April 2/Psychology & Psychiatry Journal -- Most people consume far too much salt, and a University of Iowa (UI) researcher has discovered one potential reason humans crave it: it might put them in a better mood.
UI psychologist Kim Johnson and colleagues found in their research that when rats are deficient in sodium chloride, common table salt, they shy away from activities they normally enjoy, like drinking a sugary substance or pressing a bar that stimulates a pleasant sensation in their brains.
"Things that normally would be pleasurable for rats didn't elicit the same degree of relish, which leads us to believe that a salt deficit and the craving associated with it can induce one of the key symptoms associated with depression," Johnson said.
The UI researchers can not say it is full-blown depression because several criteria factor into such a diagnosis, but a loss of pleasure in normally pleasing activities is one of the most important features of psychological depression. Plus, the idea that salt is a natural mood-elevating substance could help explain why humans are so tempted to over-ingest it, even though it is known to contribute to high blood pressure, heart disease and other health problems.
Past research has shown that the worldwide average for salt intake per individual is about 10g per day, which is greater than the U.S. Food and Drug Administration recommended intake by about 4g, and may exceed what the body actually needs by more than 8g.
Johnson, who holds appointments in psychology and integrative physiology in the College of Liberal Arts and Sciences and in pharmacology in the Carver College of Medicine, published a review of these findings in the July issue of the journal Physiology & Behavior with Michael J. Morris and Elisa S. Na, UI graduate students. In addition to reporting their own findings, the authors reviewed others' research on the reasons behind salt appetite.
The body needs salt and knows how to find it and how to conserve it. However, scientists are finding evidence that it is an abused, addictive substance -- almost like a drug.
One sign of addiction is using a substance even when it is known to be harmful. Many people are told to reduce sodium due to health concerns, but they have trouble doing so because they like the taste and find low-sodium foods bland.
Another strong aspect of addiction is the development of intense cravings when drugs are withheld. Experiments by Johnson and colleagues indicate similar changes in brain activity whether rats are exposed to drugs or salt deficiency.
"This suggests that salt need and cravings may be linked to the same brain pathways as those related to drug addiction and abuse," Johnson said.
From the April 13, 2009, Prepared Foods E-dition