Scientists have found that lower levels of sodium in the blood - known as hyponatremia - is linked with a decline in cognitive function with advancing age.

Hyponatremia occurs when the sodium level in the blood falls below 135 Millimoles Per Litre (mmol/L), according to a study published in the Clinical Journal of the American Society of Nephrology.

Mild hyponatremia was once thought to be asymptomatic, but recent studies suggest that it may be associated with higher risks of attention deficits, gait disturbances, falls, cardiovascular events, and even premature death.

“Slightly lower sodium levels in the blood are likely to be unnoticed in clinical practice,” said Kristen Nowak, from the University of Colorado Anschutz Medical Campus in the US.

“Because both slightly lower serum sodium levels and mild changes in cognitive function are common occurrences with advancing age, future research on this topic is important- including determining whether correcting lower sodium levels affects cognitive function,” Nowak said.

The researchers examined information on 5,435 asymptomatic community-dwelling men aged 65 years and above who were followed for a median of 4.6 years.

A total of 100 men had serum levels indicative of hyponatremia.

They found that slightly lower sodium levels in the blood were related to both cognitive impairment and declines in cognitive function over time. Compared with men with sodium levels of 141-142 mmol/L, men with levels of 126-140 mmol/L were 30 % more likely to have cognitive impairment at baseline and 37 % more likely to experience cognitive decline over time.

They also found an association of high serum sodium (143 -153 mmol/L) with cognitive decline over time.