Replacing 25 per cent of "discretionary" foods with fruit, vegetables, lean meats and whole grains would save just 74 calories, a study has found.

Swapping junk food for healthier options would do little to help shrink the waistlines of Australians, a study suggests.

The finding by researchers at the University of South Australia was based on analysis of the diets of more than 12,000 Australians who took part in the 2001-13 Australian Health Survey.

The researchers examined the diets and estimated the impact of cutting the amount of "discretionary" foods - including cakes, desserts, fried foods and processed meats - and swapping zero calorie drinks for water-based, sugar-sweetened ones.

They also looked at the impact of slicing the amount of added sugar and sodium in discretionary foods.

Cutting the amount of "discretionary" foods someone ate by a quarter would potentially slice 184 calories (or nine per cent) off their daily energy intake, the researchers found.

Replacing 25 per cent of those foods - often high in calories, fat, sugar and salt - with healthier choices such as fruit, vegetables, lean meats, whole grains and dairy products would save just 74 calories (or 3.6 per cent).

Cutting the sugar content in biscuits, cakes, muffins and scones by a quarter would also save just 8.6 calories (0.4 per cent), while switching to zero-calorie drinks from sugar-sweetened ones saves 60 calories (2.9 per cent).

Reducing sodium by a quarter in grain-based discretionary foods would lead to a three per cent fall in sodium consumption.

The researchers said their findings showed that reducing consumption of discretionary foods or cutting their sugar and sodium content would "in theory have small to moderate impacts on diet quality of the overall Australian population..."

"The impact of these strategies in combination, or for sub-populations with proportionally higher discretionary food intake may be more sustainable."

Discretionary foods contribute more than a third of the calories in the average Australian's diet.

Reducing the amount of discretionary foods or reformulating them so they have fewer calories have been suggested by health experts as possible ways to tackle obesity and chronic disease.

The researchers were due to present their study at the European Association for the Study of Diabetes meeting in Munich, Germany, on Tuesday.