In a prospective , randomized cross-over design separated by 1 month between diets , 70 normotensive healthy young adults ( F/M : 44/26 , aged 18-38 years ) consumed a 5-day low ( 10 mmol/day ) , normal ( 150 mmol ) , and high ( 400 mmol ) sodium diet followed by heart rate variability ( HRV ) recordings at rest and during 5-min computerized mental arithmetic .