When scientific studies are reported on in the media they can appear far more conclusive than they really are. For example, many clinical studies in humans have shown a link between high sodium (salt) intake and high blood pressure. Based on that evidence, the American Heart Association has recommended a maximum daily intake of less than 2300 milligrams of sodium. Yet there are still those scientists who argue, based on other studies, that improving overall dietary quality, including high intake of fruits and vegetables and adequate intake of minerals like potassium and calcium, is just as important as salt intake to reducing high blood pressure in humans.
These conflicting interpretations of scientific studies in humans are perfectly reasonable when you realize that the effects of changes in the human diet are affected by multiple variables, including age, gender, body weight, smoking status, and more. It's simply not feasible to control all other variables so that the effects of salt intake can be assessed as accurately as possible.
That said, it is possible to control all other dietary variables - if you don't use humans.
An international group of scientists studied the effects of varying levels of salt intake in chimpanzees, our closest genetic neighbors (Circulation. 2007;116:1563-1568). Unlike studies of humans, the scientists were able to control the monkeys' dietary intake with great accuracy, as their diet consisted solely of vegetables and fruits supplemented with dietary "biscuits." These biscuits provided the chimps with potassium, calcium, and salt in specific amounts, so the biscuits could be manipulated to provide varying levels of sodium in the monkeys' diet. Obviously humans would find it difficult to tolerate such a restricted diet - nor could they participate in such a study for the term of this study, which was over two years in length.
The results are striking: Increases in sodium intake were clearly linked to increases in blood pressure. Similarly, decreases in blood pressure were seen along with dietary decreases in salt intake. The amount of salt in the monkeys' diet, unlike other, previous studies, was similar to the amount of salt in that of humans and represented the high sodium intake of a typical Western diet (as much as 6-8 grams of sodium per day) versus that of current recommendations (about 2 grams per day).
What this means for you
It's hard to draw conclusions from animal studies that clearly apply to humans, so you won't see me talking about them all that much. However, this one is pretty compelling, as it removes a lot of the complicating variables and controls the participants' salt intake very accurately and for an extended period of time. No study is absolutely conclusive, but this one adds a hefty weight to the sodium-intake side of the blood pressure debate.