University of Iowa study debunks ‘French fry’ theory

IOWA CITY - Experts for eons have warned Americans to skip the salt, and federal regulators recently have pushed for tighter consumption recommendations, but a new University of Iowa study pokes holes is some preconceived sodium notions.

Take the 'French fry theory” that UI researchers set out to investigate. The thought was that fat and salt, when paired together, would increase food consumption and thus promote weight gain.

But they found the opposite to be true - at least for mice.

Adding high salt to a high-fat diet actually prevented weight gain in their research subjects, pointing to the key role non-caloric dietary nutrients can play in energy balance and weight gain and backing the need for more exploration and care when setting consumption guidelines.

'These results really prompt continued - and nuanced - discussions of dietary recommendations,” said Justin Grobe, assistant UI professor of pharmacology and co-author of the study, published June 11 in the journal Scientific Reports.

As part of a proposed update to the nutrition facts label found on most food packages in the United States, the U.S. Food and Drug Administration is looking to tighten its daily sodium consumption recommendations from 2,400 mg to as low as 1,500 mg.

And UI researchers caution high levels of salt are linked with cardiovascular disease in humans, stressing their data shouldn't be used directly to dictate safe levels of sodium intake in humans.

'The question facing policy makers, though, is how beneficial further reductions in sodium are for our overall health,” Grobe said. 'These data hint that further reductions in sodium may have unintended consequences. More research into the interactions of dietary nutrients is definitely warranted.”

The UI study tested the impact of high salt by feeding different diets to groups of mice. Some were given normal food and others were given high-fat food, all with varying levels of salt - from .25 to 4 percent. They found the mice on the high-fat diet with the lowest salt gained the most weight, while those on the high-fat, high-salt diet had minimal weight gain - on par with those eating normal food.

'We found out that our French fry hypothesis was perfectly wrong,” said Grobe, who also is a member of the UI Fraternal Order of Eagles Diabetes Research Center and fellow of the American Heart Association.

To investigate why, researchers examined several factors that influence energy balance in animals. They looked at changes in feeding behavior - all the mice ate the same amount, regardless of the salt content in their diet. And they found no difference in resting metabolism or physical activity between the mice.

But researchers did find varying salt levels had a significant effect on the amount of fat absorbed by the body - more salt reduced digestive efficiency.

'In other words, with increased sodium content, fewer of the calories were actually absorbed by the animals, so the animals gained less weight,” Grobe said.

The discovery might explain the widely-known adverse effects on the digestive system after consuming fast food high in both fat and salt, according to the researchers. It also could have implications for anti-obesity treatments.

And, although the UI research does not dictate sodium levels for humans, it does add to the conversation around whether salt intake should be reduced even further, Grobe said. A 2013 Institute of Medicine report found studies to be inconsistent and insufficient to conclude that lowering sodium levels will increase or decrease the risk of heart disease or mortality.

'There is not a lot of evidence about what the ramifications long-term of having even lower sodium would be,” Grobe said.

The UI study and its findings in mice highlights the need for more research, said Michael Lutter, co-senior study author and UI assistant professor of psychiatry. It shows the science community is just beginning to understand the complex interactions between nutrients and calorie absorption, and Lutter said researchers need to do more analysis of diets more accurately reflecting human eating behavior.

'This is a study in mice, and we have no idea if a similar thing happens in humans,” he said. 'I think the major take-away is that non-caloric nutrients can have an important effect on absorption of calories, but there is still a lot of work to be done.”