The researchers were surprised to find mice eating a high-fat, high-salt diet put on a similar amount of weight to mice eating a standard diet.
The findings of the study, published in Scientific Reports, may suggest that public health efforts to continue lowering sodium intake might have unexpected and unintended consequences, according to one of the study's authors.
"People focus on how much fat or sugar is in the food they eat, but [in our experiments] something that has nothing to do with caloric content - sodium - has an even bigger effect on weight gain," says co-senior author Justin Grobe, an assistant professor of pharmacology at the University of Iowa Carver College of Medicine.
The team began their research wanting to test the hypothesis that fat and salt would act together to increase both food consumption and weight gain, following on from suggestions that fast food and processed foods could stimulate reward mechanisms in the body.
To test the theory, the researchers fed groups of mice different diets and assessed the amount of weight each group gained. One group of mice consumed a diet of standard rodent food, while other groups consumed food with a high-fat content and varying concentrations of salt, ranging from 0.25-4%.
The researchers found the mice that gained the most weight were those given high-fat food with the lowest salt content, gaining around 15 grams over the 16-week study period. In contrast, the mice on the high-fat, highest salt diet had a level of weight gain - around 5 grams - similar to that experienced by the mice eating standard rodent food.
"We found out that our 'french fry' hypothesis was perfectly wrong," admits Grobe.
Weight gain suppression attributed to reduced digestive efficiency
The researchers then set out to investigate why high levels of dietary salt led to minimal weight gain. After analyzing four key factors that affect energy balance in animals, they found that the level of dietary salt influenced how effective the mice digested their food and how much fat was 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. He believes that the finding could also explain why certain fast foods high in both fat and salt can cause digestive ill effects.
Although the findings of the study might initially suggest that there are positives to be taken from a diet containing high levels of salt, the researchers are keen to point out that there are significant dangers that come with such a diet, including an increased risk for cardiovascular disease.
Instead, the findings could potentially lead to the development of new anti-obesity treatments. The team was already aware that salt levels affect the activity of an enzyme in the body called renin, part of the renin-angiotensin hormone system that is frequently targeted in the treatment of cardiovascular diseases.
The new study indicates that adjusting the renin-angiotensin system can in turn reduce dietary efficiency, and this discovery could give researchers a new target for obesity treatment.
Dr. Michael Lutter, co-senior study author and assistant professor of psychiatry at the University of Iowa, says that it is important for scientists researching the health effects of diets to analyze diets that more accurately reflect normal human eating behaviors, rather than simplified ones often used in animal experiments.
"Our findings, in conjunction with other studies, are showing that there is a wide range of dietary efficiency, or absorption of calories, in the populations," he states, "and that may contribute to resistance or sensitivity to weight gain."