But, in a wide-ranging study, they also discovered that just the right amount each day can help preserve our memory and thinking skills as we age.
It is known that small amounts of sodium are needed to help balance the level of fluids in the body and the main dietary source is salt.
The NHS says adults should eat no more than 6g – just over one fifth of an ounce – of salt containing 2.4g (0.08ozs) of sodium a day.
But the average Briton consumes 8g of salt (0.28ozs) or about 3.2g (0.1oz) of sodium, much more than the body needs.
The study by the University of Colorado Anschutz Medical Campus found both too much or too little sodium in the blood were linked to thought process impairment and decline over time in generally healthy older men.
And the researchers warned that either imbalance can go unnoticed by family doctors as we age.
The study involved 5,435 men over 65 living at home with no symptoms who were followed for just under five years.
A total of 100 men had serum levels indicative of hyponatremia – too little sodium.
Slightly lower sodium levels in the blood were related to both cognitive impairment and function decline and there was also a link between high serum sodium and cognitive decline.
University assistant professor Dr Kristen Nowak said: “To our knowledge, our findings are the first to demonstrate an independent association between lower serum sodium and cognitive decline in generally healthy community-dwelling older men.”
Dr Sara Imarisio, of Alzheimer’s Research UK, said: “The body needs small amounts of sodium in order to function properly, while this research highlights a potential link between lower levels of sodium in the blood and a decline in memory and thinking skills.”
The study was published in the Clinical Journal of the American Society of Nephrology