic changes, or chemical modifications to DNA.

But the scientists were still working to understand the underlying processes behind the results.

''It could be that we have changes in tissues even in the animals who are on the healthy diet, we may not be able to see them when we just look at body weight but maybe they're functioning quite differently,'' she said.

Professor Morris said her team was also interested in working out if there was a way to reverse the impact of a heritage of obesity.

''All this work is pointing towards advising both men and women to do everything they can before they get pregnant in terms of their diet, their exercise levels, their nutrition, their smoking, their alcohol use, because if you start with the optimal state you're probably more likely to have a good outcome,'' she said.

Professor Morris said it was not yet known how many generations the impacts of poor diet could reach.

In the experiment, the middle generation of rats were fed healthy diets to ensure the impact was coming from the paternal grandfather only.