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We're constantly told that too much salt is bad for us.

But in a study that seems defies conventional wisdom, scientists have found salt may actually help people lose weight if they're already a high-fat diet, the Daily Mail reports.

As exciting as this may sound to fast food lovers, the researchers caution that very high levels of salt are till associated with increased risk for cardiovascular disease in humans.

But they do claim these findings point to the profound effect non-caloric dietary nutrients can have on energy balance and weight gain.

'People focus on how much fat or sugar is in the food they eat,' said Justin Grobe, assistant professor of pharmacology at the University of Iowa.

'But [in our experiments] something that has nothing to do with caloric content - sodium - has an even bigger effect on weight gain.'

The University of Iowa team started the study based on the theory that fat and salt, both being tasty to humans, would act together to increase appetite and cause weight gain.

They tested the idea by feeding groups of mice different diets: normal chow or high-fat chow with varying levels of salt from 0.25 to four per cent.

To their surprise, the mice on the high-fat diet with the lowest salt gained the most weight, about 15 grams over 16 weeks.

Animals on the high-fat, highest salt diet had low weight gain that was similar to the chow-fed mice, about give grams.

'We found out that our 'french fry' hypothesis was perfectly wrong,' said Grobe.

'The findings also suggest that public health efforts to continue lowering sodium intake may have unexpected and unintended consequences.'

To find out why the high salt prevented weight gain, the researchers looked at four key factors that influence energy balance in animals.

On the energy input side, they ruled out changes in feeding behaviour - all the mice ate the same amount of calories regardless of the salt content in their diet.

On the energy output side, there was no difference in resting metabolism or physical activity between the mice on different diets.

In contrast, varying levels of salt had a significant effect on digestive efficiency - the amount of fat from the diet that is absorbed by the body.

'This suppression of weight gain with increased sodium was due entirely to a reduced efficiency of the digestive tract to extract calories from the food that was consumed,' explains Grobe.